Abstract: The novel crown-jewel (CJ) structured PdCu nanoalloys have attracted considerable interest in high-performance single-atom catalysis. The characteristics of demanding high-temperature calcination in the synthesis of these samples disable us from experimentally understanding the details of the thermal evolution behavior of PdCu nanoclusters during the heating process. In this work, by analyses of potential energy surface, bond order parameter, and radial distribution function, we have theoretically studied the thermodynamic stabilities and structural evolution of Pd-decorated Cu-based CJ nanoclusters with various compositions and sizes by molecular dynamics simulations. PdCu nanoclusters undergo a cuboctahedral (Cubo) to icosahedral (Ico) structural transformation before melting. This transformation is size- and Pd-composition dependent. The small size and high Pd-composition of PdCu nanoclusters facilitate this transformation. In addition, we find that the surface and interface effects of clusters have an important impact on the structural transformation and Cubo–Ico structural transformation is strongly related to the release of excess energy. read less

Abstract: We introduce a method to visualize dislocations along grain boundaries at the atomic level. It uses an atomic-level Nye tensor, representing the dislocation density. To calculate the Nye tensor at grain boundaries, we extend the Hartley-Mishin strain gradient calculation to the displacement shift complete lattice. We show that the method is effective in visualizing disconnections and the dislocation content of grain boundary phase junctions in bodycentered cubic tungsten, as well as face-centered cubic copper. In addition, we use the method to characterize the morphology of a two-dimensional grain boundary phase nucleus in a symmetric tilt grain boundary in tungsten. This method can be applied to both bulk dislocations and grain boundary disconnections, which makes it ideal for studying the interactions and reactions of bulk dislocations with grain boundaries, and grain boundary disconnections. read less

Abstract: Metal–semiconductor contacts in silicon carbide (SiC) diodes endure damages at the interface when exposed to harsh radiation environments. Due to the rapid rise in temperature and ultrafast cooling that follows the radiation impact, the structural properties of the materials can be altered through melting, recrystallization, and amorphization. A detailed understanding of the material failure modes at the interface is lacking, specifically at the nanoscale. We use molecular simulations to investigate the ultrafast melting at tungsten (W)–SiC interfaces following radiation damage and apply deep learning techniques to track the transient evolution of the local molecular structures. We show that W near the radiation track undergoes melting and, eventually, most of it recrystallizes with a noticeable degree of undercooling, while SiC is rendered permanently amorphous. The observation of local undercooling in W films is important as it can affect the device performance even before the bulk melting temperature of the material is reached. We also show that at high temperatures, the interface undergoes a fracture-like failure. The results presented here are significant in understating the different failure modes of SiC diode materials. read less

Abstract: Significance This work demonstrates dislocation dynamics in body-centered-cubic (BCC) high-entropy alloys (HEAs). The local composition inhomogeneity in these concentrated solutions leads to an unconventional rate-controlling mechanism that governs dislocation mobility. The activated process becomes nanoscale detrapping, of kinks on screw dislocations and of trapped segments of edge dislocations, presenting an activation barrier of similar magnitude for both dislocation types. Their sluggish mobility explains the elevated strength and strain hardening in BCC HEAs. Atomistic simulations of dislocation mobility reveal that body-centered cubic (BCC) high-entropy alloys (HEAs) are distinctly different from traditional BCC metals. HEAs are concentrated solutions in which composition fluctuation is almost inevitable. The resultant inhomogeneities, while locally promoting kink nucleation on screw dislocations, trap them against propagation with an appreciable energy barrier, replacing kink nucleation as the rate-limiting mechanism. Edge dislocations encounter a similar activated process of nanoscale segment detrapping, with comparable activation barrier. As a result, the mobility of edge dislocations, and hence their contribution to strength, becomes comparable to screw dislocations. read less

Abstract: In order to clarify the physical meaning of the eigenvector of the atomic elastic stiffness matrix, Bija=Δσia/Δej, static calculations of uniaxial tension are performed on various fcc, bcc, and hcp metals with four different embedded atom method (EAM) potentials. Many fcc metals show instability for the constant volume mode, or the eigenvector of (Δexx, Δeyy, Δezz) = (±1, ∓1, 0), under the [001] tension. Bcc also loses resistance against other constant volume mode, (Δexx, Δeyy, Δezz) = (±1, ±1, ∓2), in the [001] tension. Hcp shows shear modes Δγyz and Δγzx under the [0001] tension, which correspond to atom migration by dislocation on the slip plane. Similar shear modes appear in the [111] tension of fcc and [110] tension of bcc. Hcp also changes the mode to constant volume and shear in the [1¯010] tension, which imply the deformation in the pyramidal and prismatic planes. read less

Abstract: In the present study, the impact of surface roughness on the wettability behavior of Al droplets has been investigated via molecular dynamics (MD) simulations. In this work, amorphous carbon (AC) and graphite substrates with different depths and widths were considered. The results show that the increased width of grooves causes the transition of the wetting state from Cassie to Wenzel. Thermodynamic property analysis results indicate that the solid-liquid adhesion and the work done for the removal of the Al droplet from the solid surface decrease as the roughness increases. However, the adhesion in the Wenzel wetting state is better than that in the Cassie wetting state. Therefore, the contact angle increases with the increased roughness in the Cassie wetting systems, while in the Wenzel wetting systems, the contact angle is less than that in other rough systems. In addition, due to the heterogeneity of the surfaces, the density of Al droplets in the solid-liquid interface is decreased with the increased roughness. The anisotropic spreading of Al liquid can be explained by the MSD curves along the X and Y directions. read less

Abstract: In this work, we propose a classical equivalent two-body (ETB) model that can capture more detailed dynamic features arising from energy dissipation and atom oscillations, by introducing the Langevin equation of a harmonic oscillator. The trapping probability, scattering angle and the residence time of Ar interacting with Pt (111) and W (110) surfaces predicted by the ETB model agree well with the measured experimental data or molecular dynamics simulations. Moreover, the ETB model is also used to study the influence of oscillating and dissipating properties on the thermal accommodation coefficients and rainbow scattering of gas atoms colliding with the surface. It is found that the dependence of energy accommodation coefficients and rainbow scattering on the oscillating and dissipating parameters shows nonlinear behaviors, and the associated mechanisms are disclosed. ETB model further provides the possibility to explore the physics beyond the existing two-body models for a better description of energy transferring during atom surface interactions. read less

Abstract: So far, there have been few studies on the microscopic wetting behavior of aluminum liquid on cathode surfaces, which is critical for developing wettable cathode materials. In the present study, an investigation on the coalescence and wetting mechanism of Al droplets on different carbonaceous substrates has been performed via molecular dynamics (MD) simulation for developing wettable cathodes. The growth rate of liquid bridge, the mean squared displacement, the balanced contact angle, and the time of full coalescence were calculated to describe the coalescence and wetting of the Al droplets. The results illustrate the sequence of full coalescence time for the Al droplets: DG < HCNT < VCNT ≈ AC and the corresponding balanced contact angles were 47.98°, 53.32°, 55.02°, and 63.12°, respectively. Furthermore, the presence of defects on DG will increase the time of coalescence and the contact angle but the directions of defects have little influence. The free energy analysis indicates that the defects reduce the solid-liquid interaction and the work done for removing the Al droplet from the substrates so that the wettability is weaker than that for perfect graphene, which also explains the balanced wettability of Al droplets on the other substrates. In addition, the surface roughness increases the contact angle of Al liquid on AC (from 62° to 113°-120°) and hence, the wettability is changed from good to poor. In general, our results can improve the understanding of the wetting of AC and graphene by Al liquid at the atomic level, which can provide direction and theoretical guidance for further research on wettable cathodes. read less

Abstract: ABSTRACT The properties of ohmic contact and thermal boundary conductance between Al and GaN have been studied extensively, but the interface structures and deformation mechanisms in the Al/GaN multilayer can be rarely found in literatures. By molecular dynamics (MD) simulations, we systematically studied the interface structures and structural deformations in the Al/GaN multilayer. Two kinds of interface structures are identified according to the different terminal surfaces of GaN; glide-set terminal interface and shuffle-set terminal interface. Further analysis shows that interface has the maximum stress and misfit lines have the maximum stress values, which serve as the dislocation sources in the Al layer due to the larger stress in the interface. The mechanical responses of the Al/GaN multilayer exhibit a minor stage and some distinctive drops in the stress–strain curve. The first stage is associated with the dislocation nucleation from the interface. Upon further compression, more slip systems appear in the Al layer and dislocation nucleation in GaN could induce drops in curves. Meanwhile, the multiplications of dislocations cause strain hardening behaviours. read less

Abstract: As a representative boundary, interphase-interface may affect the strength or ductility of multilayered composites dramatically. However, the effect of the interface with mismatch dislocations on the mechanical behavior of multilayered composites is still not clear. In the present work, we performed molecular dynamics simulations to investigate the effect of interface structures and layer spacing on the mechanical properties of the Ti/Al nanolaminate. The results indicate that there are two transitions of the plastic deformation mechanism in the Ti layer with the increase of layer spacing in the sample with a coherent interface. The plastic deformation mechanism evolves from one that is dominated by dislocation to the phase transformation from the hcp-Ti to the fcc-Ti mode, which transfers to the dislocation slip deformation again. For the samples with an incoherent interface, the plastic deformation is dominated by the transformation from hcp-Ti to fcc-Ti, regardless of the variation of layer spacing, while the plastic deformations in the Al layers are mainly dislocations confined in the layer slip in the samples with both coherent and incoherent interfaces. When the layer spacing is larger than 6.6 nm, an obvious second hardening is observed due to the superior dislocation storage ability of the Ti/Al laminate with the incoherent interface. Meanwhile, extraordinary ductility is obtained when optimal layer spacing is employed in the Ti/Al laminate. Moreover, the phase transformation mechanism of hcp-Ti to bcc-Ti has also been explicated in the present work. The general conclusions derived from this work may provide a guideline for the design of high-performance Ti/Al multilayer and alloy devices.As a representative boundary, interphase-interface may affect the strength or ductility of multilayered composites dramatically. However, the effect of the interface with mismatch dislocations on the mechanical behavior of multilayered composites is still not clear. In the present work, we performed molecular dynamics simulations to investigate the effect of interface structures and layer spacing on the mechanical properties of the Ti/Al nanolaminate. The results indicate that there are two transitions of the plastic deformation mechanism in the Ti layer with the increase of layer spacing in the sample with a coherent interface. The plastic deformation mechanism evolves from one that is dominated by dislocation to the phase transformation from the hcp-Ti to the fcc-Ti mode, which transfers to the dislocation slip deformation again. For the samples with an incoherent interface, the plastic deformation is dominated by the transformation from hcp-Ti to fcc-Ti, regardless of the variation of layer spacing, wh... read less

Abstract: Al-Cu alloys are widely used in aeronautics and aerospace engineering because they exhibit outstanding performance. However, their casting characteristics are poor. Heterogeneous nucleation plays a significant role in controlling crystal growth. MD simulations are performed to explore the heterogeneous nucleation of liquid Al-Cu on a copper substrate including the heat-up process, preservation process, and freezing process. The simulation results show that the Al-Cu melt becomes layered at the liquid-solid interface and tends to be ordered in the structure because of the induced effect from the substrate. The crystal structure information is found to be gradually delivered from the substrate to the liquid, and this transmission capacity of information decays with increasing distance from the substrate. The liquid Al-Cu alloy with high copper content frozen on the single substrate tends to form a perfect crystal structure more easily, but the Al-Cu melt with low copper content between two copper substrates tends to form an arch-shaped structure, which can disappear when the copper content reaches a specific proportion. Moreover, different angles of the grooved substrates also affect the heterogeneous nucleation of the Al-Cu melt and its solidified structure. Our findings provide new insights into the defect and structural changes during heterogeneous nucleation, and our findings also promote leading-edge studies, which can provide new ideas to mechanical research. read less

Abstract: Fe‐Ni‐Cr stainless‐steels are important structural materials because of their superior strength and corrosion resistance. Atomistic studies of mechanical properties of stainless‐steels, however, have been limited by the lack of high‐fidelity interatomic potentials. Here using density functional theory as a guide, we have developed a new Fe‐Ni‐Cr embedded atom method potential. We demonstrate that our potential enables stable molecular dynamics simulations of stainless‐steel alloys at high temperatures, accurately reproduces the stacking fault energy—known to strongly influence the mode of plastic deformation (e.g., twinning vs. dislocation glide vs. cross‐slip)—of these alloys over a range of compositions, and gives reasonable elastic constants, energies, and volumes for various compositions. The latter are pertinent for determining short‐range order and solute strengthening effects. Our results suggest that our potential is suitable for studying mechanical properties of austenitic and ferritic stainless‐steels which have vast implementation in the scientific and industrial communities. Published 2018. This article is a U.S. Government work and is in the public domain in the USA. read less

Abstract: The Fe-on-Ti and Ti-on-Fe interfaces were studied experimentally by Mössbauer spectroscopy (MS), transmission electron microscopy (TEM) and x-ray reflectometry (XRR) on Ti/Fe/Ti trilayers grown on Si(1 1 1) substrates by vacuum evaporation. The nanoscale structure and composition were explored in cross sections using TEM, the layer structure and the interface widths by specular x-ray reflectometry. MS was applied to identify the interface alloy phases and to determine the pure and alloyed Fe layer fractions. The experimental results were compared with molecular dynamics (MD) simulations of layer growth on Fe or Ti underlayers of different orientations. The concentration distributions provided by MD simulations show an asymmetry at the interfaces in the layer growth direction. The transition is atomically sharp at the Ti-on-Fe interface for the (0 0 1) and (1 1 0) crystallographic orientations of the Fe underlayer, while it spreads over a few atomic layers for Fe(1 1 1) underlayer and for all studied Ti underlayer orientations at the Fe-on-Ti interface. MS and XRR data on Ti/Fe/Ti trilayers confirm the asymmetry between the bottom and top Fe interface, but the inferred interface widths considerable exceed those deduced from the MD simulations. read less

Abstract: Polycrystalline copper is considered by the industries to be the best TSV electroplating material and other interconnected structure material because of its ultra-low resistivity, high conductivity, low electro-migration rate and good compatibility with the multilayer interconnects process. Its mechanical properties are completely different from the bulk monocrystalline copper, these properties are critically vital to evaluate the thermomechanical reliability of TSV and interconnected structure in 3D packaging. To investigate the effect of the work temperature and grain size on the mechanical properties of polycrystalline Cu, The MD simulations of uniaxial tensile test are performed. The results showed that the elastic modulus gradually increases with the augment of the mean grain size, and the corresponding flow stress concurrently increases, and the flow stress is proportional to the square-root of the grain size, which satisfies the inverse-Hall-Petch relation. It also turned that the elastic modulus decreases with increasing of ambient temperatures, the flow stress is negatively correlated with the temperature. From all the tensile simulation tests, it was confirmed that the mechanism of plastic deformation for polycrystalline copper with 4.65–9.31nm grain sizes is mainly the grain’s rotation and grain- boundary sliding, the dislocation nucleation and migration is no longer the dominant factor of plastic deformation. read less

Abstract: ABSTRACT Fe75Zr25/Cu64Zr36 amorphous multilayers were prepared by magnetron sputtering. Atom probe tomography was employed to analyze the atomic inter-diffusion at the interface of the multilayers before and after annealing (573 K, 60 min). An unexpected enhanced inter-diffusion of the immiscible elements Fe and Cu was detected at the interface of the multilayers. As the inter-diffusion in amorphous multilayers is much faster than that in the crystalline counterparts, this process may open a way to manipulate or create amorphous multilayers with new properties. This idea agrees with the observation of the variation of magnetic properties of Fe75Zr25/Cu64Zr36 amorphous multilayers. GRAPHICAL ABSTRACT IMPACT STATEMENT This paper reveals the enhanced atomic inter-diffusion at the interface of amorphous materials, and may open a way to manipulate or create amorphous multilayers with new properties. read less

Abstract: The plastic deformation mechanism of Cu/Ag multilayers is investigated by molecular dynamics (MD) simulation in a nanoindentation process. The result shows that due to the interface barrier, the dislocations pile-up at the interface and then the plastic deformation of the Ag matrix occurs due to the nucleation and emission of dislocations from the interface and the dislocation propagation through the interface. In addition, it is found that the incipient plastic deformation of Cu/Ag multilayers is postponed, compared with that of bulk single-crystal Cu. The plastic deformation of Cu/Ag multilayers is affected by the lattice mismatch more than by the difference in stacking fault energy (SFE) between Cu and Ag. The dislocation pile-up at the interface is determined by the obstruction of the mismatch dislocation network and the attraction of the image force. Furthermore, this work provides a basis for further understanding and tailoring metal multilayers with good mechanical properties, which may facilitate the design and development of multilayer materials with low cost production strategies. read less

Abstract: The size-dependent melting behaviors and mechanisms of Ag nanoparticles (NPs) with diameters of 3.5–16 nm were investigated by molecular dynamics (MD). Two distinct melting modes, non-premelting and premelting with transition ranges of about 7–8 nm, for Ag NPs were demonstrated via the evolution of distribution and transition of atomic physical states during annealing. The small Ag NPs (3.5–7 nm) melt abruptly without a stable liquid shell before the melting point, which is characterized as non-premelting. A solid-solid crystal transformation is conducted through the migration of adatoms on the surface of Ag NPs with diameters of 3.5–6 nm before the initial melting, which is mainly responsible for slightly increasing the melting point of Ag NPs. On the other hand, surface premelting of Ag NPs with diameters of 8–16 nm propagates from the outer shell to the inner core with initial anisotropy and late isotropy as the temperature increases, and the close-packed facets {111} melt by a side-consumed way which is responsible for facets {111} melting in advance relative to the crystallographic plane {111}. Once a stable liquid shell is formed, its size-independent minimum thickness is obtained, and a three-layer structure of atomic physical states is set up. Lastly, the theory of point defect-pair (vacancy-interstitial) severing as the mechanism of formation and movement of the solid-liquid interface was also confirmed. Our study provides a basic understanding and theoretical guidance for the research, production and application of Ag NPs. read less

Abstract: Herein, we report a self-organized core/shell (CS) structure consisting of an Al core and a Pb shell in the liquid Pb alloys at a constant temperature. This is contrary to a believed opinion that the thermal gradient acts as the only driving force for the formation of this kind of structure. The results show that its forming ability greatly depends on the composition and temperature. Importantly, when the alloys are placed in the confined space, a structure of Al–Pb–Al sandwich construction and a completely different CS structure composed of a Pb core and an Al shell are obtained; this suggests a new strategy for controlling the phase-separated structures at the nanoscale. More interestingly, an abnormal phenomenon in the low Pb alloys with the amorphous thin Pb shell and the crystal Al core is observed after solidification, which does not occur in the high Pb alloys that just possess a secondary wrapped structure that never appears in the liquid state. The result of this study may help to shed light on understanding the formation of this core/shell structure and controlling it at an atomic view that have a potential application in the fabrication of these structural materials through nanotechnology. read less

Abstract: Demand for pragmatic alternatives to carbon‐intensive fossil fuels is growing more strident. Hydrogen represents an ideal zero‐carbon clean energy carrier with high energy density. For hydrogen fuel to compete with alternatives, safe and high capacity storage materials that are readily cycled are imperative. Here, development of such a material, comprised of nickel‐doped Mg nanocrystals encapsulated by molecular‐sieving reduced graphene oxide (rGO) layers, is reported. While most work on advanced hydrogen storage composites to date endeavor to explore either nanosizing or addition of carbon materials as secondary additives individually, methods to enable both are pioneered: “dual‐channel” doping combines the benefits of two different modalities of enhancement. Specifically, both external (rGO strain) and internal (Ni doping) mechanisms are used to efficiently promote both hydriding and dehydriding processes of Mg nanocrystals, simultaneously achieving high hydrogen storage capacity (6.5 wt% in the total composite) and excellent kinetics while maintaining robustness. Furthermore, hydrogen uptake is remarkably accomplished at room temperature and also under 1 bar—as observed during in situ measurements—which is a substantial advance for a reversible metal hydride material. The realization of three complementary functional components in one material breaks new ground in metal hydrides and makes solid‐state materials viable candidates for hydrogen‐fueled applications. read less

Abstract: The structure of bimetallic NiCu nanoparticles (NP) is investigated as a function of their composition and size by means of Lattice MonteCarlo (LMC) and molecular dynamics (MD) simulations. According to our results, copper segregation takes place at any composition of the particles. We found that this feature is not size-dependent. In contrast, nickel segregation depends on the NP size. When the size increases, Ni atoms tend to remain in the vicinity of the surface and deeper. For smaller NPs, Ni atoms are located at the surface as well. Our results also showed that most of the metal atoms segregated at the surface area were found to decorate edges and/or form islands. Our findings agree qualitatively with the experimental data found in the literature. In addition, we comment on the reactivity of these nanoparticles. read less

Abstract: This work has been funded from the Spanish Ministerio de Educacion y Ciencia through Grants No. FIS2013-47328 and No. MAT2016-78625 and the Conselleria d’Educacio, Investigacio, Cultura i Esport de la Generalitat Valenciana, PROMETEO/2017/139. C.S. gratefully acknowledges financial support from SEPE Servicio Publico de Empleo Estatal. W.D. acknowledges funding from the National Research Foundation of South Africa through the Innovation Doctoral scholarship programme, Grant UID 102574. read less

Abstract: Three-dimensional (3D) integration technology using Cu interconnections has emerged as a promising solution to improve the performance of silicon microelectronic devices. However, Cu diffuses into SiO2 and requires a barrier layer such as Ta to ensure acceptable reliability. In this paper, the effects of temperature and strain normal to the interface on the inter-diffusion of Cu and Ta at annealing conditions are investigated using a molecular dynamics (MD) technique with embedded atomic method (EAM) potentials. Under thermal annealing conditions without strain, it is found that a Cu-rich diffusion region approximately 2 nm thick is formed at 1000 K after 10 ns of annealing. Ta is capable of diffusing into the interior of Cu but Cu hardly diffuses into the inner lattice of Ta. At the Cu side near the interface an amorphous structure is formed due to the process of diffusion. The diffusion activation energy of Cu and Ta are found to be 0.9769 and 0.586 eV, respectively. However, when a strain is applied, a... read less

Abstract: One of the intriguing characteristics of honeycomb lattices is the appearance of a pseudomagnetic field as a result of mechanical deformation. In the case of graphene, the Landau quantization resulting from this pseudomagnetic field has been measured using scanning tunneling microscopy. Here we show that a signature of the pseudomagnetic field is a local sublattice symmetry breaking observable as a redistribution of the local density of states. This can be interpreted as a polarization of graphene's pseudospin due to a strain induced pseudomagnetic field, in analogy to the alignment of a real spin in a magnetic field. We reveal this sublattice symmetry breaking by tunably straining graphene using the tip of a scanning tunneling microscope. The tip locally lifts the graphene membrane from a SiO2 support, as visible by an increased slope of the I(z) curves. The amount of lifting is consistent with molecular dynamics calculations, which reveal a deformed graphene area under the tip in the shape of a Gaussian. The pseudomagnetic field induced by the deformation becomes visible as a sublattice symmetry breaking which scales with the lifting height of the strained deformation and therefore with the pseudomagnetic field strength. Its magnitude is quantitatively reproduced by analytic and tight-binding models, revealing fields of 1000 T. These results might be the starting point for an effective THz valley filter, as a basic element of valleytronics. read less

Abstract: Molecular dynamics (MD) simulations are performed to investigate the coalescence of the liquid Al and Pb drops in the graphene (G) walls and pillared-graphene (PG) walls. The confining walls can affect the coalescence dynamics of two adjacent films by restricting the movement of one of the metal drops; however, the coalescence behavior is different in the G-walls and the PG-walls. Two un-contacted films can still merge into one bigger drop because of the restricting effect of the walls, in which the movement of the Pb drop plays a predominant role. The coalescence time decreases with the decrease of the confining space. Our findings demonstrate that the coalescence dynamics can be controlled by tuning the confining space or wall surface. read less

Abstract: In this chapter, a series of molecular dynamics simulations have been carried out to explore structural and dynamical features of monatomic liquid metallic films during rapid cooling. Results show a semi‐ordered inhomogeneous morphology containing crystal‐like and disordered regions. The icosahedron contributes to nucleation through the synergy with other short‐range ordered structures and participates in crystal growth via assimilation, but the pinning effect should be overcome. The second‐peak splitting in pair correlation functions is found as the result of a statistical average of crystal‐ like and disordered structural regions, not just the amorphous structure. The splitting can be viewed as a prototype of crystal‐like peaks exhibiting distorted and vestigial features. Besides, we use the parameter P(a, τ, ν) for predicting both local structural order and motion propensity. The fraction of crystalline clusters follows a negative power‐law scaling with the cooling rate increasing, which is the inverse of P(a, τ, ν). read less

Abstract: This paper addresses the in situ growth stress evolution and phase transformation of bcc to hcp Ti in Ti/W multilayered thin films. A series of equal layer thicknesses from 20 nm to 1 nm were deposited. As the bilayer thickness reduced, the overall film stress became less compressive until the Ti transformed from hcp (at the larger layer thicknesses) to bcc in the 1 nm/1 nm multilayer. The pseudomorphic bcc stabilization resulted in a recovery of the compressive stress to values near that for the bulk phase stabilized for the 5 nm/5 nm multilayer. A discernable change in stress slope was noted for the bcc to hcp Ti transition as a function of Ti layer thickness. The stress states for each film, during film growth, are rationalized by the lattice matching of the phase with the growth surface. These results are coupled to a molecular dynamics deposition simulation which revealed good agreement with the experimentally observed transformation thickness. read less

Abstract: MD simulations are performed to study the solidification of Al melt in confined nanoslits (NSs) constructed by identical or different substrates, as well as on Fe substrates. Compared to the single substrate, the confined NS could promote the crystallization of Al melt, and its size has a significant impact on the solidified structure. In symmetrical NSs, liquid Al atoms would stack based on the atomic arrangement mode of the substrate, however in asymmetrical confined NSs, the atomic arrangement mode of liquid Al is governed by the constitution of asymmetrical substrates. Specifically, for the NS formed by Fe(110) and Fe(111) substrates, the induced region from the Fe(110) substrate is much bigger than that from Fe(111). Moreover, the freezing of liquid Al in asymmetrical NSs constructed from copper and iron has also been studied. These results throw light on heterogeneous nucleation in confined space. read less

Abstract: Understanding the deposition mechanism of ne solid particles is essential for the effective use of the cold spray (CS) technique, which is used to synthesize dense and thick metallic coatings. As such, in this study, the deposition behaviors of spherical pure aluminum particles were investigated in detail in order to understand the deposition mechanism; these particles had a diameter of 1 mm and were deposited on ve metallic substrate materials, during the CS-emulated high-velocity impact process. A single particle impact testing system, which is a modi ed single-stage light gas gun, was used to evaluate the deposition process. This evaluation con rmed that the critical velocities of Al particles vary signi cantly with the substrate material. In order to identify the dominant factors, the bonding energies, rebound velocities, plastic deformation experienced by the particles and substrates, and removability of the natural oxide lms were evaluated. The results revealed that the critical velocities increased signi cantly with increasing Ar sputtering time required for complete removal of the natural oxide lm; this time represents the removability of the lm. This result con rms that, of the factors considered, the removability of the natural oxide lm exerts the most in uence on Al particle deposition on metallic substrates. [doi:10.2320/matertrans.T-M2016803] read less

Abstract: We study the thermal stability of hollow copper nanowires using molecular dynamics simulation. We find that the plasticity-mediated structural evolution leads to transformation of the initial hollow structure to a solid wire. The process involves three distinct stages, namely, collapse, recrystallization and slow recovery. We calculate the time scales associated with different stages of the evolution process. Our findings suggest a plasticity-mediated mechanism of collapse and recrystallization. This contradicts the prevailing notion of diffusion driven transport of vacancies from the interior to outer surface being responsible for collapse, which would involve much longer time scales as compared to the plasticity-based mechanism. read less

Abstract: The effect of tilt interfaces and layer thickness of Cu/Ni multilayer nanowires on the deformation mechanism are investigated by molecular dynamics simulations. The results indicate that the plasticity of the sample with a 45° tilt angle is much better than the others. The yield stress is found to decrease with increasing the tilt angle and it reaches its lowest value at 33°. Then as the tilt angle continues to increase, the yield strength increases. Furthermore, the studies show that with the decrease of layer thickness, the yield strength gradually decreases. The study also reveals that these different deformation behaviors are associated with the glide of dislocation. read less

Abstract: A continuum model of the metal melt fracture is formulated on the basis of the continuum mechanics and theory of metastable liquid. A character of temperature and strain rate dependences of the tensile strength that is predicted by the continuum model is verified, and parameters of the model are fitted with the use of the results of the molecular dynamics simulations for ultra-high strain rates (≥1–10/ns). A comparison with experimental data from literature is also presented for Al and Ni melts. Using the continuum model, the dynamic tensile strength of initially uniform melts of Al, Cu, Ni, Fe, Ti, and Pb within a wide range of strain rates (from 1–10/ms to 100/ns) and temperatures (from melting temperature up to 70–80% of critical temperature) is calculated. The model is applied to numerical investigation of a problem of the high-current electron irradiation of Al, Cu, and Fe targets. read less

Abstract: Melting of the Au-Ag nanoparticle with core-shell structure was investigated by molecular dynamic simulation. Structural behavior of the nanoparticle was investigated in the range of temperature from 300 to 1300K. To detect the melting point Lindemann index was calculated during the simulation in the whole temperature range. It is shown that the melting of the Au-Ag nanoparticle under consideration occurs at the temperature of about 900 K. To detect the structural changes the radial distribution function was computed before and after melting. It is shown that the core-shell structure of the nanoparticle is not preserved due to the increased diffusion. read less

Abstract: A computationally efficient modelling method called quasi-coarse-grained dynamics (QCGD) is developed to expand the capabilities of molecular dynamics (MD) simulations to model behaviour of metallic materials at the mesoscales. This mesoscale method is based on solving the equations of motion for a chosen set of representative atoms from an atomistic microstructure and using scaling relationships for the atomic-scale interatomic potentials in MD simulations to define the interactions between representative atoms. The scaling relationships retain the atomic-scale degrees of freedom and therefore energetics of the representative atoms as would be predicted in MD simulations. The total energetics of the system is retained by scaling the energetics and the atomic-scale degrees of freedom of these representative atoms to account for the missing atoms in the microstructure. This scaling of the energetics renders improved time steps for the QCGD simulations. The success of the QCGD method is demonstrated by the prediction of the structural energetics, high-temperature thermodynamics, deformation behaviour of interfaces, phase transformation behaviour, plastic deformation behaviour, heat generation during plastic deformation, as well as the wave propagation behaviour, as would be predicted using MD simulations for a reduced number of representative atoms. The reduced number of atoms and the improved time steps enables the modelling of metallic materials at the mesoscale in extreme environments. read less

Abstract: The interaction of thermal Ar plasma particles with Si and W surfaces is modeled using classical molecular dynamics (MD) simulations. At plasma energies above the threshold for ablation, the ablation yield can be calculated directly from MD. For plasma energies below threshold, the ablation yield becomes exponentially low, and direct MD simulations are inefficient. Instead, we propose an integration method where the yield is calculated as a function of the Ar incident kinetic energy. Subsequent integration with a Boltzmann distribution at the temperature of interest gives the thermal ablation yield. At low plasma temperatures, the ablation yield follows an Arrhenius form in which the activation energy is shown to be the threshold energy for ablation. Interestingly, equilibrium material properties, including the surface and bulk cohesive energy, are not good predictors of the threshold energy for ablation. The surface vacancy formation energy is better, but is still not a quantitative predictor. An analysis of the trajectories near threshold shows that ablation occurs by different mechanisms on different material surfaces, and both the mechanism and the binding of surface atoms determine the threshold energy. read less

Abstract: We present a new artificial driving force for the determination of grain boundary mobility by molecular dynamics. The new driving force is a symmetric version of the synthetic driving force formerly introduced by Janssens et al 2006 Nature Mater. 5 124–7. The new version depends on two orientation parameters instead of one. We analyze the advantages and disadvantages of these two driving force methods. Grain boundary mobilities are simulated for eight symmetric CSL tilt grain boundaries in Al, Cu and γ-Fe, and two MD potentials for each of these materials. Boundary conditions are kept as similar as possible to show the influence of the different materials and to compare to the influence of the different MD potential types on simulated GB mobilities. We find that the newly introduced artificial driving force is a slight improvement, but it cannot remove the shortcomings of the original approach. Also, it is found that the differences in calculated MD mobilities between different materials are of the same order as those between different MD potentials of any one element. Sources for such differences are identified and classified by severity. read less

Abstract: The treatment of atomistic scale interactions via molecular dynamics simulations has recently found favour for multiscale modelling within engineering. The estimation of stress at a continuum point on the atomistic scale requires a pre-defined kernel function. This kernel function derives the stress at a continuum point by averaging the contribution from atoms within a region surrounding the continuum point. This averaging volume, and therefore the associated stress at a continuum point, is highly dependent on the bandwidth and shape of the kernel. In this paper we propose an effective and entirely data-driven strategy for simultaneously computing the optimal shape and bandwidth for the kernel. We thoroughly evaluate our proposed approach on copper using three classical elasticity problems. Our evaluation yields three key findings: firstly, our technique can provide a physically meaningful estimation of kernel bandwidth; secondly, we show that a uniform kernel is preferred, thereby justifying the default selection of this kernel shape in future work; and thirdly, we can reliably estimate both of these attributes in a data-driven manner, obtaining values that lead to an accurate estimation of the stress at a continuum point. read less

Abstract: Asperities play a central role in the mechanical and electrical properties of contacting surfaces. Changes in trends of uniaxial compression of an asperity tip in contact with a polycrystalline substrate as a function of substrate geometry, compressive stress and applied voltage are investigated here by implementation of a coupled continuum and atomistic approach. Surprisingly, an unmodified Au polycrystalline substrate is found to be softer than one containing a void for conditions of high stress and an applied voltage of 0.2 V. This is explained in terms of the temperature distribution and weakening of Au as a function of temperature. The findings in this communication are important to the design of materials for electrical contacts because applied conditions may play a role in reversing relative hardness of the materials for conditions experienced during operation. read less

Abstract: The temperature-dependent coefficients of self-diffusion for liquid metals are simulated by molecular dynamics methods based on the embedded-atom-method (EAM) potential function. The simulated results show that a good inverse linear relation exists between the natural logarithm of self-diffusion coefficients and temperature, though the results in the literature vary somewhat, due to the employment of different potential functions. The estimated activation energy of liquid metals obtained by fitting the Arrhenius formula is close to the experimental data. The temperature-dependent shear-viscosities obtained from the Stokes—Einstein relation in conjunction with the results of molecular dynamics simulation are generally consistent with other values in the literature. read less

Abstract: Recent developments in multiscale modelling include the treatment of atomistic scale interactions via molecular dynamics simulations. The atomistic stress definition at a given continuum point contains a space-averaging volume over nearby atoms to provide an averaged macroscopic stress measure. Previous work on atomistic stress measures introduce the size of this volume as an a priori given parameter. In this contribution we let the atomistic data speak for itself by hypothesizing that the influence between atoms can be effectively estimated from their relative spatial position and stress. Atoms with highly similar spatial position and stress should therefore be contained within the same space-averaging volume. We motivate the application of Gaussian mixture modelling as a principled probabilistic means of estimating this similarity directly from the atomistic data. This model enables the discovery of homogeneous sub-populations of atoms in an entirely data-driven manner. The size of the space-averaging volume then follows naturally from the average maximum extent of the sub-populations. Furthermore, we demonstrate how the model can be used to compute the stress at arbitrary continuum points. Thorough evaluation is conducted on a numerical example of an edge dislocation in a single crystal. We find that our results are in excellent agreement with the corresponding analytical solution. read less

Abstract: Formation of triangular‐shaped Pt adatom islands on a Pt(111) surface is investigated using a kinetic Monte Carlo approach. The energy barriers are calculated with the generalized embedded atom method and the nudged elastic band approach. The numerical results reveal that the preferential orientation of the triangles cannot be explained solely by the differences in the diffusion coefficients of the atoms along the topologically non‐equivalent edges of the islands. For a self‐consistent explanation of the triangle orientations, one has to examine the topological and energetic details of the diffusion paths for all the edge diffusion processes, kink‐formation or kink‐breaking events, and corner to edge jumps. read less

Abstract: The process of creating an atomically defined and robust metallic tip is described and quantified using measurements of contact conductance between gold electrodes and numerical simulations. Our experiments show how the same conductance behavior can be obtained for hundreds of cycles of formation and rupture of the nanocontact by limiting the indentation depth between the two electrodes up to a conductance value of approximately 5G0 in the case of gold. This phenomenon is rationalized using molecular dynamics simulations together with density functional theory transport calculations which show how, after repeated indentations (mechanical annealing), the two metallic electrodes are shaped into tips of reproducible structure. These results provide a crucial insight into fundamental aspects relevant to nanotribology or scanning probe microscopies. read less

Abstract: We have investigated the structural phase transition of FeCo and FeNi nanoparticles by molecular dynamics (MD) simulation using the generalized embedded atom potential (GEAM). It is found that the phase varies with the atomic compositions and annealing processes. By using the Honeycutt and Andersen index (HA index), bond order parameters (BOP) and pair correlation function (PCF), we found that a BCC to defective icosahedra phase transition occurs in the FeCo nanoparticle when Co composition is increased to about 60 at %. In the FeNi nanoparticle, three phases have been identified, namely, the BCC phase, the mixed BCC/FCC phase, and the multilayer defective icosahedral phase, which correspond to the Ni compositions of 0–20 at %, 20–70 at %, and 70–100 at %, respectively. Our simulations have well reproduced the phase transition points and most of the phases observed in recent experiments. read less

Abstract: Nanostructured Co with large lattice extension and contraction was produced by electrodepositing Co on nanoporous Au. The Co deposited showed a low magnetic saturation of 76 emu/g and a high coercivity of 462 Oe. First-principles calculations showed that the magnetic moment of a Co atom is significantly decreased by lattice contraction. Therefore, the noteworthy magnetic properties of the Co deposited are attributed to the large lattice strain. Also, molecular dynamics simulation showed that the lattice extension and contraction of about 10% are generated in the overall Co crystal. This is in agreement with the experimental results of HRTEM observation. The constraint of the movement of Co atoms by the concave structure of nanoporous Au leads to a wide spread of large strain region. read less

Abstract: Based on semiempirical generalized embedded atom method (GEAM), we carried out molecular dynamics and Monte Carlo simulations to study the structural properties of FeCo and FeNi nanoparticles. It is found that these two kinds of nanoparticles possess a new stable quasicore-shell structure, no matter whether they are in molten or condensed state and whether they are prepared by annealing or quenching. In FeCo (FeNi) nanoparticles of various sizes and atom compositions, the quasicore-shell structure is always preferred, with the shell formed only by Fe atoms and the core formed by randomly distributed Co(Ni) and Fe atoms. We have also investigated the formation mechanism of the quasicore-shell structure by energy difference analysis of the pure and doped icosahedra structure of FeCo and FeNi nanoparticles. read less

Abstract: This paper studies the melting of icosahedral Ag—Pd bimetallic clusters by using molecular dynamics with the embedded atom method. It finds that the mixed Ag—Pd cluster shows an irregular phenomenon before melting, i.e., the atomic energy decreases with the increase of temperature. It indicates that the segregation of Ag atoms results in this phenomenon by analysing atomic radius distribution. Since the surface energy of Ag is lower than that of Pd, this leads to the result that the decreased energy by the Ag atomic segregation is larger than the increased energy by the heating. This provides a new method to obtain irregular thermodynamic properties. read less

Abstract: The correlated effects of the insertion of a Pt spacer between ferromagnetic and antiferromagnetic layers and of the variation of the Co layers' thickness on the structural and magnetic properties of multilayers have been studied. Samples with n = 1 and 7, tCo = 0.4 and 0.6 nm, tPt = 0 and 0.4 nm have been investigated by tomographic atom probe and superconducting quantum interference device magnetometry. For spacer-free samples (tPt = 0), the structural investigation shows that when tCo = 0.4 nm, Mn and Ir atoms diffuse deeply into the (Pt/Co)3 multilayers. In contrast for tCo = 0.6 nm, the Mn and Ir diffusion is much reduced. Because Pt acts as a barrier against the Mn and Ir diffusion, this difference is less pronounced in samples with Pt insertion. The hysteresis loops shapes, the exchange bias fields and the saturation magnetization values were correlated with the structural properties of these samples and discussed, taking into account the susceptibility, exchange stiffness and perpendicular magnetic anisotropy. read less

Abstract: The extreme cooling rates in material processing can be achieved in a number of current and emerging femtosecond laser techniques capable of highly localized energy deposition. The mechanisms of rapid solidification of a nanoscale region of a metal film transiently melted by a localized photoexcitation are investigated in a large-scale atomistic simulation. The small size of the melted region, steep temperature gradients, and fast two-dimensional electron heat conduction result in the cooling rate exceeding 1013 K/s and create conditions for deep undercooling of the melt. The velocity of the liquid/crystal interface rises up to the maximum value of ∼80 m/s during the initial stage of the cooling process and stays approximately constant as the temperature of the melted region continues to decrease. When the temperature drops down to the level of ∼0.6Tm, a massive homogeneous nucleation of the crystal phase inside the undercooled liquid region takes place and prevents the undercooled liquid from reaching th... read less

Abstract: The deposition behaviour for a Fe–Cu magnetic multilayer system in an early stage of the deposition process was investigated by molecular dynamics (MD) simulation. Specifically, the steering effect was quantitatively investigated through extensive measurements of the trajectory, deposition flux and force of atoms near the artificially structured Fe or Cu step positioned on the Cu(0 0 1) or Fe(0 0 1) surface. Near the step edges of the planar structure at a low incident energy of 0.1 eV, the steering effect for the case of Fe/Cu(0 0 1) was observed more significantly than for Cu/Fe(0 0 1). Additionally, the mechanism of down-diffusion from the step was discussed and the corresponding energetic was calculated using the molecular statics method. read less

Abstract: The dependence of icosahedral transformations on the composition, concentration and configuration in Cu-based bimetallic clusters was studied by using molecular dynamics with the embedded atom method. The results show that the transformation is strongly related to the release of excess energy and can be controlled by tuning the composition, concentration and configuration. The transformation can be easily induced by the addition of Co while it is difficult in the case of Ni. The transformation temperature Ttrans decreases with the increase in Co concentration and increases with the increase in Ni concentration. The transformation can be controlled by fabricating different configurations and distributions of the composition. read less

Abstract: Structural investigation of Ta3 nm/[(Pt2 nm/Co0.4 nm)3/Ptx/IrMn7 nm]7/Pt10 nm multilayers without (x=0 nm) and with (x=0.4 nm) a Pt spacer has been performed by laser-assisted tomographic atom probe. Without a Pt spacer a strong intermixing is observed at the Co/IrMn interface. In the multilayer containing a Pt spacer the Co/Pt/IrMn interface is very weakly intermixed. It thus appears that the Pt spacer acts as a diffusion barrier that prevents the Ir and Mn atoms from diffusing into the Co layer. The consequences of this effect on the magnetic properties are discussed. The exchange bias field and the anisotropy direction of these two multilayers are analyzed and correlated with the structural investigation. read less

Abstract: This paper studies the structural evolution of (AgCo)201 clusters with different Co concentrations under various temperature conditions by using molecular dynamics with the embedded atom method. The most stable position for Co atoms in the cluster is the subsurface layer at low temperature (lower than 200 K for the Ag200Co1 cluster). The position changes to the core layer with the increase of temperature, but there is an energy barrier in the middle layer. This makes the Ag–Co cluster form an Ag–Co–Ag three-shell onion-like configuration. When the temperature is high enough [higher than 800 K for (AgCo)201 clusters with 50% Co], Co atoms can obtain enough energy to overcome the energy barrier and the cluster forms an Ag–Co core-shell configuration. Amorphization for the onion-like and core-shell clusters is induced by the large lattice misfit at Ag–Co interfaces. The structural evolution in the Ag–Co cluster is related to the release of excess energy. read less

Abstract: We study the icosahedral transformations of solid Cu–Co clusters with different initial configurations by using molecular dynamics with the embedded atom method. It is found that the formation of symmetric icosahedral cluster is strongly related to the atomic number and initial configuration. The transformation originates from the surface into the interior of the cluster and is a structural change which is rapid and diffusionless. The icosahedral clusters with any composition and configuration, such as core-shell or three-shell cluster, can be prepared by the means of solid-solid phase transition in bimetallic clusters. read less

Abstract: Using molecular dynamics simulation, the structural characteristics of Fe and Cu thin films grown on Cu and Fe(001) substrates, respectively, were investigated with respect to the incident energy of adatoms and substrate temperature. In the case of Cu on Fe(001), no surface alloying at the interface was observed in the early stage of thin-film deposition, and growth generally followed the layer-by-layer growth mode. For Fe on a Cu(001) surface, a mixture confined to a single atomic layer at the Cu(001) surface was found to form at room temperature while films showed island growth. The steering effect due to atomic attraction was also observed at low incident energy, resulting in a rougher surface. Fe/Cu(001) growth changed to a layer-by-layer mode for an incident energy of 6 eV. The different aspects of surface morphology between Fe/Cu(001) and Cu/Fe(001) systems were explained in terms of surface free energy and impact cascade diffusion. read less

Abstract: The mixing situation of Fe or Co atoms implanting onto Cu(001) substrate is investigated with regard to substrate temperature and deposition rate by molecular dynamics. The tight-binding-second-momentum-approach many-body potential is used to model the atomic interaction. The results indicate that the morphology of the layer is under epitaxial growth as the substrate temperature is 700 or 1000 K, while it is not epitaxial at the substrate temperature of 300 or 450 K. The quality of epitaxial film is better when the substrate temperature is increased. The intermixing at the deposited layers becomes clear as the substrate temperature increases. It also indicates that there are more Co atoms penetrating into the substrate than the Fe atoms, regardless of the substrate temperature. Hence, one could say that the interface mixing of Co and Cu atoms is better than that of Fe and Cu atoms. When the deposition rate is raised from 5 to 10 atoms/ps, there is no increase in the interface mixing at both systems except... read less

Abstract: In the present work a detailed atomic-level analysis of some of the main diffusion mechanisms which take place during cobalt adatom deposition are studied within atom dynamics (AD) and the nudged elastic band (NEB) method. Our computer simulations reveal a very fast exchange between Co and Au atoms when the deposit is a single cobalt adatom. However, when the nucleus size increases, a decrease in the exchange probability is observed. Activation energies for different transitions are obtained using AD in combination with the NEB method. read less

Abstract: Self-assembled FePt nanoparticle arrays are candidate structures for ultrahigh density magnetic storage media. One of the factors limiting their application to this technology is particle-to-particle compositional variation. This variation will affect the A1 to L10 transformation as well as the magnetic properties of the nanoparticles. In the present study, an analysis is provided for the formation mechanism of these nanoparticles when synthesized by the superhydride reduction method. Additionally, a comparison is provided of the composition distributions of nanoparticles synthesized by the thermal decomposition of Fe(CO)5 and the reduction of FeCl2 by superhydride. The latter process produced a much narrower composition distribution. A thermodynamic analysis of the mechanism is described in terms of free energy perturbation Monte Carlo simulations. read less

Abstract: We have investigated the impact of Ag surface coating on the microstructure of CoAg core-shell nanoparticles, and the consequences for the magnetic properties. Atomic structures were simulated using a molecular dynamics approach utilizing the embedded atom method. The magnetic properties were then simulated using an atomistic approach using a classical spin Hamiltonian, taking into account the long-range nature, atomic separation and directional and phase dependence of the exchange interactions in Co. For pure cobalt nanoparticles with a diameter less than approximately 3 nm the internal crystal structure showed multiple twinned regions and the morphology of an icosahedron. The addition of a monolayer silver coating alters the internal cobalt crystal structure to a regular planar form along the cubic [111] direction with a mixture of face centred cubic (fcc) and hexagonal close packed (hcp) atomic arrangements. The local atomic environment was used to assign anisotropies to the atoms on a site by site basis. Moreover, the capping layer influences the shape of the particle and yielded a morphology similar to that of a truncated octahedron. Taking into account the effects of surface anisotropy in addition gives an overall picture of anisotropy for CoAg nanoparticles. We present the results of calculations estimating the energy barrier to magnetization reversal and the intrinsic coercivity for these particles read less

Abstract: A combination of molecular dynamics simulations and experiments has been used to investigate the use of various low energy ion assisted vapor deposition approaches for controlling the interfacial structures of a model copper∕tantalum multilayer system. Films were grown using argon ion beam assistance with either a fixed or modulated ion energy during metal deposition. The effect of sequential ion assistance (after layer’s deposition) was also investigated. The argon ion energy was varied between 0 and 50eV and the effect on the atomic scale structure of Ta∕Cu film interfaces and the film electrical resistivity were studied. The use of simultaneous argon ion assistance with an ion energy of ∼10eV and an ion∕metal atom flux ratio of ∼6 resulted in atomically sharp interfaces with little intermixing, consistent with simulation predictions. Ion impacts in this range activated surface atom jumping and promoted a step flow film growth mode. Higher energies were also successful at interface flattening, but they ... read less

Abstract: The temperature dependences of the electron heat capacity and the electron-phonon coupling factor are investigated for Au based on the electron density of states obtained from ab initio electronic structure calculations. Thermal excitation of d band electrons leads to a significant (up to an order of magnitude) increase in the electronphonon coupling factor and makes a considerable contribution to the electron heat capacity in the range of electron temperatures typically realized in femtosecond laser material processing applications. Simulations performed with a combined atomistic-continuum method demonstrate that the increase in the strength of the electron-phonon coupling at high electron temperatures leads to a faster lattice heating, generation of stronger thermoelastic stresses, and a significant decrease in the time of the onset of the melting process. The timescale of the melting process predicted in the simulation accounting for the thermal excitation of d band electrons is in excellent agreement with the results of recent time-resolved electron diffraction experiments. A simulation performed with commonly used approximations of a constant electron-phonon coupling factor and a linear temperature dependence of the electron heat capacity, on the other hand, significantly overpredicts the time of the beginning of the melting process, supporting the importance of the electron density of states effects and thermal excitation of lower band electrons for realistic modeling of femtosecond pulse laser processing. read less

Abstract: The formation of platinum nanoparticles from a supersaturated vapour phase is investigated by molecular dynamics simulations. Argon is added as carrier gas, removing the condensation heat from the nucleating system. The interactions between the platinum atoms are modelled by the multi-body embedded atom method. The nucleation rates in highly supersaturated systems are estimated as well as properties of the critical clusters from the nucleation theorems. Furthermore, the coalescence of platinum nanoparticles is investigated and a coalescence activated structural transition in platinum nanoparticles is described. read less

Abstract: Current interatomic potentials for compound semiconductors, such as GaAs, fail to correctly predict the ab initio calculated and experimentally observed surface reconstructions. These potentials do not address the electron occupancies of dangling bonds associated with surface atoms and their well established role in the formation of low-energy surfaces. The electron counting rule helps account for the electron distribution among covalent and dangling bonds, which, when applied to GaAs surfaces, requires the arsenic dangling bonds to be fully occupied and the gallium dangling bonds to be empty. A simple method for linking this electron counting constraint with interatomic potentials is proposed and used to investigate energetics of the atomic scale structures of the GaAs(001) surface using molecular statics methods. read less

Abstract: The mechanisms of melting and photomechanical damage/spallation occurring under extreme superheating/deformation rate conditions realized in short pulse laser processing are investigated in a computational study performed with a hybrid atomistic-continuum model. The model combines classical molecular dynamics method for simulation of non-equilibrium processes of lattice superheating and fast phase transformations with a continuum description of the laser excitation and subsequent relaxation of the conduction band electrons. The kinetics and microscopic mechanisms of melting are investigated in simulations of laser interaction with free-standing Ni films and bulk targets. A significant reduction of the overheating required for the initiation of homogeneous melting is observed and attributed to the relaxation of laser-induced stresses, which leads to the uniaxial expansion and associated anisotropic lattice distortions. The evolution of photomechanical damage is investigated in a large-scale simulation of laser spallation of a 100 nm Ni film. The evolution of photomechanical damage is observed to take place in two stages, the initial stage of void nucleation and growth, when both the number of voids and the range of void sizes are increasing, followed by the void coarsening, coalescence and percolation, when large voids grow at the expense of the decreasing population of small voids. In both regimes the size distributions of voids are found to be well described by the power law with an exponent gradually increasing with time. A good agreement of the results obtained for the evolution of photomechanical damage in a metal film with earlier results reported for laser spallation of molecular systems and shock-induced back spallation in metals suggests that the observed processes of void nucleation, growth and coalescence may reflect general characteristics of the dynamic fracture at high deformation rates. read less

Abstract: In order to assure the reliability of advanced gas turbine systems, it is very important to evaluate the damage of high temperature materials such as Ni-base superalloys under creep and fatigue conditions quantitatively. Since the micro texture of the gamma-prime (γ′ ) phase was found to vary during the creep damage process, it is possible, therefore, to evaluate the creep damage of this material quantitatively by measuring the change of the micro texture. The mechanism of the directional coarsening of γ′ phasesof Ni-base superalloy under uni-axial strain at high temperatures, which is called rafting, was analyzed by using molecular dynamics (MD) analysis. The stress-induced anisotropic diffusion of Al atoms perpendicular to the finely dispersed γ/γ′ interface in the superalloy was observed clearly in a Ni(001)/Ni3 Al(001) interface structure. The stress-induced anisotropic diffusion was validated by experiment using the stacked thin films structures which consisted of the (001) face-centered cubic (FCC) interface. The reduction of the diffusion of Al atoms perpendicular to the interface is thus, effective for improving the creep and fatigue resistance of the alloy. It was also found by MD analysis that the dopant elements in the superalloy also affected the strain-induced diffusion of Al atoms. Both palladium and tantalum were effective elements which restrain Al atoms from moving around the interface under the applied stress, while titanium and tungsten accelerated the strain-induced anisotropic diffusion, and thus, the rafting phenomenon.Copyright © 2011 by ASME read less

Abstract: In order to make clear the mechanism of the directional coarsening of γ' phases (rafting) of Ni-base superalloy under uni-axial strain, molecular dynamics (MD) analysis was applied to analyze the effect of strain on the diffusion characteristics around the interface between different materials. In a Ni (001)/Al (001) interface structure, the stress induced diffusion of Al atoms perpendicular to the interface was found. The stress induced anisotropic diffusion of Al was also found in a Ni (001)/Ni3Al (001) interface. These results imply that it is highly possible the rafting occurs predominantly by the stress induced anisotropic diffusion of Al atoms. read less

Abstract: We carried out the nanoindentation simulations for the Ru (superlayer) / Cu (film) / SiO2 (substrate) system using the finite temperature MD-FEM coupling method. The calculations are performed for the different adhesion energies of Cu/SiO2 ranging from 0.2 to 0.6 J/m 2 . During loading, it was found that the interfacial crack nucleation occurs at three to four times the contact radius, driven by the tensile stress acted on the Cu/SiO2 interface. We also show that the asymmetric defect behavior have a great effect on giving birth to the crack nucleation. The observation of our simulation indicates that the mechanism of the crack nucleation strongly depends on the interfacial bonding energy. read less

Abstract: The formation and mechanical properties of amorphous copper are studied using molecular dynamics simulation. The simulations of tension and shearing show that more pronounced plasticity is found under shearing, compared to tension. Apparent strain hardening and strain rate effect are observed. Interestingly, the variations of number density of atoms during deformation indicate free volume creation, especially under higher strain rate. In particular, it is found that shear induced dilatation does appear in the amorphous metal. read less

Abstract: Modification of titanium microstructure after propagation of a melting shock wave (SW) generated by a femtosecond laser pulse is investigated experimentally and analyzed using hydrodynamic and atomistic simulations. Scanning and transmission electron microscopy with analysis of microdiffraction is used to determine the microstructure of modified subsurface layers of titanium. We found that two layers are modified beneath the surface. A top surface polycrystalline layer of nanoscale grains is formed from shock-molten material via rapid crystallization. In a deeper subsurface layer, where the shock-induced melting changes into plastic deformation due to attenuation of SW, the grain structure of solid is considerably affected, which results in a grain size distribution differing from that in the intact titanium. Molecular dynamics simulation of single-crystal titanium reveals that the SW front continues to melt even after its temperature drops below the melting curve Tm(P). The enormous shear stress of ∼12 GPa generated in a narrow SW front leads to free slip of atomic planes, collapse of the crystal lattice, and formation of a supercooled metastable melt. Such melt crystallizes in an unloading tail of SW. The mechanical melting ceases after drop in the shear stress giving rise to the shock-induced plastic deformation. The last process triggers a long-term rearrangement of atomic structures in solid. The overall depth of modified layers is limited by SW attenuation to the Hugoniot elastic limit and can reach several micrometers. The obtained results reveal the basic physical mechanisms of surface hardening of metals by ultrashort laser pulses. read less

Abstract: In the paper, the tensile mechanical properties and deformation behaviour of different crystal structures (face-centered cubic (FCC) metal ~Al and closely packed hexagonal (HCP) metal ~Ti) were studied using molecular dynamics calculation through the LAMMPS module of Materials and Processes Simulations (MAPS) software, and the effect of microstructure on their mechanical properties was explored from the atomic level, in order to reveal the deformation mechanism of different lattice types of metals at the atomic scale. The simulation results using MAPS showed that the elastic modulus and the tensile strength of Al was calculated with 45.0 GPa and 6.2 GPa, respectively, the flow stress of Al has a stable value of around 0.69-1.29 GPa. And the elastic modulus and the tensile strength of Ti was calculated with 73.1 GPa and 8.5 GPa, respectively. According to calculation by Ovite, the occurrence of dislocations in Ti was later than that in Al, indicating that the strain energy that can be accumulated in the Ti lattice of HCP structure was higher and the ability to resist deformation was greater. As the periodic boundary conditions were used in all three directions of the two model, there was no macroscopic breaking phenomenon. To sum up, the lattice of HCP structure (~Ti) can accumulate higher strain energy and have greater deformation resistance ability than that of FCC structure (~Al), which was the reason for the elastic modulus and tensile strength of Ti higher than those of Al. read less

Abstract: ABSTRACT High-entropy materials are composed of multiple elements on comparatively simpler lattices. Due to the multi-component nature of such materials, atomic-scale sampling is computationally expensive due to the combinatorial complexity. This study proposes a genetic algorithm-based methodology for sampling such complex chemically disordered materials. Genetic Algorithm-based Atomistic Sampling Protocol (GAASP) variants can generate low as well as high-energy structures. GAASP low-energy variant in conjugation with metropolis criteria avoids premature convergence as well as ensures detailed balance condition. GAASP can be employed to generate low-energy structures for thermodynamic predictions, and diverse structures can be generated for machine-learning applications. read less

Abstract: Among the properties that distinguish nanoparticles (NPs) from their bulk counterparts is their lower melting points. It is also common knowledge that relatively low melting points enhance the coalescence of (usually) nascent nanoclusters toward larger NPs. Finally, it is well established that the chemical ordering of bi- (or multi-) metallic NPs can have a profound effect on their physical and chemical properties, dictating their potential applications. With these three considerations in mind, we investigated the coalescence mechanisms for Ni and Pt NPs of various configurations using classical molecular dynamics (MD) computer simulations. Benchmarking the coalescence process, we identified a steeper melting point depression for Pt than for Ni, which indicates a reversal in the order of melting for same-size NPs of the two elements. This reversal, also evident in the nano-phase diagram thermodynamically constructed using the regular solution model, may be useful for utilising NP coalescence as a means to design and engineer non-equilibrium NPs via gas-phase synthesis. Indeed, our MD simulations revealed different coalescence mechanisms at play depending on the conditions, leading to segregated chemical orderings such as quasi-Janus core-satellite, or core–(partial) shell NPs, despite the expected theoretical tendency for elemental mixing. read less

Abstract: In computational materials science, a common means for predicting macroscopic (e.g., mechanical) properties of an alloy is to define a model using combinations of descriptors that depend on some material properties (elastic constants, misfit volumes, etc.), representative for the macroscopic behavior. The material properties are usually computed using special quasi-random structures, in tandem with density functional theory (DFT). However, DFT scales cubically with the number of atoms and is thus impractical for a screening over many alloy compositions. Here, we present a novel methodology which combines modeling approaches and machine-learning interatomic potentials. Machine-learning interatomic potentials are orders of magnitude faster than DFT, while achieving similar accuracy, allowing for a predictive and tractable high-throughput screening over the whole alloy space. The proposed methodology is illustrated by predicting the room temperature ductility of the medium-entropy alloy Mo–Nb–Ta. Graphical abstract read less

Abstract: The sliding motion of gold slabs adsorbed on a graphite substrate is simulated using molecular dynamics. The central quantity of interest is the mean lateral force, that is, the kinetic friction rather than the maximum lateral forces, which correlates with the static friction. For most setups, we find Stokesian damping to resist sliding. However, velocity-insensitive (Coulomb) friction is observed for finite-width slabs sliding parallel to the armchair direction if the bottom-most layer of the three graphite layers is kept at zero stress rather than at zero displacement. Although the resulting kinetic friction remains much below the noise produced by the erratic fluctuations of (conservative) forces typical for structurally lubric contacts, the nature of the instabilities leading to Coulomb friction could be characterized as quasi-discontinuous dynamics of the Moiré patterns formed by the normal displacements near a propagating contact line. It appears that the interaction of graphite with the second gold layer is responsible for the symmetry break occurring at the interface when a contact line moves parallel to the armchair rather than to the zigzag direction. read less

Abstract: In this work, we study the deformation behavior in amorphous/crystalline metallic composites (A/C-MCs) through nanoindentation experiments and molecular dynamic (MD) simulations. The atomic deformation processes in both crystalline (C-) and amorphous (A-) phases near the amorphous-crystalline interface (ACI) are investigated and correlated with the material’s overall constitutive behavior at the microscale. Our major findings are (i) the ACIs enable a co-deformation of the A- and C-phases through “stiffening” the soft phases but “softening” the stiff phases in A/C-MCs through different micro-mechanisms; (ii) there exists an ACI-induced transition zone with a thickness of ~ 10 nm; (iii) the strong coupling between shear transformation zones (STZs) and dislocations can be quantified through carefully designed indentation experiments and simulations; and (iv) the nanoscale MD-simulation-predicted mechanisms can be mapped to the “pop-in” or “excursion” events on the force–indentation depth curves extracted from microscale experiments, although there is a length-scale gap in between. read less

Abstract: Understanding the size- and shape-dependent properties of platinum nanoparticles is critical for enabling the design of nanoparticle-based applications with optimal and potentially tunable functionality. Toward this goal, we evaluated nine different empirical potentials with the purpose of accurately modeling faceted platinum nanoparticles using molecular dynamics simulation. First, the potentials were evaluated by computing bulk and surface properties-surface energy, lattice constant, stiffness constants, and the equation of state-and comparing these to prior experimental measurements and quantum mechanics calculations. Then, the potentials were assessed in terms of the stability of cubic and icosahedral nanoparticles with faces in the {100} and {111} planes, respectively. Although none of the force fields predicts all the evaluated properties with perfect accuracy, one potential-the embedded atom method formalism with a specific parameter set-was identified as best able to model platinum in both bulk and nanoparticle forms. read less

Abstract: Frictional contacts lead to the formation of a surface layer called the third body, consisting of wear particles and structures resulting from their agglomerates. Its behavior and properties at the nanoscale control the macroscopic tribological performance. It is known that wear particles and surface topography evolve with time and mutually influence one another. However, the formation of the mature third body is largely uncharted territory and the properties of its early stages are unknown. Here we show how a third body initially consisting of particles acting as roller bearings transitions into a shear-band-like state by forming adhesive bridges between the particles. Using large-scale atomistic simulations on a brittle model material, we find that this transition is controlled by the growth and increasing disorganization of the particles with increasing sliding distance. Sliding resistance and wear rate are at first controlled by the surface roughness, but upon agglomeration wear stagnates and friction becomes solely dependent on the real contact area in accordance with the plasticity theory of contact by Bowden and Tabor. read less

Abstract: Classical molecular dynamics (MD) simulations are used to investigate the role of phase separation (PS) on the Rayleigh-Plateau (RP) instability. Ni–Ag bulk structures are created at temperatures (... read less

Abstract: The orientation between twin boundary (TB) and loading direction may play an intriguing role in the deformation behaviors of twinned metallic materials. In this aspect, its essential effect on the high-entropy alloy (HEA) nanocrystals is elusive. Attention herein is focused on the atomicscaled deformation mechanisms and fracture behaviors of HEA nanocrystals containing twins of even smaller spacings via a combined approach of in situ tensile tests inside a high-resolution transmission electron microscope and molecular dynamics simulations. The results indicate that the deformation mechanisms (especially dislocation activities) of HEA nanocrystals depend on the load orientation with respect to TBs. Because of the low activation energy and uneven local composition of HEA, the surface acts as an effective dislocation source and, together with Schmid factor, dominate the activated dislocation slip system. The load orientation-dependent TB-dislocation interactions may transform the type of fracture from semi-brittle to ductile. Our results indicate that the deformation mechanisms and the types of fracture in HEA nanocrystals can be controlled by changing the orientation. The nanotwinned materials attract extensive attention because of their exceptional combination of high strength and ductility. The deep understanding of the role of twin will bring advances to design new strong and ductile materials. The orientation between twin boundary (TB) and loading direction plays an intriguing role in the deformation behaviors. In this work, we use a novel method, in situ transmission electron microscope (TEM) melting, to produce nanotwin with special load/TB orientation. The strong load/TB orientation dependences of deformation mechanisms and fracture modes are revealed via in situ TEM strain and molecular dynamics simulations. The free surface together with Schmid factor dominate the activated dislocation slip system. This work points out an alternative route in designing advanced twin-inducedplasticity materials by controlling load/TB orientations. read less

Abstract: The dislocation plasticity of ductile materials in a dynamic process of cold gas spraying is a relatively new research topic. This paper offers an insight into the microstructure and dislocation mechanism of the coating using simulations of molecular dynamics (MD) because of the short MD simulation time scales. The nano-scale deposition of ductile materials onto a deformable copper substrate has been investigated in accordance with the material combination and impact velocities in the particle/substrate interfacial region. To examine the jetting mechanisms in a range of process parameters, rigorous analyses of the developments in pressure, temperature, dislocation plasticity, and microstructure are investigated. The pressure wave propagation’s critical function was identified by the molecular dynamics’ simulations in particle jet initiation, i.e., exterior material flow to the periphery of the particle and substrate interface. The initiation of jet occurs at the point of shock waves interact with the particle/substrate periphery and leads to localization of the metal softening in this region. In particular, our findings indicate that the initial particle velocity significantly influences the interactions between the material particles and the substrate surface, yielding various atomic strain and temperature distribution, processes of microstructure evolution, and the development of dislocation density in the particle/substrate interfacial zone for particles with various impact velocities. The dislocation density in the particle/substrate interface area is observed to grow much more quickly during the impact phase of Ni and Cu particles and the evolution of the microstructure for particles at varying initial impact velocities is very different. read less

Abstract: The formation mechanism and the influence of laser power on welding deformations of welded joints in Ag nanowire connectors were investigated using molecular dynamic simulations. Simulation results revealed that some regions of voids in the welded joint were filled due to the heat in the heat affected zone. The laser irradiation with the appropriate power can decrease dislocations and phase transitions generated by cold welding. The welding strength possessed an upward trend with the increase in laser power. With the increase of the laser power, the shear resistance of the welded joint improved. However, the welded joint after the laser with the excessive power weakened. Furthermore, by comparing the laser welding and the thermal annealing welding, we found that the laser welded joint possesses fewer amorphous atoms, phase transitions, dislocations, residual stress, and possibility of the shear deformation via simulation. read less

Abstract: In the past, experimentally observed dislocations were often interpreted using an isolated dislocation assumption because the effect of background dislocation density was difficult to evaluate. Contrarily, dislocations caused by atomistic simulations under periodic boundary conditions can be better interpreted because linear elastic theory has been developed to address the effect of periodic dislocation array in the literature. However, this elastic theory has been developed only for perfect dislocations, but not for dissociated dislocations. The periodic boundary conditions may significantly change the dissociation energy of dislocations and stacking fault width, which in turn, change the deformation phenomena observed in simulations. To enable materials scientists to understand the dislocation behavior under the periodic boundary conditions, we use isotropic elastic theory to analyze the thermodynamics of dissociated periodic dislocations with an arbitrary dislocation character angle. Analytical expressions for force, stacking fault width, and energies are presented in the study. Results obtained from the periodic dislocation array were compared with those obtained from isolated dislocations to shed light on the interpretation of experimentally observed and simulated dislocations. read less

Abstract: We map the three-dimensional strain heterogeneity within a single core-shell Ni nanoparticle using Bragg coherent diffractive imaging. We report the direct observation of both uniform displacements and strain within the crystalline core Ni region. We identify non-uniform displacements and dislocation morphologies across the core–shell interface, and within the outer shell at the nanoscale. By tracking individual dislocation lines in the outer shell region, and comparing the relative orientation between the Burgers vector and dislocation lines, we identify full and partial dislocations. The full dislocations are consistent with elasticity theory in the vicinity of a dislocation while the partial dislocations deviate from this theory. We utilize atomistic computations and Landau–Lifshitz–Gilbert simulation and density functional theory to confirm the equilibrium shape of the particle and the nature of the (111) displacement field obtained from Bragg coherent diffraction imaging (BCDI) experiments. This displacement field distribution within the core-region of the Ni nanoparticle provides a uniform distribution of magnetization in the core region. We observe that the absence of dislocations within the core-regions correlates with a uniform distribution of magnetization projections. Our findings suggest that the imaging of defects using BCDI could be of significant importance for giant magnetoresistance devices, like hard disk-drive read heads, where the presence of dislocations can affect magnetic domain wall pinning and coercivity. read less

Abstract: Two-dimensional (2D) materials are full of surprises and fascinating potential. Motivated by a recent discovery that sub-nanometer PdMo bimetallenes can realize exceptional performance in the oxygen reduction reaction [Nature 2019, 574, 81–85], we explore the potential of 2D bimetallenes for catalyzing the CO2 electroreduction reaction (CO2RR). Following extensive first-principles calculations on more than a hundred bimetallenes, we identify 17 Cu- and Ag-based bimetallenes, which are highly active and selective toward the formation of formic acid and simultaneously suppress the competing hydrogen evolution reaction. Equally important, we find that CO2RR products via intermediates of COOH and CO are disfavored on these bimetallenes. Although surface strains are developed on the bimetallenes, their contribution to the catalytic activities is moderate as compared to that of the alloying effect. This work opens the door to future applications of bimetallenes as active and selective catalysts for the CO2RR. read less

Abstract: We investigate the atomistic mechanism of homogeneous nucleation during solidification in molybdenum employing transition path sampling. The mechanism is characterized by the formation of a pre-structured region of high bond-orientational order in the supercooled liquid followed by the emergence of the crystalline bulk phase within the center of the growing solid cluster. This precursor plays a crucial role in the process as it provides a diffusive interface between the liquid and crystalline core, which lowers the interfacial free energy and facilitates the formation of the bulk phase. Furthermore, the structural features of the pre-ordered regions are distinct from the liquid and solid phases and preselect the specific polymorph that nucleates. The similarity in the nucleation mechanism of Mo with that of metals that exhibit different crystalline bulk phases indicates that the formation of a precursor is a general feature observed in these materials. The strong influence of the structural characteristics of the precursors on the final crystalline bulk phase demonstrates that for the investigated system, polymorph selection takes place in the very early stages of nucleation. read less

Abstract: PtM alloy catalysts (e.g., PtFe, PtCo), especially in an intermetallic L10 structure, have attracted considerable interest due to their respectable activity and stability for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). However, metal‐catalyzed formation of ·OH from H2O2 (i.e., Fenton reaction) by Fe‐ or Co‐containing catalysts causes severe degradation of PEM/catalyst layers, hindering the prospects of commercial applications. Zinc is known as an antioxidant in Fenton reaction, but is rarely alloyed with Pt owing to its relatively negative redox potential. Here, sub‐4 nm intermetallic L10‐PtZn nanoparticles (NPs) are synthesized as high‐performance PEMFC cathode catalysts. In PEMFC tests, the L10‐PtZn cathode achieves outstanding activity (0.52 A mgPt−1 at 0.9 ViR‐free, and peak power density of 2.00 W cm−2) and stability (only 16.6% loss in mass activity after 30 000 voltage cycles), exceeding the U.S. DOE 2020 targets and most of the reported ORR catalysts. Density function theory calculations reveal that biaxial strains developed upon the disorder‐order (A1L10) transition of PtZn NPs would modulate the surface PtPt distances and optimize PtO binding for ORR activity enhancement, while the increased vacancy formation energy of Zn atoms in an ordered structure accounts for the improved stability. read less

Abstract: Only few available interatomic interaction potentials implement the α ↔ γ phase transformation in iron by featuring a stable low-temperature bcc and high-temperature fcc lattice structure. Among these are the potentials by Meyer and Entel (1998 Phys. Rev. B 57 5140), by Müller et al (2007 J. Phys.: Condens. Matter 19 326220) and by Lee et al (2012 J. Phys.: Condens. Matter 24 225404). We study how these potentials model the phase transformation during heating and cooling; in order to help initiating the transformation, the simulation volume contains a grain boundary. For the martensitic transformation occurring on cooling an fcc structure, we additionally study two potentials that only implement a stable bcc structure of iron, by Zhou et al (2004 Phys. Rev. B 69 144113) and by Mendelev et al (2003 Philos. Mag. 83 3977). We find that not only the transition temperature depends on the potential, but that also the height of the energy barrier between fcc and bcc phase governs whether the transformation takes place at all. In addition, details of the emerging microstructure depend on the potential, such as the fcc/hcp fraction formed in the α → γ transformation, or the twinning induced in and the lattice orientation of the bcc phase in the γ → α transformation. read less

Abstract: We use first-principles calculations to investigate the structural and transport properties of various Cu/Ta(N)/Cu interface stacks, which are representative of the metal interfaces located at the bottom of vertical interconnects in state-of-the-art back-end-of-line technology. In particular, we consider approximately 2-nm thick layers of several different Ta-based barrier layers sandwiched between two Cu(111) layers, including TaN, α-Ta, β-Ta, and a bilayer TaN/ α-Ta structure. Our results highlight that the bilayer Cu/TaN/ α-Ta/Cu structure shows both an attractive combination of low electrical resistance and superior dielectric adhesion. We also find that inelastic phonon transport across the interface structures is largely determined by the frequency overlap of the bulk-like phonon density of states of each metal layer. Our results are fed into a simple interconnect performance benchmarking model based on a single-driver signal wire, where we find that metal barrier optimization can result in a net 2.5% stage delay reduction without comprising reliability. read less

Abstract: ABSTRACT As opposed to traditional laboratory testing, Molecular Dynamics (MD) offers an atomistic scale method to estimate the mechanical properties of metals. However, there is limited literature that shows the effect of interatomic potentials when determining mechanical properties. Hence, the present research was conducted to investigate the accuracy of various interatomic potentials in estimating mechanical properties of aluminium. Several types of potentials, including Embedded Atom Method (EAM), Modified EAM (MEAM) and Reactive Force Field (ReaxFF) were compared with available experimental data for pure aluminium to determine the most accurate interatomic potential. A uniaxial tensile test was performed at room temperature using MD simulations for nanoscale aluminium. Results demonstrated that those potentials parameterised with elastic constants at physically realisable temperatures were consistently more accurate. Overall, the Mishin et al. EAM potential was the most accurate when compared to single-crystal experimental values. Regardless of the potential type, the error was significantly higher for those potentials that did not consider elastic constants during development. In brief, the application of the interatomic potentials to estimate mechanical properties of a nanoscale aluminium was investigated. read less

Abstract: Significance Precipitates in a material are traditionally thought of as dislocation obstacles that lead to elevated strength and reduced ductility. In contrast, recent experiments suggest that nanoscale precipitates facilitate both high strength and large ductility. To help resolve this apparent paradox, here we reveal that nanoprecipitates provide a unique type of dislocation sources that are activated at sufficiently high stress levels and render uniform plasticity by simultaneously serving as efficient dislocation sources and obstacles to dislocation motion, giving rise to sustained deformability. The findings can guide development of next generation of materials such as multiple-element alloys with precipitate engineering. Traditionally, precipitates in a material are thought to serve as obstacles to dislocation glide and cause hardening of the material. This conventional wisdom, however, fails to explain recent discoveries of ultrahigh-strength and large-ductility materials with a high density of nanoscale precipitates, as obstacles to dislocation glide often lead to high stress concentration and even microcracks, a cause of progressive strain localization and the origin of the strength–ductility conflict. Here we reveal that nanoprecipitates provide a unique type of sustainable dislocation sources at sufficiently high stress, and that a dense dispersion of nanoprecipitates simultaneously serve as dislocation sources and obstacles, leading to a sustainable and self-hardening deformation mechanism for enhanced ductility and high strength. The condition to achieve sustainable dislocation nucleation from a nanoprecipitate is governed by the lattice mismatch between the precipitate and matrix, with stress comparable to the recently reported high strength in metals with large amount of nanoscale precipitates. It is also shown that the combination of Orowan’s precipitate hardening model and our critical condition for dislocation nucleation at a nanoprecipitate immediately provides a criterion to select precipitate size and spacing in material design. The findings reported here thus may help establish a foundation for strength–ductility optimization through densely dispersed nanoprecipitates in multiple-element alloy systems. read less

Abstract: Shock-induced consolidation of tungsten nanoparticles to form a bulk material was modeled using molecular dynamics simulation. By arranging the nanoparticles in a three-dimensional model of body-centered cubic super-lattice, the calculated shock velocity-particle velocity Hugoniot data are in good agreement with the experiments. Three states, including solid-undensified, solid-densified, and liquid-densified, can be sequentially obtained with the increase of the impact velocity. It is due to the flow deformation at the particle surface that densifies the cavity, and the high pressure and temperature that join the particles together. Melting is not a necessary factor for shock consolidation. Based on whether or not melting takes place, the consolidation mechanisms are liquid-diffusion welding or solid-pressure welding.Shock-induced consolidation of tungsten nanoparticles to form a bulk material was modeled using molecular dynamics simulation. By arranging the nanoparticles in a three-dimensional model of body-centered cubic super-lattice, the calculated shock velocity-particle velocity Hugoniot data are in good agreement with the experiments. Three states, including solid-undensified, solid-densified, and liquid-densified, can be sequentially obtained with the increase of the impact velocity. It is due to the flow deformation at the particle surface that densifies the cavity, and the high pressure and temperature that join the particles together. Melting is not a necessary factor for shock consolidation. Based on whether or not melting takes place, the consolidation mechanisms are liquid-diffusion welding or solid-pressure welding. read less

Abstract: Nanolayered metallic composites have attracted intensive scientific interests due to their ultrahigh strength. However, the deformation incompatibility among the component layers with high mechanical contrast leads to extremely low tensile ductility in the nanolayered composites, which is a great setback for their engineering applications. Here, by molecular dynamics simulations, we show that a heterogeneous nanolayered design by combining 2.5 nm and 24 nm Cu/Ni bilayers in a composite in an appropriate way can promote the dislocation activity of the hard phase, i.e., the Ni layers. In the new heterogeneous structure, each 24 nm Cu or Ni layer is coated on both surfaces by one 2.5 nm Cu/Ni bilayer. The simulations show that the dislocations in the 24 nm Ni layers can nucleate and glide almost synchronously with those in the 24 nm Cu layers. The enhanced dislocation activities are attributed to the presence of the 2.5 nm Cu layer that can promote the dislocation nucleation and motion in the 24 nm Ni layer by forming more nodes in the dislocation network of the interface.Nanolayered metallic composites have attracted intensive scientific interests due to their ultrahigh strength. However, the deformation incompatibility among the component layers with high mechanical contrast leads to extremely low tensile ductility in the nanolayered composites, which is a great setback for their engineering applications. Here, by molecular dynamics simulations, we show that a heterogeneous nanolayered design by combining 2.5 nm and 24 nm Cu/Ni bilayers in a composite in an appropriate way can promote the dislocation activity of the hard phase, i.e., the Ni layers. In the new heterogeneous structure, each 24 nm Cu or Ni layer is coated on both surfaces by one 2.5 nm Cu/Ni bilayer. The simulations show that the dislocations in the 24 nm Ni layers can nucleate and glide almost synchronously with those in the 24 nm Cu layers. The enhanced dislocation activities are attributed to the presence of the 2.5 nm Cu layer that can promote the dislocation nucleation and motion in the 24 nm Ni layer ... read less

Abstract: Grain refinement has been extensively used to strengthen metallic materials for decades. Grain boundaries act as effective barriers to the transmission of dislocations, consequently leading to strengthening. Conventional grain boundaries have a thickness of 1-2 atomic layers, typically ∼0.5 nm for most metallic materials. Here, we report, however, the formation of ∼3 nm thick grain boundaries in a nanocrystalline Ni alloy. In situ micropillar compression studies coupled with molecular dynamics simulations suggest that the thick grain boundaries are stronger barriers than conventional grain boundaries to the transmission of dislocations. This study provides a fresh perspective for the design of high strength, deformable nanostructured metallic materials. read less

Abstract: Concentrated solid-solution alloys (CSAs) demonstrate excellent mechanical properties and promising irradiation resistance depending on their compositions. Existing experimental and simulation results indicate that their heterogeneous structures induced by the random arrangement of different elements are one of the most important reasons responsible for their outstanding properties. Nevertheless, the details of this heterogeneity remain unclear. Specifically, which properties induced by heterogeneity are most relevant to their irradiation response? In this work, we scrutinize the role of heterogeneity in CSAs played in damage evolution in different aspects through atomistic simulations, including lattice misfit, thermodynamic mixing, point defect energetics, point defect diffusion, and dislocation properties. Our results reveal that structural parameters, such as lattice misfit and enthalpy of mixing, are generally not suitable to assess their irradiation response under cascade conditions. Instead, atomic-level defect properties are the keys to understand defect evolution in CSAs. Therefore, tuning chemical disorder to tailor defect properties is a possible way to further improve the irradiation performance of CSAs. read less

Abstract: Strengthening, i.e. increased stress required to move a dislocation, in dilute or complex alloys arises from the totality of the interaction energies between the solutes and an individual dislocation. Prevailing theories for strengthening in bcc alloys consider only solute interactions in the core of the screw dislocation while computations suggest longer-range interactions. Here, a full statistical solute/screw interaction energy parameter relevant for predicting strengthening in random bcc alloys is presented. The parameter is valid for any number of constituent atoms and at any concentrations, thus including the range from dilute binary alloys to high-entropy alloys. The interaction energy parameter is then calculated for many bcc alloys in the Nb–Ta-V–Ti–Zr family using the Zhou-Johnson EAM potentials to demonstrate the spatial range of solutes contributing to this key quantity and to assess accuracy of previous simplified models. The interaction energy parameter is found to converge if solutes out to sixth neighbors are included while the simplified models are generally not very accurate. A recently-proposed correlation between solute/dislocation interaction energy and the solute/[111]/6 unstable stacking fault (USF) interaction energy is then assessed in detail. A very good correlation is found between the full interaction energy parameter introduced here and the solute/USF interaction energy. This points toward a simplified approach to estimating the interaction energy parameter using first-principles methods. read less

Abstract: The commercialization of proton exchange membrane fuel cells (PEMFCs) relies on highly active and stable electrocatalysts for oxygen reduction reaction (ORR) in acid media. The most successful catalysts for this reaction are nanostructured Pt-alloy with a Pt-skin. Here, we report the synthesis of ultrasmall and ordered L1 0 -PtCo nanoparticle ORR catalysts further doped with a few percent of transition metals (i.e., W, Ga, and Zn). Compared to commercial Pt/C catalyst, the L1 0 -W-PtCo/C catalyst shows significant improvement in both initial activity and high-temperature stability. Importantly, the L1 0 -W-PtCo/C catalyst achieves high activity and stability in the PEMFC after 50k voltage cycles at 80 °C, superior to the DOE 2020 targets. Extended X-ray absorption fine structure analysis and density functional theory calculations reveal that W doping not only stabilizes the ordered intermetallic structure, but also tunes the Pt-Pt distances in such a way to optimize the binding energy between Pt and O intermediates on the surface. This work demonstrates a new strategy of stabilizing the intermetallic nanoparticles with transition metal doping to improve the performance of electrocatalysts. read less

Abstract: In this work, we present a concurrent atomistic-continuum (CAC) method for modeling and simulation of crystalline materials. The CAC formulation extends the Irving-Kirkwood procedure for deriving transport equations and fluxes for homogenized molecular systems to that for polyatomic crystalline materials by employing a concurrent two-level description of the structure and dynamics of crystals. A multiscale representation of conservation laws is formulated, as a direct consequence of Newton's second law, in terms of instantaneous expressions of unit cell-averaged quantities using the mathematical theory of distributions. Finite element (FE) solutions to the conservation equations, as well as fluxes and temperature in the FE representation, are introduced, followed by numerical examples of the atomic-scale structure of interfaces, dynamics of fracture and dislocations, and phonon thermal transport across grain boundaries. In addition to providing a methodology for concurrent multiscale simulation of transport processes under a single theoretical framework, the CAC formulation can also be used to compute fluxes (stress and heat flux) in atomistic and coarse-grained atomistic simulations.In this work, we present a concurrent atomistic-continuum (CAC) method for modeling and simulation of crystalline materials. The CAC formulation extends the Irving-Kirkwood procedure for deriving transport equations and fluxes for homogenized molecular systems to that for polyatomic crystalline materials by employing a concurrent two-level description of the structure and dynamics of crystals. A multiscale representation of conservation laws is formulated, as a direct consequence of Newton's second law, in terms of instantaneous expressions of unit cell-averaged quantities using the mathematical theory of distributions. Finite element (FE) solutions to the conservation equations, as well as fluxes and temperature in the FE representation, are introduced, followed by numerical examples of the atomic-scale structure of interfaces, dynamics of fracture and dislocations, and phonon thermal transport across grain boundaries. In addition to providing a methodology for concurrent multiscale simulation of transpo... read less

Abstract: ABSTRACT Tensile and creep properties of dissimilar cold weld joints (Al (metal)–Cu50Zr50 (metallic glass)) are investigated using molecular dynamics simulations. Embedded atom method potential is used to model the interactions between Al–Cu–Zr atoms. Cold welding is carried out at three different velocities (20, 30 and 40 m/s) and for three interferences (0.4, 1.3 and 2.3 nm). The strength of the welded joints is measured using the tensile test carried out at a strain rate of 1.5 × 109/s. Structure studies by radial distribution function analysis indicate amorphisation of Al in the weld regions. Tensile studies show that the maximum strength is obtained in the sample that is welded for 1.3 nm interference. Creep studies carried out over range of stresses (200–350 MPa) and temperatures (200–500 K) show very short primary creep and significant steady-state creep. The stress exponent n has two values; at lower stress, n = 1.2, and at higher stress, n = 4.06, respectively. The deformation mechanisms are observed to be slip by Shockley partial dislocation and by twinning in Al region. The icosahedral cluster population in metallic glass decreases as the temperature increases and contributes to large plastic strain. read less

Abstract: Shock-induced plasticity and phase transition in single crystal lead are investigated by nonequilibrium molecular dynamics simulations. Under dynamic shock loading, the appearance of plasticity in materials precedes that of phase transition. Plasticity mainly causes two effects: one is that plasticity has a significant relaxation effect on shear stress, and the other is that deformation twinning serves as important nucleation sites for the phase transition. This twinning is caused by mutual impediments among different cross-slips and {111} slips. There are three main stages in the dynamic phase transition process of lead: fcc → bcc-like phase transition, plasticity, and hcp phase formation and growth. Moreover, phase transition has a more significant relaxation effect on shear stress, which relaxes the shear stress to a minimum value. The spall strength of lead decreases as the shock intensity increases, but its rate of decrease under different shock intensities is different. Plasticity, especially phase transition, would obviously result in a lower rate of decrease in spall strength.Shock-induced plasticity and phase transition in single crystal lead are investigated by nonequilibrium molecular dynamics simulations. Under dynamic shock loading, the appearance of plasticity in materials precedes that of phase transition. Plasticity mainly causes two effects: one is that plasticity has a significant relaxation effect on shear stress, and the other is that deformation twinning serves as important nucleation sites for the phase transition. This twinning is caused by mutual impediments among different cross-slips and {111} slips. There are three main stages in the dynamic phase transition process of lead: fcc → bcc-like phase transition, plasticity, and hcp phase formation and growth. Moreover, phase transition has a more significant relaxation effect on shear stress, which relaxes the shear stress to a minimum value. The spall strength of lead decreases as the shock intensity increases, but its rate of decrease under different shock intensities is different. Plasticity, especially phase ... read less

Abstract: The nanofriction of graphene is critical for its broad applications as a lubricant and in flexible electronics. Herein, using a Au substrate as an example, we have investigated the effect of the grain boundary on the nanofriction of graphene by means of molecular dynamics simulations. We have systematically examined the coupling effects of the grain boundary with different mechanical pressures, velocities, temperatures, contact areas, and relative rotation angles on nanofriction. It is revealed that grain boundaries could reduce the friction between graphene and the gold substrate with a small deformation of the latter. Large lateral forces were observed under severe deformation around the grain boundary. The fluctuation of lateral forces was bigger on surfaces with grain boundaries than that on single-crystal surfaces. Friction forces induced by the armchair grain boundaries was smaller than those by the zigzag grain boundaries. read less

Abstract: Structural changes of Ni–Au core–shell nanoparticles with increasing temperature are studied at atomic resolution. The bimetallic clusters, synthesized in superfluid helium droplets, show a centralized Ni core, which is an intrinsic feature of the growth process inside helium. After deposition on SiNx, the nanoparticles undergo a programmed temperature treatment in vacuum combined with an in situ transmission electron microscopy study of structural changes. We observe not only full alloying far below the actual melting temperature, but also a significantly higher stability of core–shell structures with decentralized Ni cores. Explanations are provided by large-scale molecular dynamics simulations on model structures consisting of up to 3000 metal atoms. Two entirely different diffusion processes can be identified for both types of core–shell structures, strikingly illustrating how localized, atomic features can still dictate the overall behavior of a nanometer-sized particle. read less

Abstract: Classical molecular dynamics (MD) simulations were used to investigate how free surfaces, as well as supporting substrates, affect phase separation in a NiAg alloy. Bulk samples, droplets, and droplets deposited on a graphene substrate were investigated at temperatures that spanned regions of interest in the bulk NiAg phase diagram, i.e., miscible and immiscible liquid, liquid-crystal, and crystal-crystal regions. Using MD simulations to cool down a bulk sample from 3000 K to 800 K, it was found that phase separation below 2400 K takes place in agreement with the phase diagram. When free surface effects were introduced, phase separation was accompanied by a core-shell transformation: spherical droplets created from the bulk samples became core-shell nanoparticles with a shell made mostly of Ag atoms and a core made of Ni atoms. When such droplets were deposited on a graphene substrate, the phase separation was accompanied by Ni layering at the graphene interface and Ag at the vacuum interface. Thus, it should be possible to create NiAg core-shell and layer-like nanostructures by quenching liquid NiAg samples on tailored substrates. Furthermore, interesting bimetallic nanoparticle morphologies might be tuned via control of the surface and interface energies and chemical instabilities of the system. read less

Abstract: ABSTRACT Molecular dynamics simulation was used to stretch Cu nanoplates along its [100] direction at various strain rates and temperatures. Under high strain rate and beyond the elastic limit, the Cu nanoplates underwent an unusual deformation mechanism with expansion along free surface lateral direction and contraction along the other lateral direction, which leaded to the face-centred-cubic phase transforming into unstressed body-centred-cubic phase. Under low strain rate, the deformation of the nanoplate went back to well-known dislocation mechanism. The face-centred-cubic to body-centred-cubic phase transformation mechanism was further discussed in terms of elastic stability theory and free surface stress effect. read less

Abstract: We evaluate the structure of liquid NiTi under various pressures from 0 GPa to 40 GPa in the atomic level using molecular dynamics simulations. The structure factor and radial distribution function are used to investigate the general structural change of the system. Further identification of the local structures is examined by the bond-angle method and bond-angle distribution analysis. From our results, we found that the count of the local structure of fcc, hcp, bcc, and icosahedral short-range order monotonically increase when the pressures increase. We also observed in our results that the size of the local cluster grows as the pressure increases, and the long-range connectivity of the quasi-crystal is achieved at high pressure. read less

Abstract: The chemical structure and the localized surface plasmon resonance (LSPR) of size-selected AgxFe1–x (x = 0.5, 0.8) nanoparticles (NPs), produced by laser vaporization, were investigated experimenta... read less

Abstract: this paper describes thermal stabilities (573 K for 8 h) of pressureless-sintered Copper (Cu) on four kinds of top metallization layers (Ni, Cu, Ag, and Au) by experiments. Evolutions of sintering process of Cu nanoparticles and diffusion coefficients of interfaces between a bulk Cu layer and metallization layers were also evaluated by molecular dynamics (MD) simulations. After aging at 573 K for 8 h in terms of bonding samples, the shear strengths of sintered Cu on Ni and Cu layer increased, whereas those of sintered Cu on Ag and Au layer decreased. It was confirmed that interdiffusion occurred in the interfaces between sintered Cu layer and Ag layer or Au layer by energy dispersive X-ray spectroscopy (EDX), which increased the porosities of sintered Cu near the interfaces. The increases of interfacial porosities on sintered Cu/Ag and sintered Cu/Au decreased the shear strengths. In contrast, the porosities near the interface between sintered Cu layer and Ni layer or Cu layer hardly changed after aging. MD simulations revealed that Kirkendall voids were promoted by higher interdiffusion coefficients and higher ratio of intrinsic diffusion coefficients between a bulk Cu layer and metallization layers, which consequently increased the porosities of sintered Cu near the interfaces. The interdiffusion coefficients, which seem to have a correlation with the shear strengths of sintered Cu, can be used as an index value to find metallization layers that are suitable for the sintered Cu layer by calculations of MD simulations. read less

Abstract: The effects of segregation of impurity molybdenum (Mo) atoms on the tensile mechanical properties of nanocrystalline nickel (Ni) are investigated with molecular dynamics simulation. The results show that the segregation of Mo atoms induces an obvious increase in the elastic modulus and strength of nanocrystalline Ni, and the strengthening effect is more significant with smaller grain size. When the grain size decreases below a critical value, at which the softening occurs in non-segregated Ni-Mo alloy, no evident softening phenomenon is observed in Mo-segregated systems. Furthermore, based on a bicrystal configuration, it is found that Mo atoms segregating to the grain boundary reduce the energy and mobility of the grain boundary, increasing the grain boundary stability and thus accommodating the strengthening. The present findings will shed light on the fabrication of high strength nanocrystalline materials by controlling the segregation of atoms. read less

Abstract: Engineering the crystal structure of Pt–M (M = transition metal) nanoalloys to chemically ordered ones has drawn increasing attention in oxygen reduction reaction (ORR) electrocatalysis due to their high resistance against M etching in acid. Although Pt–Ni alloy nanoparticles (NPs) have demonstrated respectable initial ORR activity in acid, their stability remains a big challenge due to the fast etching of Ni. In this work, sub‐6 nm monodisperse chemically ordered L10‐Pt–Ni–Co NPs are synthesized for the first time by employing a bifunctional core/shell Pt/NiCoOx precursor, which could provide abundant O‐vacancies for facilitated Pt/Ni/Co atom diffusion and prevent NP sintering during thermal annealing. Further, Co doping is found to remarkably enhance the ferromagnetism (room temperature coercivity reaching 2.1 kOe) and the consequent chemical ordering of L10‐Pt–Ni NPs. As a result, the best‐performing carbon supported L10‐PtNi0.8Co0.2 catalyst reveals a half‐wave potential (E1/2) of 0.951 V versus reversible hydrogen electrode in 0.1 m HClO4 with 23‐times enhancement in mass activity over the commercial Pt/C catalyst along with much improved stability. Density functional theory (DFT) calculations suggest that the L10‐PtNi0.8Co0.2 core could tune the surface strain of the Pt shell toward optimized Pt–O binding energy and facilitated reaction rate, thereby improving the ORR electrocatalysis. read less

Abstract: In search of materials with better properties, polycrystalline materials are often found to be superior to their respective single crystalline counterparts. Reduction of grain size in polycrystalline materials can drastically alter the properties of materials. When the grain sizes reach the nanometer scale, the improved mechanical response of the materials make them attractive in many applications. Multicomponent solid-solution alloys have shown to have a higher radiation tolerance compared with pure materials. Combining these advantages, we investigate the radiation tolerance of nanocrystalline multicomponent alloys. We find that these alloys withstand a much higher irradiation dose, compared with nanocrystalline Ni, before the nanocrystallinity is lost. Some of the investigated alloys managed to keep their nanocrystallinity for twice the irradiation dose as pure Ni. read less

Abstract: This paper investigates the compressive and shear properties of nanoporous gold (np-Au) coated with different ultrathin metallic materials (i.e., platinum and silver) via molecular dynamics simulations. Atomistic models used for the geometric representation of coated and uncoated np-Au structures are generated through a modeling technique based on the Voronoi tessellation method. Three different coating thickness values are used to examine the role of thickness for the coating performance under compressive and shear loading by comparing the mechanical characteristics of the atomistic models such as Young's modulus, yield, and ultimate strengths. Moreover, adaptive common neighbor analyses are carried out by monitoring the evolution of the crystal structure of the specimens during the loading process. In this way, the deformation mechanisms of coated and uncoated nanoporous specimens are identified thoroughly. As a key finding from the simulation results, it is observed that the mechanical properties of np-Au are crucially dependent on the type of the coating material. However, a significant improvement on the toughness within the plastic regime is demonstrated for all types of coating materials and loading conditions.This paper investigates the compressive and shear properties of nanoporous gold (np-Au) coated with different ultrathin metallic materials (i.e., platinum and silver) via molecular dynamics simulations. Atomistic models used for the geometric representation of coated and uncoated np-Au structures are generated through a modeling technique based on the Voronoi tessellation method. Three different coating thickness values are used to examine the role of thickness for the coating performance under compressive and shear loading by comparing the mechanical characteristics of the atomistic models such as Young's modulus, yield, and ultimate strengths. Moreover, adaptive common neighbor analyses are carried out by monitoring the evolution of the crystal structure of the specimens during the loading process. In this way, the deformation mechanisms of coated and uncoated nanoporous specimens are identified thoroughly. As a key finding from the simulation results, it is observed that the mechanical properties of np... read less

Abstract: Recent irradiation experiments on concentrated random solid solution alloys (CSAs) show that some CSAs can undergo disorder-to-order transition, i.e., the atoms that are initially randomly distributed on a face centered cubic crystal lattice undergo ordering (e.g., L10 or L12) due to irradiation. In this work, we elucidate that the atomic structure could affect the segregation properties of grain boundaries. While working on Ni and Ni-Fe alloys, from static atomistic simulations on 138 grain boundaries, we show that despite identical alloy composition, Cr segregation is higher in the disordered structures compared to ordered structures in both Ni0.50Fe0.50 and Ni0.75Fe0.25 systems. We also show that grain boundary (GB) energy could act as a descriptor for impurity segregation. We illustrate that there is a direct correlation between Cr segregation and grain boundary energy, i.e., segregation increases with the increase in the GB energy. Such correlation is observed in pure Ni and in the Ni-Fe alloys studied in this work.Recent irradiation experiments on concentrated random solid solution alloys (CSAs) show that some CSAs can undergo disorder-to-order transition, i.e., the atoms that are initially randomly distributed on a face centered cubic crystal lattice undergo ordering (e.g., L10 or L12) due to irradiation. In this work, we elucidate that the atomic structure could affect the segregation properties of grain boundaries. While working on Ni and Ni-Fe alloys, from static atomistic simulations on 138 grain boundaries, we show that despite identical alloy composition, Cr segregation is higher in the disordered structures compared to ordered structures in both Ni0.50Fe0.50 and Ni0.75Fe0.25 systems. We also show that grain boundary (GB) energy could act as a descriptor for impurity segregation. We illustrate that there is a direct correlation between Cr segregation and grain boundary energy, i.e., segregation increases with the increase in the GB energy. Such correlation is observed in pure Ni and in the Ni-Fe alloys studi... read less

Abstract: this paper describes the sintering properties and bonding properties of copper (Cu) die-bonding sinter paste for power devices operating at high temperatures. The Cu paste can be sintered pressure less in 100% H2 or under pressure in 100% N2 atmospheres. The as-sintered density, thermal conductivity and resistivity of pressure less-sintered Cu (in 100% H2, 300 °C, 1 h) is found to be 7S%, 180 Wm^-1K^-1 and 4.3 $\mu\Omega\cdot cm$, respectively. The pressurelesssintered Cu has higher 0.2% proof stress than the pressure-sintered Ag (sintered density =87%, in air, 300 °C 10 MPa, 10min) as a comparison material in a three-point bending test. The die-shear strength of appropriate pressurelesssintered Cu on four different metal adherends (Cu, Ni, Ag and Au) was 30 MPa or higher. The die-shear strength of pressure- sintered Cu in 100% N2 was 36 MPa or higher. A thermal cycle tolerance of 1000 cycles or greater was shown in a power device test package which was bonded using the pressurelesssintered Cu and encapsulated with an epoxy molding compound. The Cu sinter paste can be used as a reliable die-bonding material for power modules operating at high temperatures. read less

Abstract: Molecular dynamics simulations on the indentation process of freestanding and Pt(111)-supported black phosphorus (BP) monolayer were conducted to study the fracture mechanism of the membrane. For the freestanding BP monolayer, crack grows firstly along armchair direction and then zigzag direction during the indentation process. Whereas, for the Pt(111)-supported BP monolayer, crack growth shows no obvious directionality, with irregular distribution of crack tips. Further study on stress distribution shows that maximum normal stress component at elastic stage is in zigzag direction for the freestanding BP monolayer, and in vertical direction for the Pt(111)-supported BP monolayer. As BP monolayer is remarkably anisotropic for in-plane mechanical properties and homogeneous for out-of-plane mechanical properties, the difference of stress state may be a key reason for the different fracture behavior in these two cases. These findings may help to understand the failure mechanism of BP, when applied in nano-devices. read less

Abstract: We present an embedded atom method (EAM) potential for the binary Cu–Au system. The unary phases are described by two well-tested unary EAM potentials for Cu and Au. We fitted the interaction between Cu and Au to experimental properties of the binary intermetallic phases Cu3Au, CuAu and CuAu3. Particular attention has been paid to reproducing stacking fault energies in order to obtain a potential suitable for studying deformation in this binary system. The resulting energies, lattice constant, elastic properties and melting points are in good agreement with available experimental data. We use nested sampling to show that our potential reproduces the phase boundaries between intermetallic phases and the disordered face-centered cubic solid solution. We benchmark our potential against four popular Cu–Au EAM parameterizations and density-functional theory calculations. read less

Abstract: Nanoporous metals are a class of novel nanomaterials with potential applications in many fields. Herein, we demonstrate the cold-welding mechanism of nanoporous metals with various combinations using molecular dynamics simulations. This study shows that it is possible to cold-weld two nanoporous metals to form a novel composite material. The influence of temperature, in the range of 300-900 K, on the mechanical properties of the resultant composite material was investigated. With an increase in temperature, the weld stress and the mechanical strength of the nanoporous structures significantly decreased as an increase in disorder magnitude was observed. These results could lead to bottom-up nanofabrication and nanoassembly of combined nanoporous metals for high mechanical performance. read less

Abstract: Recent advances in graphene and other two-dimensional (2D) material synthesis and characterization have led to their use in emerging technologies, including flexible electronics. However, a major challenge is electrical contact stability, especially under mechanical straining or dynamic loading, which can be important for 2D material use in microelectromechanical systems. In this letter, we investigate the stability of dynamic electrical contacts at a graphene/metal interface using atomic force microscopy (AFM), under static conditions with variable normal loads and under sliding conditions with variable speeds. Our results demonstrate that contact resistance depends on the nature of the graphene support, specifically whether the graphene is free-standing or supported by a substrate, as well as on the contact load and sliding velocity. The results of the dynamic AFM experiments are corroborated by simulations, which show that the presence of a stiff substrate, increased load, and reduced sliding velocity lead to a more stable low-resistance contact. read less

Abstract: We carry out molecular statics simulations of the indentation of bare and graphene-covered Pt (111) surfaces with smooth and rough indenters of radius 1.5 to 10 nm. Our simulations show that the plastic yield of bare surfaces strongly depends on atomic-scale indenter roughness such as terraces or amorphous disorder. Covering surfaces with graphene regularizes this response to the results obtained for ideally smooth indenters. Our results suggest that graphene monolayers and other 2D materials mitigate the effect of roughness, which could be exploited to improve the fidelity of experiments that probe the mechanical properties of interfaces. read less

Abstract: First-order interfacial phaselike transformations that break the mirror symmetry of the symmetric ∑5 (210) tilt grain boundary (GB) are discovered by combining a modified genetic algorithm with hybrid Monte Carlo and molecular dynamics simulations. Density functional theory calculations confirm this prediction. This first-order coupled structural and adsorption transformation, which produces two variants of asymmetric bilayers, vanishes at an interfacial critical point. A GB complexion (phase) diagram is constructed via semigrand canonical ensemble atomistic simulations for the first time. read less

Abstract: The plastic work of fracture in a deformable solid has been believed to be related to only the ideal brittle fracture energy. However, additional factors affecting the plastic work must also exist because the plastic work is a path function. In the present work, first-principles calculations and molecular dynamics simulations of tensile tests were performed on Σ3(1 1)[1 1 0] and Σ11(1 3)[1 1 0] symmetric tilt aluminium grain boundaries, where the grain boundary energy of the Σ11(1 3) grain boundary is higher than that of the Σ3(1 1)[1 1 0] gain boundary. The calculations showed that, although the ideal brittle fracture energy for the Σ11(1 3) grain boundary was almost the same as that for the Σ3(1 1) grain boundary, the plastic work for the former was larger than that for the latter, resulting in a larger fracture energy for the Σ11(1 3) grain boundary. Local inelastic deformation occurred around the atoms with high internal energy at the grain boundary for the Σ11(1 3) grain boundary. It is therefore suggested that the plastic work is a function of both the grain boundary energy and the ideal brittle fracture energy. read less

Abstract: Atomistic simulations were used to study conductance across the interface between a nanoscale gold probe and a graphite surface with a step edge. Conductance on the graphite terrace was observed to increase with load and be approximately proportional to contact area calculated from the positions of atoms in the interface. The relationship between area and conductance was further explored by varying the position of the contact relative to the location of the graphite step edge. These simulations reproduced a previously-reported current dip at step edges measured experimentally and the trend was explained by changes in both contact area and the distribution of distances between atoms in the interface. The novel approach reported here provides a foundation for future studies of the fundamental relationships between conductance, load and surface topography at the atomic scale. read less

Abstract: In this study, the mechanical properties of nanoporous gold (np-Au) coated with different ultrathin metallic materials (i.e., platinum and silver) are studied through molecular dynamics simulations. A new atomistic modelling technique, which is based on the Voronoi tessellation method providing periodic atomistic specimens, is used for the geometric representation of np-Au structure. Three different coating thickness values are used to examine the role of thickness on the coating performance under tensile loading at a constant strain rate. Several parameters, including Young's modulus, yield, and ultimate strengths, are utilized to compare the mechanical characteristics of coated and uncoated np-Au specimens. Moreover, adaptive common neighbor analyses are performed on the specimens for the purpose of understanding the deformation mechanisms of coated and uncoated nanoporous specimens comprehensively by monitoring the microstructural evolution of the crystal structure of the specimens within the deformati... read less

Abstract: We optimized the high pressure-temperature phase diagram of pure Co up to the liquidus temperature and 120 GPa, based on thermodynamic properties calculated using first-principles. The Gibbs energy for each phase was evaluated in the framework of a quasiharmonic approximation, with a consideration of the thermal electronic contribution at finite temperatures. Particularly, the liquidus temperature, as a function of pressure, was determined using classical Molecular Dynamics simulations. Our results in this work successfully integrated experimental observations and the previous theoretical predictions. The critical solid phase transitions of ε → γf and ε → β were clarified using the Gibbs energy as a function of pressure and temperature. In addition, the magnetism of β above 70 GPa was verified to be nonmagnetic. The difference between γp and β, which was unclear before, has been illustrated to be associated with the magnetic transformation from the paramagnetic state of the γp phase to the nonmagnetic state of the β phase rather than the structural transformation. read less

Abstract: The thermal stability of nanocrystalline Ni due to small additions of Mo or W (up to 1 at%) was investigated in computer simulations by means of a combined Monte Carlo (MC)/molecular dynamics (MD) two-steps approach. In the first step, energy-biased on-lattice MC revealed segregation of the alloying elements to grain boundaries. However, the condition for the thermodynamic stability of these nanocrystalline Ni alloys (zero grain boundary energy) was not fulfilled. Subsequently, MD simulations were carried out for up to 0.5 μs at 1000 K. At this temperature, grain growth was hindered for minimum global concentrations of 0.5 at% W and 0.7 at% Mo, thus preserving most of the nanocrystalline structure. This is in clear contrast to a pure Ni model system, for which the transformation into a monocrystal was observed in MD simulations within 0.2 μs at the same temperature. These results suggest that grain boundary segregation of low-soluble alloying elements in low-alloyed systems can produce high-temperature metastable nanocrystalline materials. MD simulations carried out at 1200 K for 1 at% Mo/W showed significant grain boundary migration accompanied by some degree of solute diffusion, thus providing additional evidence that solute drag mostly contributed to the nanostructure stability observed at lower temperature. read less

Abstract: Machine learning interatomic potentials (MLIPs) based on a large dataset obtained by density functional theory (DFT) calculation have been developed recently. This study gives both conceptual and practical bases for the high accuracy of MLIPs, although MLIPs have been considered to be simply an accurate black-box description of atomic energy. We also construct the most accurate MLIP of the elemental Ti ever reported using a linearized MLIP framework and many angular-dependent descriptors, which also corresponds to a generalization of the modified embedded atom method (MEAM) potential. read less

Abstract: Hyper-QC is a multiscale method based on the quasicontinuum (QC) method in which time is accelerated using hyperdynamics through the addition of a suitable bias potential. This paper describes the practical details of implementing and carrying out hyper-QC simulations and introduces a novel mechanism-based bias potential for deformation processes in face-centred cubic (fcc) systems. The factors limiting the maximum achievable acceleration are discussed. The method is demonstrated for nanoindentation into a thin film of single crystal fcc nickel at near experimental loading rates. Speed up factors as high as 10,000 are achieved. The simulations reveal a thermally activated dislocation nucleation mechanism with a logarithmic dependence on temperature and indenter velocity in agreement with a theoretical model. read less

Abstract: ABSTRACT We develop a new method using binary collision approximation simulating the Rutherford backscattering spectrometry in channeling conditions (RBS/C) from molecular dynamics atom coordinates of irradiated cells. The approach allows comparing experimental and simulated RBS/C signals as a function of depth without fitting parameters. The simulated RBS/C spectra of irradiated Ni and concentrated solid solution alloys (CSAs, NiFe and NiCoCr) show a good agreement with the experimental results. The good agreement indicates the damage evolution under damage overlap conditions in Ni and CSAs at room temperature is dominated by defect recombination and migration induced by irradiation rather than activated thermally. GRAPHICAL ABSTRACT IMPACT STATEMENT A new method simulating the Rutherford backscattering Spectrometry in channeling conditions (RBS/C) was proposed. The RBS/C simulations reveal that the radiation damage buildup in Ni, NiFe and NiCoCr was dominated by athermal defect reactions. read less

Abstract: In this study, a new method developed for the generation of periodic atomistic models of coated and uncoated nanoporous metals (NPMs) is presented by examining the thermodynamic stability of coated nanoporous structures. The proposed method is mainly based on the Voronoi tessellation technique, which provides the ability to control cross-sectional dimension and slenderness of ligaments as well as the thickness of coating. By the utilization of the method, molecular dynamic (MD) simulations of randomly structured NPMs with coating can be performed efficiently in order to investigate their physical characteristics. In this context, for the purpose of demonstrating the functionality of the method, sample atomistic models of Au/Pt NPMs are generated and the effects of coating and porosity on the thermodynamic stability are investigated by using MD simulations. In addition to that, uniaxial tensile loading simulations are performed via MD technique to validate the nanoporous models by comparing the effective Young’s modulus values with the results from literature. Based on the results, while it is demonstrated that coating the nanoporous structures slightly decreases the structural stability causing atomistic configurational changes, it is also shown that the stability of the atomistic models is higher at lower porosities. Furthermore, adaptive common neighbour analysis is also performed to identify the stabilized atomistic structure after the coating process, which provides direct foresights for the mechanical behaviour of coated nanoporous structures. read less

Abstract: We use both a model of dislocation energy and molecular dynamics (MD) simulations to explore the habit planes of 〈a〉-type dislocation loops, while cascade simulations are produced to investigate the effect of irradiation on those loops. Vacancy and interstitial loops are artificially created on perfect prism planes in MD, and they reorient to their preferred habit planes during a relaxation stage. The statistics presented in stereographic projections show that the preferred habit planes are close to the prism plane , consistent with experimental data from the literature. We also confirm that the angle between the Burgers vector and the loop’s plane is a useful parameter when identifying the stability of 〈a〉-type dislocation loops. read less

Abstract: We propose a multiscale computational framework to design core/shell nanoparticles (NPs) for oxygen reduction reaction (ORR). Essential to the framework are linear scaling relations between oxygen adsorption energy and surface strain, which can be determined for NP facets and edges from first-principles and multiscale QM/MM calculations, respectively. Based on the linear scaling relations and a microkinetic model, we can estimate ORR rates as a function of surface strain on core/shell NPs. Employing the multiscale framework, we have systematically examined the ORR activity on Pd-based core/shell NPs as a function of their shape, size, shell thickness, and alloy composition of the core. Three NP shapes—icosahedron, octahedron, and truncated octahedron—are explored, and the truncated octahedron is found to be the most active and the icosahedron is the least active. NixPd1–x@Pd NPs with high Ni concentrations and thin shells could exhibit higher ORR rates than the pure Pt(111) surface and/or Pt NPs. AgxPd1–x... read less

Abstract: This work investigates the shock-induced microjetting from a grooved surface (10 nm, 120 degree) of low-melting metal Pb with molecular dynamics simulations. The microjetting processes under surface/release melting conditions are presented in detail, and some properties on the microjet mass and velocity are revealed for different shock pressure and profile cases. It is found that the increase of microjet mass with shock pressure experiences three stages: rapid increase (solid phase), slowdown increase (release melting) and almost no increase (shock melting). For all cases, the ratio of the maximal jetting velocity to the surface velocity approximately keeps a constant (1.5–1.55), but this value undergoes a degree of exponential decay with time for the solid release cases. In addition, the temperature of the microjet is found to be always above the melting point (zero pressure) and keep a continuous increase towards the microjet tip. When introducing slow decaying profiles, the microjet mass begins to increase with the decay rate, which is dominated by the deformation of bubble during pull-back. When the decay rate becomes fast enough, the microspall occurs as expected, meanwhile the microjet appears to reduce because of the shock energy reduction. But that cannot cut off the microjet completely. The velocity distribution along the loading direction shows two linear regions corresponding to the microspall and microjet, and the latter seems to have a greater velocity gradient. read less

Abstract: We report the fabrication of a set of new rare-earth-free magnetic compounds, which form the Fe3Co3Ti2-type hexagonal structure with P-6m2 symmetry. Neutron powder diffraction shows a significant Fe/Co anti-site mixing in the Fe3Co3Ti2 structure, which has a strong effect on the magnetocrystalline anisotropy as revealed by first-principle calculations. Increasing substitution of Fe atoms for Co in the Fe3Co3Ti2 lattice leads to the formation of Fe4Co2Ti2, Fe5CoTi, and Fe6Ti2 with significantly improved permanent-magnet properties. A high magnetic anisotropy (13.0 Mergs/cm3) and saturation magnetic polarization (11.4 kG) are achieved at 10 K by altering the atomic arrangements and decreasing Fe/Co occupancy disorder. read less

Abstract: Two methods, deposition method and ideal modeling based on lattice constant, are used to prepare three modulation periods’ 1.8 nm Cu/3.6 nm Au, 2.7 nm Cu/2.7 nm Au, and 3.6 nm Cu/1.8 nm Au thin films for nanoindentation at different temperatures. The results show that the temperature will weaken the hardness of thin films. The deposition method and the formation of coherent interface will result in a lot of defects in thin films. These defects can reduce the residual stress in the thin films which is caused by the external force. The proposed system will provide potential benefits in designing the microstructures for thin films. read less

Abstract: We conduct molecular dynamics simulations of the ejection process from a grooved Pb surface subjected to supported and unsupported shock waves with various shock-breakout pressures (P SB) inducing a solid–liquid phase transition upon shock or release. It is found that the total ejecta mass changing with P SB under a supported shock reveals a similar trend with that under an unsupported shock and the former is always less than the latter at the same P SB. The origin of such a discrepancy could be unraveled that for an unsupported shock, a larger velocity difference between the jet tip and its bottom at an early stage of jet formation results in more serious damage, and therefore a greater amount of ejected particles are produced. The cumulative areal density distributions also display the discrepancy. In addition, we discuss the difference of these simulated results compared to the experimental findings. read less

Abstract: Melting simulation methods are of crucial importance to determining melting temperature of materials efficiently. A high-efficiency melting simulation method saves much simulation time and computational resources. To compare the efficiency of our newly developed shock melting (SM) method with that of the well-established two-phase (TP) method, we calculate the high-pressure melting curve of Au using the two methods based on the optimally selected interatomic potentials. Although we only use 640 atoms to determine the melting temperature of Au in the SM method, the resulting melting curve accords very well with the results from the TP method using much more atoms. Thus, this shows that a much smaller system size in SM method can still achieve a fully converged melting curve compared with the TP method, implying the robustness and efficiency of the SM method. read less

Abstract: The structure and magnetic properties of new magnetic Fe3Co3X2 (X  =  Ti, Nb) compounds are studied by genetic algorithm, first-principles density functional theory (DFT) calculations, and experiments. The atomic structure of a hexagonal structure with P-6m2 symmetry is determined. The simulated x-ray diffraction (XRD) spectra of the P-6m2 structures agree well with experimental XRD data for both Fe3Co3Ti2 and Fe3Co3Nb2. The magnetic properties of these structures as well as the effect of the disorder of Fe and Co on their magnetic properties are also investigated. The magnetocrystalline anisotropy energy is found to be very sensitive to the occupancy disorder between Fe and Co. read less

Abstract: The calculation of the surface tension of curved interfaces has been deeply investigated from molecular simulation during this last past decade. Recently, the thermodynamic Test-Area (TA) approach has been extended to the calculation of surface tension of curved interfaces. In the case of the cylindrical vapour-liquid interfaces of water and Lennard-Jones fluids, it was shown that the surface tension was independent of the curvature of the interface. In addition, the surface tension of the cylindrical interface is higher than that of the planar interface. Molecular simulations of cylindrical interfaces have been so far performed (i) by using a shifted potential, (ii) by means of large cutoff without periodic boundary conditions, or (iii) by ignoring the long range corrections to the surface tension due to the difficulty to estimate them. Indeed, unlike the planar interfaces there are no available operational expressions to consider the tail corrections to the surface tension of cylindrical interfaces. We propose here to develop the long range corrections of the surface tension for cylindrical interfaces by using the non-exponential TA (TA2) method. We also extend the formulation of the Mecke-Winkelmann corrections initially developed for planar surfaces to cylindrical interfaces. We complete this study by the calculation of the surface tension of cylindrical surfaces of liquid tin and copper using the embedded atom model potentials. read less

Abstract: We have performed density functional theory (DFT) based calculations of Fe-Au nanoalloys containing 113 atoms, Fe(x)Au(113-x) (x = 23, 56, 90), to determine their preferred geometric structure and the ensuing electronic structural and magnetic properties. We find that these nanoalloys prefer the formation of a core-shell structure and the Fe core maintains almost a constant magnetic moment of ∼2.8 μ(B) regardless of the Fe content, which is 27% enhancement from the bulk value and in qualitative agreement with some previous results. The local magnetic moment of Fe atoms is well correlated with the local coordination of the Fe atoms. Furthermore, the enhancement of the magnetic moment may be traced to charge depletion from the Fe atoms in the core to the Au atoms in the shell. The preference for the core-shell structure over one with segregated Fe and Au parts could be the low surface tension at the Fe-Au interface, which is larger for the core-shell structure, and can be attributed to strong Fe-Au interfacial interaction as a result of large charge transfer at the interface. read less

Abstract: Structural models for dislocation, vacancy clusters, twin boundary, stacking fault and nanocrystalline sample are constructed using copper as a model material. Positron lifetimes and momentum distributions of annihilating electron–positron pairs are calculated for these structural models. The calculated results indicate that the dislocation, twin boundary and stacking fault are shallow traps to positrons. The dislocation associated with monovacancies gives rise to a positron lifetime similar to that of monovacancies. The calculated positron lifetimes of the nanocrystalline copper show no dependence on the mean grain size. The as-constructed nanocrystalline samples contain vacancy clusters in grain boundaries, and positrons are localized by the vacancy clusters. However after relaxation the samples show only other two kinds of free volumes: one is the interatomic space in grain boundaries which is a shallow trap to positrons; the other is similar to a monovacancy. The latter contributes a positron lifetime of about 163 ps. This kind of free volume is not only observed in grain boundaries but also in the regions near grain boundaries. Positron lifetime calculation combined with the momentum distribution calculation is useful to identify the defect in the nanocrystalline Cu. read less

Abstract: The structures and magnetic properties of the Co-Zr-B alloys near the Co5Zr composition were studied using adaptive genetic algorithm and first-principles calculations to guide further experimental effort on optimizing their magnetic performances. Through extensive structural searches, we constructed the contour maps of the energetics and magnetic moments of the Co-Zr-B magnet alloys as a function of composition. We found that the Co-Zr-B system exhibits the same structural motif as the "Co11Zr2" polymorphs, which plays a key role in achieving high coercivity. Boron atoms can either substitute selective cobalt atoms or occupy the interstitial sites. First-principles calculation shows that the magnetocrystalline anisotropy energies can be significantly improved through proper boron doping. read less

Abstract: Ag is the most conductive metal but is vulnerable to electromigration (EM), which can limit its application in e.g. microelectronics. Molecular dynamics (MD) is used to simulate the migrating behavior of an Ag surface by adding an extra directional force on each atom. The migration of Ag atoms is found to be limited to the topmost 4 (002) lattice planes in the first 40 ns while atoms in the crystal bulk remain oscillating around their equilibrium positions. A Pd coating layer is shown to be a protection from EM for an Ag surface. After adding a layer of 9 Pd (002) lattice planes, the same procedure is repeated with different forces. No migration happens until the extra directional force become so large that all atoms of the model end up moving freely. The MD model presented in this paper can lead to an understanding of EM at the atomic scale and a guideline for potential reliability improvement of microelectronics by coating technology. read less

Abstract: We report on the structural evolution and atomic inter-diffusion characteristics of the bimetallic Ni-Au nanocrystals (NCs) by molecular dynamics simulations studies. Our results reveal that the thermal stability dynamics of Ni-Au NCs strongly depends on the atomic configurations. By engineering the structural construction with Ni:Au = 1:1 atomic composition, compared with core-shell Au@Ni and alloy NCs, the melting point of core-shell Ni@Au NCs is significantly enhanced up to 1215 K. Unexpectedly, with atomic ratio of Au:Ni= 1:9, the melting process initiates from the atoms in the shell of Ni@Au and alloy NCs, while starts from the core of Au@Ni NCs. The corresponding features and evolution process of structural motifs, mixing and segregation are illustrated via a series of dynamic simulations videos. Moreover, our results revealed that the face centered cubic phase Au0.75Ni0.25 favorably stabilizes in NCs form but does not exist in the bulk counterpart, which elucidates the anomalies of previously repor... read less

Abstract: Molecular dynamics simulations are employed to examine the relation between ejecta production and shock-breakout pressure for single crystal Pb subjected to a decaying shockwave loading. To better understand the physical mechanism of ejecta formation, a surface with multiple triangular grooves representing the imperfections left from machining finish is taken into consideration. It is found that the ejecta volume density distribution displays a smooth nature and the amount of ejecta increases significantly after melting on release or shock. Additionally, the ejecta particle mass distribution is captured by a power law scaling, revealing the self-similarity. These results are in reasonable agreement with the characteristics of experimentally diagnosed findings. read less

Abstract: Motivated by the recent experimental reports, we explore the formation of Rayleigh-like instability in metallic nanowires during the solid state annealing, a concept originally introduced for liquid columns. Our molecular dynamics study using realistic interatomic potential reveals instability induced pattern formation at temperatures even below the melting temperature of the wire, in accordance with the experimental observations. We find that this is driven by the surface diffusion, which causes plastic slips in the system initiating necking in the nanowire. We further find the surface dominated mass-transport is of subdiffusive nature with time exponent less than unity. Our study provides an atomistic perspective of the instability formation in nanostructured solid phase. read less

Abstract: Molecular dynamics simulations are carried out to study atomic diffusion in the explosive welding process of Ni50Ti50—Cu (at.%). By using a hybrid method which combines molecular dynamics simulation and classical diffusion theory, the thickness of the diffusion layer and the atomic concentration distribution across the welding interface are obtained. The results indicate that the concentration distribution curves at different times have a geometric similarity. According to the geometric similarity, the atomic concentration distribution at any time in explosive welding can be calculated. Ni50Ti50—Cu explosive welding and scanning electron microscope experiments are done to verify the results. The simulation results and the experimental results are in good agreement. read less

Abstract: We have carried out nanoindentation studies of gold in which the indenter is atomically characterized by field-ion microscopy and the scale of deformation is sufficiently small to be directly compared with atomistic simulations. We find that many features of the experiment are correctly reproduced by molecular dynamics simulations, in some cases only when an atomically rough indenter rather than a smooth repulsive-potential indenter is used. Heterogeneous nucleation of dislocations is found to take place at surface defect sites. Using input from atomistic simulations, a model of indentation based on stochastic transitions between continuum elastic–plastic states is developed, which accurately predicts the size distributions of plastic ‘pop-in’ events and their dependence on tip geometry. read less

Abstract: A computational strategy to design core/shell nanoparticle catalysts for oxygen reduction reactions (ORRs) is proposed based on multiscale modeling. Using a quantum mechanics/molecular mechanics (QM/MM) coupling method, we have studied the ORR on Pt-Cu core/shell nanoparticles with the size ranging from 3 to 8 nm. We have calculated the oxygen adsorption energy on the nanoparticle surface (a descriptor for ORR activity) as a function of the nanoparticle size and thickness of the Pt shell. We find that the Pt-Cu core/shell nanoparticles exhibit higher ORR activities than flat Pt(111) surfaces, consistent with experimental observations. We predict that the diameter of the core/shell nanoparticles should be larger than 7 nm to reach the peak of ORR activities. By examining the effects of ligand, quantum confinement, and surface strain, we confirm that the strain plays the dominant role on ORR activities for the core/shell nanoparticles. A universal relation between the surface strain and the oxygen adsorption energy is established based on which one can computationally screen and design core/shell nanoparticle catalysts for superior ORR activities. read less

Abstract: Fundamental physical phenomena in metals irradiated by ultrashort laser pulses with absorbed fluences higher than few tens of mJ/cm2 are investigated. For those fluences, laser‐produced electron distribution function relaxes to equilibrium Fermi distribution with electron temperature Te within a short time of 10‐100 fs. Because the electron subsystem has Te highly exceeding much the ion subsystem temperature Ti the well‐known twotemperature hydrodynamic model (2T‐HD) is used to evaluate heat propagation associated with hot conductive electron diffusion and electron‐ion energy exchange. The model coefficients of electron heat conductivity κ (ϱ, Te, Ti) and electron‐ion coupling parameter α (ϱ, Te) together with 2T equation of state E (ϱ, Te, Ti) and P (ϱ, Te, Ti) are calculated. read less

Abstract: The effect of alloying elements on the toughness and the fracture behaviour was investigated on seven kinds of Mg-0.3 at.% X (X = Ag, Al, Ca, Pb, Sn, Y and Zn) alloys with a grain size of 3–5 μm. The fracture toughness and fracture behaviour in magnesium alloys were closely related to the segregation energy. The Mg–Al and –Zn alloys that had small segregation energy showed high toughness and ductile fracture in most regions, while the Mg–Ca alloy with large segregation energy exhibited low toughness and intergranular fracture. These different tendencies resulted from solute segregation at grain boundaries (GBs). The change in the lattice parameter ratio was the influential material parameter regardless of whether the GB embrittlement was for enhancement or suppression. read less

Abstract: In order to investigate effects of stacking fault energies (SFEs) on fundamental deformation modes of slips and deformation twinnings in magnesium, we carried out molecular dynamics simulations of shear deformations for the deformation modes with two kinds of many-body interatomic potentials. The SFEs of the basal and second-pyramidal planes are lower for a generalized embedded atom method (GEAM) potential than for an embedded atom method (EAM) potential. While the basal slip quite easily occurs and the prism dislocation is activated, the first-pyramidal slip and the second-pyramidal slip are hard to be operated. However, for the GEAM simulations, the second-pyramidal slip was activated due to reduction of the second-pyramidal SFE. Additionally, the reduction of the SFEs suppresses nucleation of the f10 11g twin in the bf10 11g 2 shearing direction. The relative order of the other fundamental deformation modes in the critical shear stress is qualitatively maintained despite the reduction of the SFEs. [doi:10.2320/matertrans.MAW201311] read less

Abstract: Under shock loadings, the temperature of materials may vary dramatically during deformation and fracture processes. Thus, thermal effect is important for constructing dynamical failure models. Existing works on thermal dissipation effects are mostly from meso- to macro-scale levels based on phenomenological assumptions. The main purpose of the present work is to provide several atomistic scale perspectives about thermal dissipation during spall fracture by nonequilibrium molecular dynamics simulations on single-crystalline and nanocrystalline Pb. The simulations show that temperature arising starts from the vicinity of voids during spalling. The thermal dissipation rate in void nucleation stage is much higher than that in the later growth and coalescence stages. Both classical spallation and micro-spallation are taken into account. Classical spallation is corresponding to spallation phenomenon where materials keep in solid state during shock compression and release stages, while micro-spallation is corres... read less

Abstract: Nanocrystalline Co consisting of fcc and hcp phases was processed by electrodeposition, and its mechanical properties were investigated by hardness tests. In addition, high-resolution transmission electron microscopy observations and molecular dynamics (MD) simulations were performed to investigate the grain boundary structure and dislocation nucleation from the grain boundaries. A large amount of disorders existed at the grain boundaries and stacking faults were formed from the grain boundaries in the as-deposited Co specimen. The as-deposited specimen showed a lower hardness than did the annealed specimen, although the grain size of the former was smaller than that of the latter. The activation volume of the as-deposited specimen (=1.5b3) was lower than that of the annealed specimen (=50b3), thus indicating that nucleation of dislocations from grain boundaries is more active in the as-deposited specimen than in the annealed specimens. The MD simulations showed that dislocation nucleation was closely related to a change in the defect structures at the boundary. Therefore, it is suggested that a significant amount of defects enhance changes in the defect structures at the boundary, resulting in softening of the as-deposited specimen. read less

Abstract: 平成25年4月1日より,従来の大阪大学大学院工学研究科附属原 子分子イオン制御理工学センターが改組となり,新たなアトミッ クデザイン研究センターとなりました. 当センターでは, i) 原子・分子構造からの材料・構造・機能設計を意図した 研究に重点 ii) シミュレーションベースト・エンジニアリングの積極的な 推進 iii) 産業応用に直結させたプロトタイプリサーチに重点 をセンター趣旨の3本柱と考えております. 従来のセンターで構築してきましたアトミックテクノロジーを さらに発展させ,「原子・分子からのものづくり」をモットーと して,デザインの観点を加味した先端ものづくりのセンターとし て邁進していきたいと思っております. 当センターでの成果や関連事項を迅速に皆様にお知らせするた めに,このニュースレター(CAMT Newsletter)を配信させていた だくことにいたしました. センターのホームページも立ち上げましたので,お時間のある 時にご覧いただければ幸いです. 今後とも,関係各位の皆様方からのご指導,ご鞭撻のほど,よ ろしくお願い申し上げます. 目次 read less

Abstract: We present a molecular dynamics (MD) study of the micro-spallation of lead (Pb), which corresponds to damage and liquid fragment ejection following the reflection of a strong shock wave on the free surface of the target. First, the Hugoniot and melting curves of Pb are derived by equilibrium MD simulations, and the potential function is validated by comparing these curves with experimental results. Then nonequilibrium MD simulations are conducted to study the dynamical processes of micro-spallation. Damage and ejection processes are analyzed by a binning analysis and direct observations of atom configurations. Comparisons with classical spallation simulations or experiments are made where necessary. It is found that damages in classical spallation and micro-spallation are both dominated by cavitation, i.e. nucleation and the growth and coalescence of voids. The main difference in the cavitation process of classical and micro-spallation lies in the amount and spatial distribution of void nucleation sites. Different properties in dynamical stress evolutions between micro-spallation and classical spallation are also discussed. In addition, the properties of the surface micro-spall are found to be different from those of interior micro-spall particles in some shock intensity regimes. Factors that cause such differences are studied by analyzing in detail the thermodynamics paths of different parts of the shocked target. read less

Abstract: The mechanisms of spalling and melting in nanocrystalline Pb under shock loading are studied by molecular dynamics simulations. A wide range of shock intensity is conducted with the lowest one just above the threshold of solid spallation, and the highest one higher than the threshold of compression melting. The spallation mechanism is dominated by cavitation, i.e., nucleation, growth, and coalescence of voids. Our results show that grain boundaries have significant influences on spalling behaviors in cases of classical spallation and releasing melting. In these cases, cavitation and melting both start on grain boundaries, and they display mutual promotion: melting makes the voids nucleate at smaller tensile stress, and void growth speeds melting. Influences of microstructure, strain rate, and temperature on spall strength are qualitatively discussed. Due to grain boundary effects, the spall strength of nanocrystalline Pb varies slowly with the shock intensity in cases of classical spallation. In cases of ... read less

Abstract: It is well known that screw dislocation motion dominates the plastic deformation in body-centered-cubic metals at low temperatures. The nature of the nonplanar structure of screw dislocations gives rise to high lattice friction, which results in strong temperature and strain rate dependence of plastic flow. Thus the nature of the Peierls potential, which is responsible for the high lattice resistance, is an important physical property of the material. However, current empirical potentials give a complicated picture of the Peierls potential. Here, we investigate the nature of the Peierls potential using density functional theory in the bcc transition metals. The results show that the shape of the Peierls potential is sinusoidal for every material investigated. Furthermore, we show that the magnitude of the potential scales strongly with the energy per unit length of the screw dislocation in the material. read less

Abstract: Molecular dynamics (MD) simulation was applied to the sintering behavior of silver nanoparticles on a gold substrate in order to elucidate the sintering mechanism of the nanoparticles on the substrate. The simulation revealed that silver atoms from 1 and 2nm nanoparticles migrated freely because of their larger surface energy and then epitaxially reoriented to the gold substrate so as to reduce grain boundary energy. The silver nanoparticles were more spread out on the (011) gold substrate than on the (001) substrate, indicating that substrates with larger surface energy induce greater spreading rates. Consideration of the competition of neck growth and epitaxial growth in sintering of nanoparticles revealed that reduction of surface energy is the predominant driving force in the initiation of sintering of silver nanoparticles, and that the reduction of grain boundary energy is subsequently consequential. [doi:10.2320/matertrans.MB201201] read less

Abstract: A mechanically formed electrical nanocontact between gold and tungsten is a prototypical junction between metals with dissimilar electronic structure. Through atomically characterized nanoindentation experiments and first-principles quantum transport calculations, we find that the ballistic conduction across this intermetallic interface is drastically reduced because of the fundamental mismatch between s wave-like modes of electron conduction in the gold and d wave-like modes in the tungsten. The mechanical formation of the junction introduces defects and disorder, which act as an additional source of conduction losses and increase junction resistance by up to an order of magnitude. These findings apply to nanoelectronics and semiconductor device design. The technique that we use is very broadly applicable to molecular electronics, nanoscale contact mechanics, and scanning tunneling microscopy. read less

Abstract: In situ neutron‐diffraction experiments at the spallation neutron source, simultaneously illuminating the diffraction of the matrix and the strengthening nano precipitates, allow the determination of their plastic deformation. An irreversible neutron‐diffraction‐profile evolution of the nano precipitates is observed. However, there is no conclusive trend of the nano‐precipitate peak‐width evolution subjected to the greater stress levels. Hence, in the present work, molecular‐dynamics simulations are applied to reveal the deformation mechanisms of the nano precipitate and its interaction with the surrounding matrix. The microstructure size, dislocation content, and structural parameters of the nano precipitates, quantified by X‐ray, transmission electron microscopy, and small‐angle neutron scattering, are used as the simulation input and reference. The simulation results show that there are two competing deformation mechanisms, which lead to the fluctuation of the nano‐precipitate‐diffraction widths, occurring during the higher plastic deformation stages. read less

Abstract: The melting curves and entropy of fusion of body-centered cubic (bcc) tungsten (W) under pressure are investigated via molecular dynamics (MD) simulations with extended Finnis-Sinclair (EFS) potential. The zero pressure melting point obtained is better than other theoretical results by MD simulations with the embedded-atom-method (EAM), Finnis-Sinclair (FS) and modified EAM potentials, and by ab initio MD simulations. Our radial distribution function and running coordination number analyses indicate that apart from the expected increase in disorder, the main change on going from solid to liquid is thus a slight decrease in coordination number. Our entropy of fusion of W during melting, Delta S, at zero pressure, 7.619 J/mol.K, is in good agreement with the experimental and other theoretical data. We found that, with the increasing pressure, the entropy of fusion Delta S decreases fast first and then oscillates with pressure; when the pressure is higher than 100 GPa, the entropy of fusion Delta S is about 6.575 +/- 0.086 J/mol.K, which shows less pressure effect. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4733947] read less

Abstract: This paper reports on the transition of crystalline to amorphous structure with increasing W content in electrodeposited binary Co-W alloys. The glass forming ability of the electrodeposited Co-based alloys, including Co-W alloy, is analyzed using deep eutectic criteria based on reduced liquidus temperature and relative composition ratio. MD simulations based on solid solution models indicated glass forming ability above 20 at. % W for Co-rich Co-W alloys, which is in good agreement with the experimental observations. read less

Abstract: Recently a new possibility of welding was experimentally shown in the case of gold nanowires (NWs) at ambient temperatures, without need of additional heat and with low pressures, called cold weldi... read less

Abstract: In this study, the epitaxial growth of gallium nitride (GaN) on a GaN substrate is investigated by a molecular dynamics (MD) method. Furthermore, the difference between the surface diffusion of atoms of a strained substrate and an unstrained substrate is examined. From the results of this examination, it is found that the diffusion characteristic in the unstrained case is higher than that in the strained case. Therefore, in the unstrained case, GaN grows layer-by-layer. On the other hand, in the strained case, multiple layers of GaN grow simultaneously. Furthermore, it is also found that the wurtzite structure of GaN differs between the strained case and the unstrained case. read less

Abstract: To elucidate the mechanism responsible for structural instability at the atomic level, atomistic modeling simulation of tension in a Cu thin film containing a notch was performed using an embedded-atom method potential and dislocation nucleation was observed. Mechanical stability during tension was analyzed by solving the eigenvalue problem of the Hessian matrix taking into account all the degrees of freedom of the atoms in the system. Since an eigenvalue designates the curvature of the potential energy landscape in the direction of the corresponding eigenvector, which indicates a deformation mode, the system is unstable under vanishing temperature at the critical strain (cid:1) (cid:1) c (cid:2) when any eigenvalue is zero or negative. At a strain smaller than (cid:1) c where all the eigenvalues are positive, atomic fluctuations due to finite temperature may cause structural instability. We found that the path of activated instability (cid:1) dislocation emission from the notch (cid:2) could be written with a linear combination of the eigenvectors having small eigenvalues obtained under a corresponding external strain at zero temperature. The energy landscape has a much lower hill along the mixed-mode path than along any single-mode paths. In a molecular dynamics simulation under finite temperature, components of deformation modes having small eigenvalues fluctuate at low frequency, which dominate the activation of instability. DOI: 10. read less

Abstract: The structural and self-assembling characteristic of Pb heterostructures on the Cu (111) substrate in the early stage of the deposition process were investigated using a molecular dynamics simulation and density functional theory. The Pb islands formed on the Cu (111) surface were observed to diffuse actively in lateral directions following the layer-by-layer growth mode. A heptameric hexagonal island was found to be most stable under highly nonequilibrium conditions. This result can be explained by the tendency of Pb heterostructures, which have minimum surface energy, to have the maximum number of Pb–Pb bondings. In addition, the atomic binding energy, the surface diffusion coefficient prefactor, and the surface diffusion energy barrier for Pb adatoms were quantitatively calculated according to various shapes of Pb islands to determine the stability of the corresponding island. read less

Abstract: Using molecular dynamics simulation, the effect of Pb surfactant for the thin film growth of Co atoms on Cu(111) substrate was investigated. Specifically, the behavior of Co atoms being deposited on Cu(111) substrate with predeposited Pb layer was extensively investigated and compared with the case of without Pb layer to explain the effect of Pb surfactant. It was observed that Pb layer was floating during the Co deposition. It was, quantitatively, found that Pb surfactant played an important role in suppression of active diffusion of Co atoms, which was accomplished by the increase in the surface diffusion barrier energy. The energy change in the deposited Co adatom on the Cu(111) substrate with predeposited Pb layer showed that the approaching Co adatom penetrated into the Pb layer; then, the Co adatom settled down on the Cu(111) substrate. Consequently, Pb atoms around Co adatom suppressed the further diffusion of Co adatom. read less

Abstract: Deposition and annealing behaviors of Al atoms on rough Cu (111) surface were investigated on the atomic scale by three-dimensional classical molecular dynamics simulation. The rough Cu surface was modeled by depositing 5 ML of Cu on Ta (011) substrate. Most Al atoms deposited on the rough Cu surface placed on the atomic steps, preserving the major features of the surface during Al deposition. This behavior was discussed in terms of the smaller barrier of the surface diffusion than Ehrlich–Schwoebel barrier of Al on Cu (111) surface. By annealing at 700 K, significant intermixing between Al and Cu rapidly occurs with decrease in the surface roughness. This behavior reveals that the exchange process of Al with substrate Cu dominates during the initial stage of high temperature annealing. read less

Abstract: Using molecular dynamics simulation, the growth behavior of Co atoms on Cu substrates for two different crystallographic orientations, (001) and (111), was extensively investigated. The surface roughness became noticeably higher during 20-monolayer (ML) deposition of Co thin-film on the Cu(111) substrate compared to the case of the Cu(001) substrate. It was found that the high diffusivity of Co adatoms on Cu(111) enhanced the atomic lateral movement, and the deposited Co adatoms could be agglomerated easily. The different growth behavior could be successfully explained in terms of the lateral atomic displacement and local acceleration energy. read less

Abstract: As a sequel to the previous paper on the calculation of the crystal–melt interface free energy (2021 Materialia 15 100962), here we report the results on the kinetic coefficients using molecular dynamics simulations performed on six fcc metals and four bcc metals with the intention to compare the crystal structural influence. We found that the calculated kinetic coefficients are well described by the model by Broughton, Gilmer and Jackson (1982 Phys. Rev. Lett. 49 1496), and in particular, they exhibit varying degrees of anisotropy. We reveal that the anisotropies are related to the fluctuation of the crystal–melt interfaces, which causes the increase of the actual interface area in melting or solidification. The kinetic coefficients always display asymmetry between the solidification and melting process, and the difference is much more pronounced for the (111) interfaces in fcc metals which have the highest anisotropy. We found that the atomic mechanisms of the kinetic behaviors of these interfaces are closely related to the formation of twin-crystal domains during solidification, which delays the solidification process and consequently causes a decrease in the calculated kinetic coefficients. read less

Abstract: The objective of this study is to create an ultra-thin palladium foil with a molecular dynamic (MD) simulation technique on a copper substrate surface. The layer formed onto the surface consists of a singular 3D palladium (Pd) nanoparticle structure which, by the cold gas dynamic spray (CGDS) technique, is especially incorporated into the low-cost copper substrate. Pd and Cu have been chosen for their possible hydrogen separation technology applications. The nanoparticles were deposited to the substrate surface with an initial velocity ranging from 500 to 1500 m/s. The particle radius was 1 to 4 nm and an angle of impact of 90° at room temperature of 300 K, in order to evaluate changes in the conduct of deformation caused by effects of size. The deformation mechanisms study revealed that the particle and substrate interface is subject to the interfacial jet formation and adiabatic softening resulting in a uniform layering. However, shear instabilities at high impact speeds were confirmed by the evolution of von Mises shear strain, temperature evolution and plastic strain. The results of this study can be used to further our existing knowledge in the complex spraying processes of cold gas dynamic spray technology. read less

Abstract: Herein we report on a copper (Cu) sinter die-bonding paste that sinters under pressureless bonding conditions for power devices operating at high temperatures. The shear strengths of pressureless-sintered Cu on four different metallization layers (Ni, Cu, Ag, and Au) were studied by experiment. The sintering behavior of Cu nanoparticles and diffusion coefficients at interfaces between a bulk Cu layer and the metallization layers was also evaluated by molecular dynamics (MD) simulation. After aging (573 K, 8 h), the shear strength of the Cu sintered to the Ni and Cu layers increased, whereas that of Cu sintered to the Ag and Au layers decreased. The interdiffusion at the interfaces between the sintered Cu layer and the Ag or Au layer increased the interfacial porosity of sintered Cu, which decreased the shear strengths in the sintered Cu/Ag and Cu/Au systems. In contrast, the interfacial porosity between the sintered Cu and the Ni or Cu layer hardly changed after aging. MD simulations revealed that Kirkendall voids were promoted by higher interdiffusion coefficients and a higher ratio of intrinsic diffusion coefficients between a bulk Cu layer and an Ag or Au layer. This consequently increased the porosity of Cu sintered near the interfaces. read less

Abstract: This thesis summarizes our efforts to study the structure and properties of materials computationally. The adaptive genetic algorithm (AGA) developed by us to predict crystal/surface/interface structures is presented. Applications of AGA to a variety of systems, such as non-rare earth magnetic materials, ultra-hard transition metal borides and SrTiO3 grain boundaries, are discussed. We demonstrated by AGA the capability of solving crystal structures with more than 100 atoms per unit cell and rapidly accessing the structures and phase stabilities of different compositions in multicomponent systems. We also introduced a motif-network scheme to study the complex crystal structures in silicate cathodes. In addition, we explored different computational methods for atomistic simulations of materials behavior, such as Monte Carlo modeling of the alnico magnets. read less

Abstract: The interfacial bonding utilizing Ag2O-derived silver nanoparticles was evaluated using TEM observation and molecular dynamics simulation. The TEM observation reveals that the crystal orientation of the sintered silver corresponded to that of the gold substrate. This is considered that the epitaxial layer of silver was formed through in-situ formation of silver nanoparticles from Ag2O paste, and oriented in the direction of the gold crystal. MD simulation successfully recreated the sintering behavior of silver nanoparticles and the gold substrate. The simulation results clearly showed the epitaxial layers of silver atoms were formed on the substrate. The existence of the closed pore indicates the acceleration of the sintering between nanoparticles and the gold substrate to minimize the total sum of surface energy and grain boundary energy. read less

Abstract: A selection of nanoscale processes is studied theoretically, with the aim of identifying themechanisms that could lead to selective carbon nanotube (CNT) growth. Only mechanisms relevant to catalytic chemical vapour deposition (CVD) are considered. The selected processes are analysed with classical molecular dynamics (MD) simulations and continuum modelling. The melting and pre-melting behaviour of supported nickel catalyst particles is investigated. Favourable epitaxy between a nanoparticle and the substrate is shown to significantly raise themelting point of the particle. It is also demonstrated that substrate binding can induce solid-solid transformations, whilst the epitaxy may even determine the orientation of individual crystal planes in supported catalysts. These findings suggest that the substrate crystal structure alone can potentially be used to manipulate the properties of catalyst particles and, hence, influence the structure of CNTs. The first attempt at modelling catalyst dewetting, a process where the catalyst unbinds from the inner walls of a nucleating nanotube, is presented. It is argued that understanding this process and gaining control over itmay lead to better selectivity in CNT growth. Two mutually exclusive dewetting mechanisms, namely cap lift-off and capillary withdrawal, are identified and then modelled as elastocapillary phenomena. The modelling yields an upper bound on the diameter of CNTs that can stem from a catalyst particle of a given size. It is also demonstrated that cap lift-off is sensitive to cap topology, suggesting that it may be possible to link catalyst characteristics to the structural properties of nucleating CNTs. However, a clear link to the chiral vector remains elusive. It is shown that particle size, as well as binding affinity, plays a critical role in capillary absorption and withdrawal of catalyst nanoparticles. This size dependence is explored in detail, revealing interesting ramifications to the statics and dynamics of capillary-driven flows at the nanoscale. The findings bear significant implications for our understanding of CNT growth from catalyst particles, whilst also suggesting new nanofluidic applications and methods for fabricating composite metal-CNT materials. read less

Abstract: This paper presents the results of molecular dynamic modeling, revealing that inserting confined graphene layers into copper crystal reduces the thermal conductivity of the whole composite, and the coefficient of thermal conductivity κ decreases upon an increase in the number of graphene layers. The injection of one, two, and three layers of 15 nm graphene leads to a change in the coefficient of thermal conductivity from 380 W/(m·K) down to 205.9, 179.1, and 163.6 W/(m·K), respectively. Decreasing the length of graphene layers leads to a decrease in the density of defects on which heat is dissipated. With one, two, and three layers of 8 nm graphene, the coefficient of thermal conductivity of the composite is equal to 272.6, 246.8, and 240.8 W/(m·K), appropriately. Meanwhile the introduction of an infinite graphene layer results in the growth of κ to 414.2–803.3 W/(m·K). read less

Abstract: Shock-induced plastic deformation and spall damage in the single-crystalline FCC Co25Ni25Fe25Al7.5Cu17.5 high-entropy alloy (HEA) under varying shock intensities were systematically investigated using large-scale molecular dynamics simulations. The study reveals the significant influence of crystalline orientation on the deformation mechanism and spall damage. Specifically, the shock wave velocities in the [110] and [111] directions are significantly higher than that in the [001] direction, resulting in a two-zone elastic-plastic shock wave structure observed in the [110] and [111] samples, while only a single-wave structure is found in the [001] sample. The plastic deformation is dominated by the FCC to BCC transformation following the Bain path and a small amount of stacking faults during the compression stage in the [001] sample, whereas it depends on the stacking faults induced by Shockley dislocation motion in the [110] and [111] samples. The stacking faults and phase transformation in the [001] sample exhibit high reversibility under release effects, while extensive dislocations are present in the [110] and [111] samples after release. Interestingly, tension-strain-induced FCC to BCC phase transformation is observed in the [001] sample during the release stage, resulting in increased spall strength compared to the [110] and [111] samples. The spall strength estimated from both bulk and free surface velocity history shows reasonable consistency. Additionally, the spall strength remains stable with increasing shock intensities. The study discusses in detail the shock wave propagation, microstructure change, and spall damage evolution. Overall, our comprehensive studies provide deep insights into the deformation and fracture mechanisms of Co25Ni25Fe25Al7.5Cu17.5 HEA under shock loading, contributing to a better understanding of dynamic deformation under extreme environments. read less

Abstract: Materials processing often occurs under extreme dynamic conditions leading to a multitude of unique structural environments. These structural environments generally occur at high temperatures and/or high pressures, often under non-equilibrium conditions, which results in drastic changes in the material's structure over time. Computational techniques, such as molecular dynamics simulations, can probe the atomic regime under these extreme conditions. However, characterizing the resulting diverse atomistic structures as a material undergoes extreme changes in its structure has proved challenging due to the inherently non-linear relationship between structures as large-scale changes occur. Here, we introduce SODAS++, a universal graph neural network framework, that can accurately and intuitively quantify the atomistic structural evolution corresponding to the transition between any two arbitrary phases. We showcase SODAS++ for both solid–solid and solid–liquid transitions for systems of increasing geometric and chemical complexity, such as colloidal systems, elemental Al, rutile and amorphous TiO2, and the non-stoichiometric ternary alloy Ag26Au5Cu19. We show that SODAS++ can accurately quantify all transitions in a physically interpretable manner, showcasing the power of unsupervised graph neural network encodings for capturing the complex and non-linear pathway, a material's structure takes as it evolves. read less

Abstract: In the case where an interstitial atom is located in a close-packed atomic row of the crystal lattice, it is called a crowdion. Crowdions play an important role in the processes of mass and energy transfer resulting from irradiation, severe plastic deformation, ion implantation, plasma and laser processing, etc. In this work, supersonic N-crowdions (N=1, 2) in fcc lattices of lead and nickel are studied by the method of molecular dynamics. Modeling shows that the propagation distance of a supersonic 2-crowdion in lead at a high initial velocity is less than that of a supersonic 1-crowdion. In other fcc metals studied, including nickel, supersonic 2-crowdions have a longer propagation distance than 1-crowdions. The relatively short propagation distance of supersonic 2-crowdions in lead is due to their instability and rapid transformation into supersonic 1-crowdions. This feature of the dynamics of supersonic N-crowdions in lead explains its high radiation-shielding properties. read less

Abstract: Investigating the relationship between the glass-forming ability (GFA), mechanical properties, and structure of metallic glasses is crucial to understanding the nature of the metallic glass state. In this study, the correlation among the atomic structure, electronic valence band, and properties have been studied using Zr50Cu44.5−xAl5.5Agx (x = 0, 1.5, 3 at.%) bulk metallic glasses (BMGs). The results reveal that through the micro-addition of Ag, the GFA of Zr50Cu44.5Al5.5 BMG can be enhanced; meanwhile, the critical diameter of Zr50Cu44.5Al5.5 glass rods increases from approximately 2.5 mm to 5.0 mm with the addition of 3% Ag. Through the addition of Ag, the thermal stability of Zr50Cu44.5Al5.5 BMG is improved, and the proportion of icosahedral-like clusters increases. The plasticity of the Zr50Cu44.5−xAl5.5Agx (x = 0, 1.5, 3 at.%) BMGs decreased from 4.6% to 0.8% with the addition of Ag. The valence band spectrum of the Zr50Cu44.5−xAl5.5Agx (x = 0, 1.5, 3 at.%) BMGs indicates that with the addition of Ag, the p-d hybridization near the Fermi level is enhanced, and the binding energy will move to a lower value. read less

Abstract: Metallic multilayered nanowires have a wide application prospect in micro-nano devices because of their superior physical and chemical properties and microstructure designability. Size effects on the tensile behaviors of Ti/Cu multilayered nanowires are investigated by molecular dynamic simulations. Aspect ratios of 1:4, 1:3, 1:2, 1:1, 1:0.75, and 1:0.67 and sectional dimensions of 3, 4, 5, 6, and 7 nm are adopted to construct nanowires with different sizes. Simulation results indicate that the strength of Ti/Cu nanowires decreases with the decrease of aspect ratio in the large aspect ratio range (>1:2) and all simulated sectional dimension ranges, showing a reverse Hall-Petch effect. The Hall-Petch law can only be satisfied in a small aspect ratio range (<1:2). Deformation mechanism transition is found in the critical aspect ratio of 1:2. When the aspect ratio is larger than 1:2, crystalline phases of Ti and Cu layers dominate the plastic deformation of Ti/Cu nanowires. Crystal phases and interface both bear plastic deformation when the aspect ratio is smaller than 1:2. Interface is an important factor in the strength and deformation of Ti/Cu nanowires. The variation of interface fraction and interaction between interface and dislocation motion determine the tendency of strength variation for Ti/Cu nanowires. read less

Abstract: Thermal stability is an important feature of the materials used as components and parts of sensors and other devices of nanoelectronics. Here we report the results of the computational study of the thermal stability of the triple layered Au@Pt@Au core-shell nanoparticles, which are promising materials for H2O2 bi-directional sensing. A distinct feature of the considered sample is the raspberry-like shape, due to the presence of Au nanoprotuberances on its surface. The thermal stability and melting of the samples were studied within classical molecular dynamics simulations. Interatomic forces were computed within the embedded atom method. To investigate the thermal properties of Au@Pt@Au nanoparticles, structural parameters such as Lindemann indexes, radial distribution functions, linear distributions of concentration, and atomistic configurations were calculated. As the performed simulations showed, the raspberry-like structure of the nanoparticle was preserved up to approximately 600 K, while the general core-shell structure was maintained up to approximately 900 K. At higher temperatures, the destruction of the initial fcc crystal structure and core-shell composition was observed for both considered samples. As Au@Pt@Au nanoparticles demonstrated high sensing performance due to their unique structure, the obtained results may be useful for the further design and fabrication of the nanoelectronic devices that are required to work within a certain range of temperatures. read less

Abstract: PurposeThe changes in tensile behavior of polycrystalline nanocopper lattice with changes in temperature, average grain size (AGS) and strain rate, have been explored. The existence of a critical AGS has also been observed which shows that the Hall–Petch relationship behaves inversely.Design/methodology/approachNanoscale deformation of polycrystalline nanocopper has been done in this study with the help of an embedded atom method (EAM) potential. Voronoi construction method has been employed for creating four polycrystals of nanocopper with different sizes. Statistical analysis has been used to examine the observations with emphasis on the polycrystal size effect on melting point temperature.FindingsThe study has found that the key stress values (i.e. elastic modulus, yield stress and ultimate tensile stress) are significantly influenced by the considered parameters. The increase in strain rate is observed to have an increasing impact on mechanical properties, whereas the increase in temperature degrades the mechanical properties. In-depth analysis of the deformation mechanism has been studied to deliver real-time visualization of grain boundary motion.Originality/valueThis study provides the relationship between required grain size variations for consecutive possible variations in mechanical properties and may help to reduce the trial processes in the synthesis of polycrystalline copper based on different temperatures and strain rates. read less

Abstract: ABSTRACT Molecular dynamics simulations are performed to explore nanoindentation characteristics of tungsten, and the influences of grain size, indenter velocity, indenter size, and temperature are discussed. The results illustrate that the hardness reduces as the grain size (5.00 ∼ 24.62 nm) decreases. There is no phase change observed during the whole deformation process. For monocrystalline W, the dislocation nucleation and propagation dominate the deformation mechanisms. Differently, the primary deformation mode of nanocrystalline W is the grain split and motion of GBs. Dislocations primarily nucleate below the contact surface of the indenter and substrate and then glide in the grain core. The monocrystalline W has better pattern-forming ability than nanocrystalline. Besides, the pattern-forming ability of nanocrystalline W is negatively correlated with the average grain size (5.00 ∼ 24.62 nm). The von Mises stress is mainly concentrated in the interface between the indenter and substrate, the dislocation area for monocrystalline, and grain boundaries for nanocrystalline. The indentation force and hardness are positively correlated with indenter radius size (30 ∼ 80 Å), negatively correlated with temperature (10 ∼ 1500 K), and insensitive to the indenter velocity when velocity is lower than 3.0 Å /ps (300 m/s). read less

Abstract: Atomistic simulations are employed to investigate chemical short-range ordering in two body-centered cubic refractory multi-principal element alloys, HfMoNbTaTi and HfNbTaTiZr, and its influence on their ideal simple shear strengths. Both the alias and affine shear strengths are analyzed on the {110} and {112} planes in the two opposing [Formula: see text] directions. In both quinary alloys, local ordering of NbNb, TaTa, HfNb, HfTa, and NbTa is preferred as the annealing temperature decreases from 900 to 300 K. The pair that achieves the highest degree of local ordering is TiTi in HfMoNbTaTi and HfTi in HfNbTaTiZr. Subject to the affine shear, these alloys yield by first phase transformation at the most likely pairs followed by deformation twinning at those sites. read less

Abstract: Multicomponent alloys possessing nanocrystalline structure, often alluded to as Cantor alloys or high entropy alloys (HEAs), continue to attract the great attention of the research community. It has been suggested that about 64 elements in the periodic table can be mixed in various compositions to synthesize as many as ∼108 different types of HEA alloys. Nanomechanics of HEAs combining experimental and atomic simulations are rather scarce in the literature, which was a major motivation behind this work. In this spirit, a novel high-entropy alloy (Ni25Cu18.75Fe25Co25Al6.25) was synthesized using the arc melting method, which followed a joint simulation and experimental effort to investigate dislocation-mediated plastic mechanisms leading to side flow, pileup, and crystal defects formed in the sub-surface of the HEA during and after the scratch process. The major types of crystal defects associated with the plastic deformation of the crystalline face-centered cubic structure of HEA were 2,3,4-hcp layered such as defect coordination structures, coherent ∑3 twin boundary, and ∑11 fault or tilt boundary, in combination with Stair rods, Hirth locks, Frank partials, and Lomer–Cottrell locks. Moreover, 1/6 <112> Shockley, with exceptionally larger dislocation loops, was seen to be the transporter of stacking faults deeper into the substrate than the location of the applied cutting load. The (100) orientation showed the highest value for the kinetic coefficient of friction but the least amount of cutting stress and cutting temperature during HEA deformation, suggesting that this orientation is better than the other orientations for improved contact-mode manufacturing. read less

Abstract: We use an atomistic spin model to simulate FePt-based bilayers for heat assisted magnetic recording (HAMR) devices and investigate the effect of various degrees intermixing that might arise throughout the fabrication, growth and annealing processes, as well as different interlayer exchange couplings, on HAMR magnetisation dynamics. Intermixing can impact the device functionality, but interestingly does not deteriorate the properties of the system. Our results suggest that modest intermixing can prove beneficial and yield an improvement in the magnetisation dynamics for HAMR processes, also relaxing the requirement for weak exchange coupling between the layers. Therefore, we propose that a certain intermixing across the interface could be engineered in the fabrication process to improve HAMR technology further. read less

Abstract: This paper proposed a hybrid intelligent process model, based on a hybrid model combining the two-temperature model (TTM) and molecular dynamics simulation (MDS) (TTM-MDS). Combined atomistic-continuum modeling of short-pulse laser melting and disintegration of metal films [Physical Review B, 68, (064114):1–22.], and Gaussian process regression (GPR), for micro-electrical discharge machining (micro-EDM) were also used. A model of single-spark micro-EDM process has been constructed based on TTM-MDS model to predict the removed depth (RD) and material removal rate (MRR). Then, a GPR model was proposed to establish the relationship between input process parameters (energy area density and pulse-on duration) and the process responses (RD and MRR) for micro-EDM machining. The GPR model was trained, tested, and tuned using the data generated from the numerical simulations. Through the GPR model, it was found that micro-EDM process responses can be accurately predicted for the chosen process conditions. Therefore, the hybrid intelligent model proposed in this paper can be used for a micro-EDM process to predict the performance. read less

Abstract: Pristine specimens yield plastically under high loads by nucleating dislocations. Since dislocation nucleation is a thermally activated process, the so-called nucleation-controlled plasticity is probabilistic rather than deterministic, and the distribution of the yield strengths depends on the activation parameters to nucleate. In this work, we develop a model to predict the strength distribution in nucleation-controlled plasticity when there are multiple nucleation site types. We then apply the model to molecular dynamics (MD) simulations of Pd nanowires under tension. We found that in Pd nanowires with a rhombic cross-section, nucleation starts from the edges, either with the acute or the obtuse cross-section angles, with a probability that is temperature-dependent. We show that the distribution of the nucleation strain is approximately normal for tensile loading at a constant strain rate. We apply the proposed model and extract the activation parameters for site types from both site types. With additional nudged elastic bands simulations, we propose that the activation entropy, in this case, has a negligible contribution. Additionally, the free-energy barriers obey a power-law with strain, with different exponents, which corresponds to the non-linear elastic deformation of the nanowires. This multiple site type nucleation model is not subjected only to two site types and can be extended to a more complex scenario like specimen with rough surfaces which has a distribution of nucleation sites with different conditions to nucleate dislocations. read less

Abstract: An extended molecular dynamics simulation that incorporates classical free electron dynamics in the framework of the force-field model has been developed to enable us to describe the optical response of metal materials under the visible light electric field. In the simulation, dynamical atomic point charges follow equations of motion of classical free electrons that include Coulomb interactions with the oscillating field and surrounding atomic sites and collision effects from nearby electrons and ions. This scheme allows us to simulate an interacting system of metals with molecules using an ordinary polarizable force-field and preserves energy conservation in the case without applying an external electric field. As the first applications, we show that the presented simulation accurately reproduces (i) the classical image potential in a metal-charge interaction system and (ii) the dielectric function of bulk metal. We also demonstrate (iii) calculations of absorption spectra of metal nano-particles with and without a water solvent at room temperature, showing reasonable red-shift by the solvent effect, and (iv) plasmon resonant excitation of the metal nano-particle in solution under the visible light pulse and succeeding energy relaxation of the absorbed light energy from electrons to atoms on the metal and to the water solvent. Our attempt thus opens the possibility to expand the force-field based molecular dynamics simulation to an alternative tool for optical-related fields. read less

Abstract: The process of ultra-thin single crystal gold film irradiated by femtosecond laser is simulated by the combination of improved two-temperature model (TTM) and molecular dynamics (MD). The results indicate that under the non-Fourier effect, the gold film shows special melting characteristics and spalling mechanism. The phenomena of homogeneous melting and pore spalling of gold film are successfully distinguished by curing rate method and atomic density method respectively. Due to the tiny thickness of the gold film studied, the damage mechanism of the gold film irradiated by laser has also changed. read less

Abstract: Molecular dynamics (MD) simulation of Al ions was performed as a model of Al ion battery using 1-Ethyl-3-methylimidazolium chloride ([EMIm]Cl) as an electrolyte. It is one of the safe secondary battery candidates that can replace Li-ion battery. The structure and charge of the EMIm ions used in the MD simulation were calculated in advance by Gaussian09 based on the density functional theory (DFT). The pair distribution function, the mean square displacement, and frequency dependent diffusion coefficient are obtained to examine structure, diffusion coefficient, and vibrational spectra. The significant large vibrational peak of Al has been obtained, which may be attributed to the cage effect of Al ion in the [EMIm]Cl electrolyte. read less

Abstract: This study is an experimental research that aimed to determine the effectiveness of the Flipped Classroom Instructional Strategies in teaching Mathematics 5. The control group, comprising of 34 pupils, and the experimental group composing of 35 pupils was tasked to provide their age, and sex. The conventional teaching method taught to the control group, while the Flipped Classroom Approach taught for a period of two (2) weeks to the experimental group. Surveyed students are in appropriate age for their current grade level. The study is dominated by male respondents. Respondents from control and experimental group have low to average performance during the pre- test. Performances of control group and experimental group have increased. Though, experimental group has higher post-test score rating than control group. Pre-test scores of both groups do not have significant difference. Also, it depicts that the ability of respondents on both control and experimental is almost at the same level. An intervention should be used to improve their performance. Post-test scores of both groups have significant difference. Although there is an increase on the performance of the control group, experimental group has performed even better. The use of flipped classroom instructional strategies in teaching Mathematics is deemed effective. read less

Abstract: The ability of mechanosensing is essential for intelligent systems. Here we show by molecular dynamics (MD) simulations that a graphene flake on a bent beam exhibits amazing mechanosensing behavior, termed flexotaxis. The graphene flake can perceive the beam bending gradient which indeed leads to a gradient of atomic density that produces a driving force on the flake toward the direction of increasing density. An analytical model is developed to further confirm the mechanism, and the simulation results can be well reproduced by the model. Our findings may have general implications not only for the potential applications of graphene as sensing elements in nanoscale intelligent devices but also for the exploration of mechanosensing capability of other two-dimensional materials. read less

Abstract: Accurate methods and an efficient workflow for computing and documenting dislocation core energies are developed and applied to $\frac{1}{2}\ensuremath{\langle}111\ensuremath{\rangle}$ and $\ensuremath{\langle}100\ensuremath{\rangle}$ dislocations in five body-centered cubic (bcc) metals W, Ta, V, Mo, and $\ensuremath{\alpha}$-Fe represented by 13 model interatomic potentials. For each dislocation type, dislocation core energies are extracted for a large number of dislocation characters thoroughly sampling the entire 2-space of crystallographic line orientations of the bcc lattice. Of particular interest, core energies of the $\frac{1}{2}\ensuremath{\langle}111\ensuremath{\rangle}{110}$ dislocations are found to be distinctly asymmetric with respect to the sign of the character angle, whereas core energies of $\ensuremath{\langle}100\ensuremath{\rangle}{110}$ junction dislocations exhibit marked cusps for line orientations vicinal to the closed-packed $\ensuremath{\langle}111\ensuremath{\rangle}$ directions. Our findings furnish substantial insights for developing accurate models of dislocation core energies employed in mesoscale dislocation dynamics simulations of crystal plasticity. read less

Abstract: The structural evolution of laser-excited systems of gold has previously been measured through ultrafast MeV electron diffraction. However, there has been a long-standing inability of atomistic simulations to provide a consistent picture of the melting process, leading to large discrepancies between the predicted threshold energy density for complete melting, as well as the transition between heterogeneous and homogeneous melting. We make use of two-temperature classical molecular dynamics simulations utilizing three highly successful interatomic potentials and reproduce electron diffraction data presented by Mo et al. [Science 360, 1451–1455 (2018)]. We recreate the experimental electron diffraction data, employing both a constant and temperature-dependent electron–ion equilibration rate. In all cases, we are able to match time-resolved electron diffraction data, and find consistency between atomistic simulations and experiments, only by allowing laser energy to be transported away from the interaction region. This additional energy-loss pathway, which scales strongly with laser fluence, we attribute to hot electrons leaving the target on a timescale commensurate with melting. read less

Abstract: In this work, we report a convenient and efficient approach to improve heat conduction across the metal/graphite interface. It is demonstrated that the interfacial thermal conductance between Al and graphite can be enhanced by a factor of ∼5 after milling the graphite with a focused ion beam. Such enhancement is attributed to the decreased Fermi level of the milled graphite compared with the pristine counterpart. Once graphite is milled with the focused ion beam, surface defects are formed that induce the redistribution of electrons at the interface between Al and graphite. The formation of enormous dipoles on the milled graphite/Al interface leads to the conversion of the interfacial interaction from physisorption to chemisorption, which is beneficial for phonon transmission across the interface. Based on the measured Fermi level difference, the non-equilibrium Green's function method predicts that the interfacial interaction strength in the Al/milled graphite is increased 4-fold compared with Al/pristine graphite, which causes the increase of the interfacial thermal conductance. Our theoretical model also predicts that the interfacial thermal conductance does not increase monotonically with the interaction strength. Once the interaction strength exceeds a critical value, the interface thermal conductance will decrease. read less

Abstract: We report molecular dynamics (MD) simulation studies on the structure and tensile properties (temperature = 300 K and strain rate = 1010 s−1) of irradiated copper single crystals (101.8 Å× 50.54Å × 50.54Å) oriented along the [100] and [110] crystallographic orientations. Primary knock-on atom (PKA) method is used for radiation (0.5 keV to 5 keV). Structural studies indicate liquid like regions near the PKA region and are more in [110] orientation. The self-diffusivity (D) of copper in the PKA regions is found to increase from 4.05 × 10−5 m2/s at 1 keV irradiation energy to 27.36 × 10−5 at 5 keV irradiation energy in [110] orientation. However, no such relationship is observed in the [100] orientation. Vacancies, self-interstitials, and dislocations are more prominent in [100] orientation (dislocation density = 1 × 1016 m−2). Young’s modulus and yield strength decrease due to the formation of defects as anticipated. read less

Abstract: Directing motion of a nanoscale object on solid surfaces, in particular in an intrinsic way, is crucial for many aspects of nanotechnology applications. Here we report a novel intrinsic mechanism for nanoscale directional motion, termed angustotaxis, where a wide single walled carbon nanotube in a tapered channel drives itself toward the narrower end of the channel. The underlying physics of angustotaxis is attributed to the lower system potential when the nanotube is at a narrower region of the channel due to the increased contact area between the nanotube and the channel. Angustotaxis could lead to promising routes not only for nanoscale energy conversion from van der Waals potential to mechanical work, but also for mass transport like surface cleaning. read less

Abstract: Atomic scale defects critically limit performance of semiconductor materials. To improve materials, defect effects and defect formation mechanisms must be understood. In this paper, we demonstrate multiple examples where molecular dynamics simulations have effectively addressed these issues that were not well addressed in prior experiments. In the first case, we report our recent progress on modelling graphene growth, where we found that defects in graphene are created around periphery of islands throughout graphene growth, not just in regions where graphene islands impinge as believed previously. In the second case, we report our recent progress on modelling TlBr, where we discovered that under an electric field, edge dislocations in TlBr migrate in both slip and climb directions. The climb motion ejects extensive vacancies that can cause the rapid aging of the material seen in experiments. In the third case, we discovered that the growth of InGaN films on (0001) surfaces suffers from a serious polymorphism problem that creates enormous amounts of defects. Growth on (1120) surfaces, on the other hand, results in single crystalline wurtzite films without any of these defects. In the fourth case, we first used simulations to derive dislocation energies that do not possess any noticeable statistical errors, and then used these error-free methods to discover possible misuse of misfit dislocation theory in past thin film studies. Finally, we highlight the significance of molecular dynamics simulations in reducing defects in the design space of nanostructures. read less

Abstract: We present a continuum formulation to obtain the effects of surface residual stress and surface elastic constants on extensional and torsional stiffnesses of isotropic circular nanorods. Analytical expressions of axial force, twisting moment, and extensional and torsional stiffnesses are obtained. Unlike the case of rectangular nanorods, we show that the stiffnesses of circular nanorods also depend on surface residual stress components. This is attributed to non-zero surface curvature inherent in circular nanorods. We further normalize these expressions and analyze their asymptotic limits in the limit of the nanorod’s radius approaching both zero and infinity, corresponding to surface-dominated and bulk-dominated regimes, respectively. Finally, we use the recently proposed helical Cauchy–Born rule and perform molecular statics calculations to obtain axial force, twisting moment, and stiffnesses of the tungsten nanorod. The tungsten material is selected since its bulk crystal exhibits isotropy in the stress-free state. The results from molecular statics calculations are shown to match the derived continuum formulas accurately. read less

Abstract: Discrete breather (DB) is a time-periodic, spatially localized vibrational mode in a perfect nonlinear lattice. In this study, two new types of DBs are reported in fcc metals (Al, Cu and Ni), based on the molecular dynamics simulations using standard embedded atom method interatomic potentials. All calculations are performed at a zero temperature in a three-dimensional computational cell with the use of periodic boundary conditions. A plane DB is excited in a single (111) atomic plane by displacing the atoms from their equilibrium lattice sites according to a specific pattern corresponding to a delocalized vibrational mode in a two-dimensional triangular lattice. This plane DB is delocalized in two dimensions and localized in one dimension, normal to the excited (111) plane. It is shown that in all studied metals the plane DBs have maximal lifetimes of 17–22 ps in the range of initial amplitudes of 0.15–0.30 Å. Herewith the studied mode demonstrates a hard type of nonlinearity, i.e. its frequency increases with amplitude. The second new type of DB is the plane-radial DB obtained by imposing a radial localizing function on the plane DB. This disk-type DB is localized in all three dimensions. The time evolution of the atomic displacement amplitudes and the kinetic energy are studied. The DBs of this type can exist for 9 and 4 ps in Cu and Ni, respectively, and then they decay by dissipating their vibrational energy onto neighboring atoms. The long-lived plane-radial DB in Al is not found. read less

Abstract: In simulations of metallic interfaces, a critical aspect of metallic behavior is missing from the some of the most widely used classical molecular dynamics force fields. We present a modification of the embedded atom method (EAM) which allows for electronic polarization of the metal by treating the valence density around each atom as a fluctuating dynamical quantity. The densities are represented by a set of additional fluctuating variables (and their conjugate momenta) which are propagated along with the nuclear coordinates. This ``density readjusting EAM'' (DR-EAM) preserves nearly all of the useful qualities of traditional EAM, including bulk elastic properties and surface energies. However, it also allows valence electron density to migrate through the metal in response to external perturbations. We show that DR-EAM can successfully model polarization in response to external charges, capturing the image charge effect in atomistic simulations. DR-EAM also captures some of the behavior of metals in the presence of uniform electric fields, predicting surface charging and shielding internal to the metal. We further show that it predicts charge transfer between the constituent atoms in alloys, leading to novel predictions about unit cell geometries in layered $\mathrm{L}{1}_{0}$ structures. read less

Abstract: We report the classical molecular dynamics (MD) study of thermal stability of three two-dimensional (2D) titanium carbides Ti_2C, Ti_3C_2, and Ti_4C_3 (MXenes). Thermal properties of 2D nanomaterials are of fundamental importance and raise particular interest due to their potential applications in nanoelectronics. To investigate the behavior of Ti_ n +1C_n MXenes during heating, structural parameters such as Lindemann indexes, radial distribution functions, and atomistic configurations were calculated. The analysis of MD data allowed us to obtain approximate values of MXene degradation temperatures that are 1050, 1500, and 1700 K for Ti_2C, Ti_3C_2, and Ti_3C_4 MXenes, respectively. read less

Abstract: When solids are deformed plastically, they develop surface roughness because plastic flow is not laminar at small scales. Most natural and man-made surfaces appear to be rough on many length scales. There is presently no unifying theory of the origin of roughness or the self-affine nature of surface topography. One likely contributor to the formation of roughness is deformation, which underlies many processes that shape surfaces such as machining, fracture, and wear. Using molecular dynamics, we simulate the biaxial compression of single-crystal Au, the high-entropy alloy Ni36.67Co30Fe16.67Ti16.67, and amorphous Cu50Zr50 and show that even surfaces of homogeneous materials develop a self-affine structure. By characterizing subsurface deformation, we connect the self-affinity of the surface to the spatial correlation of deformation events occurring within the bulk and present scaling relations for the evolution of roughness with strain. These results open routes toward interpreting and engineering roughness profiles. read less

Abstract: Three independent elastic constants C11, C12, and C44 were calculated and compared using available potentials of eight different metals with FCC crystal structure; Gold, Silver, Copper, Nickel, Platinum, Palladium, Aluminum and Lead. In order to calculate the elastic constants, the second derivative of the energy density of each system was calculated with respect to different directions of strains. Each set of the elastic constants of the metals in bulk scale was compared with experimental results, and the average relative error was for each was calculated and compared with other available potentials. Then, using the Voigt-Reuss-Hill method, approximated values for Young and shear moduli and Poisson’s ratio of the FCC metals in the bulk scale were found for each potential. Furthermore, to observe the surface effects on the metals in nanoscale, surface elastic constants of the thin films of the metals have been calculated. In the study of the thin films of materials in nanoscale, the number of surface atoms is considerable compared to all atoms of the object. This leads to an increase in the surface effects, which influence the elastic properties. By considering this fact and employing related definitions and equations, the properties of the thin films of the metals were calculated, and the surface effects for different crystallographic directions were compared. Subsequently, in some cases, comparisons among characteristics of the metals in the thin film and bulk material were made. read less

Abstract: We compare the calculation of the contact area between the palladium and aluminum nanoparticles and graphene substrate by an ellipse and by the distance between atoms. Linear approximations of shear stress depending on the contact area of different metals were compared. It has been observed that aluminum nanoparticles are spherical and palladium nanoparticles have an ellipsoid shape, because the metal-metal interaction in palladium is stronger than that of aluminum. read less

Abstract: This work investigates how the interatomic surface potential influences molecular dynamics (MD)-derived thermal accommodation coefficients (TACs). Iron, copper, and silicon surfaces are considered over a range of temperatures that include their melting points. Several classes of potentials are reviewed, including two-body, three-body, and bond-order force fields. MD-derived densities and visualization of the surfaces are used to explain the differences in the parameterizations of these potentials within the context of gas–surface scattering. Finally, TACs are predicted for a range of gas–surface combinations, and recommended values of the TAC are selected that take into account the robustness and uncertainties of each of the considered parameterizations. Further, it is observed that there is a significant change in the TAC about phase changes that must be taken into account for applications with a large range of surface temperatures. read less

Abstract: In den letzten zwei Jahrzehnten übte die Atomsondentomographie (Atom Probe Tomography, APT) erheblichen Einfluss auf den Bereich der Materialwissenschaft und in jüngster Zeit auch darüber hinaus aus [1]. Es handelt sich um eine Kombination aus Mikroskop und Massenspektrometer, durch die 3D-Daten einer großen Bandbreite an Feststoffen in atomarem Maßstab bereitgestellt werden können. Die Bezeichnung „Atomsonde“ entstand ursprünglich in Anlehnung an die Erfindung der Elektronenmikrosonde zu dieser Zeit, da ihr Erfinder, Erwin Müller, die Tatsache betonen wollte, dass dieses Instrument faktisch Einzelatomempfindlichkeit erreichen konnte. Der Begriff beschreibt die Erfindung nicht vollumfänglich, da nicht wirklich eine Sonde zum Einsatz kommt, sondern vielmehr ein elektrisches Feld, das oft mit einer Wärmeeinwirkung durch einen Laser kombiniert wird, um Atome von einer scharfen Spitze zu extrahieren und sie in Richtung eines für einzelne Atome empReceived: May 25, 2018 Accepted: June 13, 2018 read less

Abstract: A Stillinger-Weber potential is computationally very efficient for molecular dynamics simulations. Despite its simple mathematical form, the Stillinger-Weber potential can be easily parameterized to ensure that crystal structures with tetrahedral bond angles (e.g., diamond-cubic, zinc-blende, and wurtzite) are stable and have the lowest energy. As a result, the Stillinger-Weber potential has been widely used to study a variety of semiconductor elements and alloys. When studying an A-B binary system, however, the Stillinger-Weber potential is associated with two major drawbacks. First, it significantly overestimates the elastic constants of elements A and B, limiting its use for systems involving both compounds and elements (e.g., an A/AB multilayer). Second, it prescribes equal energy for zinc-blende and wurtzite crystals, limiting its use for compounds with large stacking fault energies. Here, we utilize the polymorphic potential style recently implemented in LAMMPS to develop a modified Stillinger-Weber potential for InGaN that overcomes these two problems.A Stillinger-Weber potential is computationally very efficient for molecular dynamics simulations. Despite its simple mathematical form, the Stillinger-Weber potential can be easily parameterized to ensure that crystal structures with tetrahedral bond angles (e.g., diamond-cubic, zinc-blende, and wurtzite) are stable and have the lowest energy. As a result, the Stillinger-Weber potential has been widely used to study a variety of semiconductor elements and alloys. When studying an A-B binary system, however, the Stillinger-Weber potential is associated with two major drawbacks. First, it significantly overestimates the elastic constants of elements A and B, limiting its use for systems involving both compounds and elements (e.g., an A/AB multilayer). Second, it prescribes equal energy for zinc-blende and wurtzite crystals, limiting its use for compounds with large stacking fault energies. Here, we utilize the polymorphic potential style recently implemented in LAMMPS to develop a modified Stillinger-Weber... read less

Abstract: We report a computational discovery of novel grain boundary structures and multiple grain boundary phases in elemental body-centered cubic (bcc) metals represented by tungsten, tantalum and molybdenum. While grain boundary structures created by the γ-surface method as a union of two perfect half crystals have been studied extensively, it is known that the method has limitations and does not always predict the correct ground states. Herein, we use a newly developed computational tool, based on evolutionary algorithms, to perform a grand-canonical search of high-angle symmetric tilt and twist boundaries, and we find new ground states and multiple phases that cannot be described using the conventional structural unit model. We use molecular dynamics (MD) simulations to demonstrate that the new structures can coexist at finite temperature in a closed system, confirming that these are examples of different grain boundary phases. The new ground state is confirmed by first-principles calculations. read less

Abstract: The validation of classical potentials for describing multicomponent materials in complex geometries and their high temperature structural modifications (disordering and melting) requires to verify both a faithful description of the individual phases and a convincing scheme for the mixed interactions, like it is the case of the embedded atom scheme. The present paper addresses the former task for an embedded atom potential for copper, namely the widely adopted parametrization by Zhou, through application to bulk, surface and nanocluster systems. It is found that the melting point is underestimated by 200 degrees with respect to experiment, but structural and temperature-dependent properties are otherwise faithfully reproduced. read less

Abstract: 本文采用分子动力学方法模拟Ag及其掺杂团簇升温过程，通过平均原子势能曲线、团簇快照图分析团簇的结构和熔化行为。研究发现Ag团簇在熔化前出现二十面体结构转变，转变温度随团簇尺寸减小而降低。在(AgCo)561掺杂团簇中，Co原子的数量和位置对团簇结构和性质有重要作用：掺杂Co原子促进(AgCo)561团簇的二十面体结构转变，引起转变温度降低，且掺杂原子数越多二十面体结构转变温度越低；中心掺杂团簇的二十面体结构转变温度最高，而外层掺杂可诱导出无序态异常结构，导致团簇升温无二十面体结构转变；掺杂对团簇熔点有影响，掺杂导致团簇熔点降低，但在中心位置掺杂55个Co原子引起熔点升高。 In this work, the Ag and doping clusters during the heating processes were studied using molecular dynamics simulations. Based on the potential-temperature curves, the snapshots are obtained and the structural transitions and melting behavior are analyzed. Our results show an icosahedral structure change during the heating processes of Ag clusters and the temperature of structure change is decreased with the size reduction of clusters. For doping Ag clusters, we found that the structure and property were influenced by the number and doping position of Co atoms in (AgCo)561 cluster. The structure change temperature of icosahedral is decreased with the increasing Co atoms and increased with center position doping by Co atoms. For a special case, there is an irregular structure by doping Co atoms in outer layer of clusters and the icosahedral structure change is disappear in this cluster. And the melting temperature is decreased by doping Co atoms, but the higher melting temperature is obtained in Co55Ag506 by Co center doping. read less

Abstract: We investigate the formation of extended defects during molecular-dynamics (MD) simulations of GaN and InGaN growth on (0001) and ([Formula: see text]) wurtzite-GaN surfaces. The simulated growths are conducted on an atypically large scale by sequentially injecting nearly a million individual vapor-phase atoms towards a fixed GaN surface; we apply time-and-position-dependent boundary constraints that vary the ensemble treatments of the vapor-phase, the near-surface solid-phase, and the bulk-like regions of the growing layer. The simulations employ newly optimized Stillinger-Weber In-Ga-N-system potentials, wherein multiple binary and ternary structures are included in the underlying density-functional-theory training sets, allowing improved treatment of In-Ga-related atomic interactions. To examine the effect of growth conditions, we study a matrix of >30 different MD-growth simulations for a range of In x Ga 1-x N-alloy compositions (0 ≤ x ≤ 0.4) and homologous growth temperatures [0.50 ≤ T/T*m (x) ≤ 0.90], where T*m (x) is the simulated melting point. Growths conducted on polar (0001) GaN substrates exhibit the formation of various extended defects including stacking faults/polymorphism, associated domain boundaries, surface roughness, dislocations, and voids. In contrast, selected growths conducted on semi-polar ([Formula: see text]) GaN, where the wurtzite-phase stacking sequence is revealed at the surface, exhibit the formation of far fewer stacking faults. We discuss variations in the defect formation with the MD growth conditions, and we compare the resulting simulated films to existing experimental observations in InGaN/GaN. While the palette of defects observed by MD closely resembles those observed in the past experiments, further work is needed to achieve truly predictive large-scale simulations of InGaN/GaN crystal growth using MD methodologies. read less

Abstract: High thermal conductivity is one important factor in the selection or development of ceramics or composite materials. Predicting the thermal conductivity would be useful to the design and application of such materials. In this paper, a multi-scale model is developed to predict the effective thermal conductivity in SiC particle-reinforced aluminum metal matrix composite. A coupled two-temperature molecular dynamics model is used to calculate the thermal conductivity of the Al/SiC interface. The electronic effects on the interfacial thermal conductivity are studied. A homogenized finite element model with embedded thin interfacial elements is used to predict the properties of bulk materials, considering the microstructure. The effects of temperatures, SiC particle sizes, and volume fractions on the thermal conductivity are also studied. A good agreement is found between prediction results and experimental measurements. The successful prediction of thermal conductivity could help a better understanding and an improvement of thermal transport within composites and ceramics. read less

Abstract: Thin films generally exhibit unusual kinetics leading to chemical reactions far from equilibrium conditions. Binary metallic multilayer thin films with miscible elements show some similar behaviors with respect to interdiffusion and phase formation mechanisms. Interfacial density, lattice defects, internal stresses, layer morphologies and deposition conditions strongly control the mass transport between the individual layers. In the present work, Ag/Al multilayer thin films are used as a simple model system, in which the effects of the sputtering power and the bilayer period thickness on the interdiffusion and film reactions are investigated. Multilayers deposited by DC magnetron sputtering undergo calorimetric and microstructural analyses. In particular, atom probe tomography is extensively used to provide quantitative information on concentration gradients, grain boundary segregations, and reaction mechanisms. The magnitude of interdiffusion was found to be inversely proportional to the period thickness... read less

Abstract: Molecular Dynamics (MD) simulations are suitable to study the details of atomistic processes. Identification of interstitials is one of the main difficulties in the analysis of defect dynamics. To identify the interstitials, different methods use different criteria. All the methods, except the Wigner-Seitz method, assume some input parameters which may lead to an erroneous number of interstitials. We describe an unsupervised clustering algorithm, called Max Space clustering method, to identify interstitials without assuming any input parameters. We show that the algorithm is suited for identifying interstitials at high temperatures when amplitude of atomic oscillations become larger, a fact ignored by other models. read less

Abstract: Choice of appropriate force field is one of the main concerns of any atomistic simulation that needs to be seriously considered in order to yield reliable results. Since, investigations on mechanical behavior of materials at micro/nanoscale has been becoming much more widespread, it is necessary to determine an adequate potential which accurately models the interaction of the atoms for desired applications. In this framework, reliability of multiple embedded atom method based interatomic potentials for predicting the elastic properties was investigated. Assessments were carried out for different copper, aluminum and nickel interatomic potentials at room temperature which is considered as the most applicable case. Examined force fields for the three species were taken from online repositories of National Institute of Standards and Technology (NIST), as well as the Sandia National Laboratories, the LAMMPS database. Using molecular dynamic simulations, the three independent elastic constants, C11, C12 and C44 were found for Cu, Al and Ni cubic single crystals. Voigt-Reuss-Hill approximation was then implemented to convert elastic constants of the single crystals into isotropic polycrystalline elastic moduli including Bulk, Shear and Young's modulus as well as Poisson's ratio. Simulation results from massive molecular dynamic were compared with available experimental data in the literature to justify the robustness of each potential for each species. Eventually, accurate interatomic potentials have been recommended for finding each of the elastic properties of the pure species. Exactitude of the elastic properties was found to be sensitive to the choice of the force fields. Those potentials were fitted for a specific compound may not necessarily work accurately for all the existing pure species. read less

Abstract: A multi-scale model of the tensile fracture of metal melts is developed based on a combination of molecular dynamics (MD) simulations and continuum description of kinetics and dynamics of voids; the model considerably extends the time and length scales of MD. Nucleation of voids due to thermal fluctuations is taken into account. Growth of a void in melts is well described by the Rayleigh-Plesset equation, while in the case of a solid metal we propose a dislocation-based model of the void growth. Based on the MD simulations, we investigate the nucleation rates in the uniform monocrystalline metals and metal melts, dynamics of pre-existing voids and compare them with the continuum model (equations of nucleation and growth). Using of the literature data on the surface tension and viscosity of melts allows us to get a correspondence between the continuum description and MD. With the use of the model, we calculated the strength of the uniform melts of Al, Cu, Ni, Pb, Fe and Ti within a wide range of strain rates (from 103-104 to 109-1011 s-1) and temperatures (from melting temperature to 70-80% of critical temperature). Calculations show that the tensile strength of homogeneous melts decreases slowly with the strain rate decrease. As a result, within the range of strain rates of 106-108 s-1, a homogeneous nucleation mode can be realized, in which the dynamic strength of a melt can be comparable to, or even higher than the strength of a solid metal. read less

Abstract: Heterogeneous nucleation is of significance in controlling the crystal growth, but the key route is still limited. Simulations are performed to explore the microscopic details of how the wedge substrate spreads its structural information to a growing crystal and further affects the solidification process. The simulation results show that, owing to the induced effect from the substrate, the copper atoms become layered at the liquid-solid interface in a "V"-shaped pattern and tend to form a twin crystal. The structural information delivery of the substrate decays with the distance away from the substrate, and the final solidified structure would gradually recover its inherent structure. Interestingly, the wedge angle of 90° seems to be an exception at which the solidified structure exhibits a perfect crystal due to the nearly perfect match with the Cu atoms. Moreover, the cooling rate and the atomic structure of the substrate are also found to have striking correlations with the final structures. These research results are favorable for a better understanding of the inherent relation between the solidified structure and the substrate. read less

Abstract: The freezing behavior of a monatomic liquid film in confined conditions during rapid cooling was studied by molecular dynamics simulations. We illustrated the synergy and pinning effects of the local icosahedral order during freezing. Our results show that the icosahedron contributes to nucleation through the synergy with other short-range ordered structures and participates in crystal growth via assimilation, but the pinning effect should be overcome when crystals grow. Furthermore, a semi-ordered morphology with maze-like nano-patterns emerged due to the cooperation between the synergy effect and the pinning effect. Our findings shed light on the correlation between the local icosahedral order and the crystalline medium-range order, providing a better understanding of the rapid solidification. read less

Abstract: The relationship between eutectic compositions and glass-forming ability (GFA) in Cu100-xZrx (x = 8–83) system is studied by molecular dynamic (MD) simulation based on the embedded atom method (EAM). Local maxima of GFA occur in Cu56Zr44, Cu47Zr53 and Cu31Zr69 alloys, close to eutectic points in Cu-Zr binary phase diagram. The structural parameters indicate that the heterogeneous Cu-Zr pairs exhibit stronger and more stable interaction than that of homogeneous pairs. The small peak with a negative amplitude ahead of the main peak in SCuZr(Q) also implies the preference of Cu-Zr pairs. The avoidance of Cu-Cu neighbours and Zr-Zr neighbours, which correspond to the prepeak prior to the main peak of SCuCu(Q) and SZrZr(Q), leads to structural order on intermediate length scales. Perfect and defect icosahedra constitute medium range order (MRO) clusters by sharing atoms, which promotes the glass formation. read less

Abstract: The non-local spin signals of Co2Fe(Ga0.5Ge0.5)/Cu lateral spin valves with sub-micron size dimensions were measured with varying temperatures. The non-local spin signal reaches 54 mΩ at 4 K, while it degrades down to 13 mΩ at room temperature. Analysis based on the one-dimensional spin diffusion model clarifies the dominant source for degrading of the spin signal is suppression of the spin diffusion length in Cu, not the spin polarization, indicating Co2Fe(Ga0.5Ge0.5) keeps half-metallic nature even at room temperature. The temperature dependence of non-local spin signal was found to exhibit a downturn at 36 K. The presence of magnetic impurities, detrimental effect of which becomes more pronounced for diffusive transport in long Cu wires, is suggested to cause the observed downturn in non-local spin signals. read less

Abstract: The formation and rupture of atomic-sized contacts is modelled by means of molecular dynamics simulations. Such nano-contacts are realized in scanning tunnelling microscope and mechanically controlled break junction experiments. These instruments routinely measure the conductance across the nano-sized electrodes as they are brought into contact and separated, permitting conductance traces to be recorded that are plots of conductance versus the distance between the electrodes. One interesting feature of the conductance traces is that for some metals and geometric configurations a jump in the value of the conductance is observed right before contact between the electrodes, a phenomenon known as jump-to-contact. This paper considers, from a computational point of view, the dynamics of contact between two gold nano-electrodes. Repeated indentation of the two surfaces on each other is performed in two crystallographic orientations of face-centred cubic gold, namely (001) and (111). Ultimately, the intention is to identify the structures at the atomic level at the moment of first contact between the surfaces, since the value of the conductance is related to the minimum cross-section in the contact region. Conductance values obtained in this way are determined using first principles electronic transport calculations, with atomic configurations taken from the molecular dynamics simulations serving as input structures. read less

Abstract: The melting of CuPd bimetallic clusters was studied by using molecular dynamics with the embedded-atom method. The results show that the same number of Cu atoms with Pd clusters, their temperature curve of clusters and the melting point value can be induced by the process of Cu atomic segregation to appear big differences. According to the analysis of HA index and clusters snapshot image, the singular phenomenon is caused by hierarchical differences between Cu atomic distances from the inner surface segregation. It is concluded that the difference of the melting point of Cu-Pd bimetallic clusters provides an effective method of controllable preparation. read less

Abstract: In this paper, by using the classical molecular dynamics method and the GEAM potential, the geometric structure and the melting properties of the 19-atom Ni–Co clusters with different compositions are studied. It is found that all the clusters have the double icosahedron structures although some of the structures are slightly deformed. With the increase of the temperature, a pre-melting phenomenon is observed. The pre-melting temperatures of the pure cobalt and nickel clusters are very close. But on the whole, the pre-melting temperature decreases with the increase of the number of the nickel atom for the mixed clusters. The effects of the substitution atoms on the melting temperature of the clusters are similar to that on the pre-melting temperature although there are some oscillations in the decrease process. The mechanism of these findings are also investigated and analyzed. read less

Abstract: It is a concerned problem that what influences of cooling rate on evolution of microstructures of the treated and untreated Cu are during solidification process after Cu’s being melted by laser grooving. Based on the embedded-atom method, molecular dynamics simulations have been performed on the cooling processes of liquid metal Cu by using crystal-liquid configuration method at six different cooling rates. Through the two-phase structural features, pair correlation function, average atomic energy and mean square displacement analysis, it is found that the cooling rate plays a critical role on the solidification process to the liquid Cu. The liquid Cu becomes its crystalline state during relatively low cooling rate, while the glass transition of the liquid Cu is performed with the relatively high cooling rate. The solidification process of the liquid Cu has effect on the solid crystal Cu and leads to form the indented interface between two-phase structures. read less

Abstract: Numerical calculations of radiation damages in beryllium, alpha-iron and tungsten irradiated by fusion neutrons were performed using molecular dynamics (MD) simulations. The displacement cascades efficiency has been calculated using the Norgett-Robinson-Torrens (NRT) formula, the universal pair-potential of Ziegler-Biersack-Littmark (ZBL) and the EAM inter-atomic potential. The pair potential overestimates the defects production by a factor of 2. The ZBL pair potential results and the EAM are comparable at higher primary knock-on atom (PKA) energies (E > 100 keV). We found that the most common types of defects are single vacancies, di-vacancies, interstitials and small number of interstitial clusters. On the bases of calculated results, the behavior of vacancies, empty nano-voids and nano-voids with hydrogen and helium were discussed. read less

Abstract: Carrying out molecular dynamics (MD) simulations is a relevant way to understand growth phenomena at the atomic scale. Initial conditions are defined for reproducing deposition conditions of plasma sputtering experiments. Two case studies are developed to highlight the implementation of MD simulations in the context of plasma sputtering deposition: ZrxCu1−x metallic glass and AlCoCrCuFeNi high entropy alloy thin films deposited onto silicon. Effects of depositing atom kinetic energies and atomic composition are studied in order to predict the evolution of morphologies and atomic structure of MD grown thin films. Experimental and simulated x-ray diffraction patterns are compared. read less

Abstract: The bonding properties of three kinds of multi-alloy materials Al0·9Cu0·1, Al0·9Cu0·035Mg0·065 and Al0·9Cu0·03Mg0·06Fe0·01 in nano-scale are studied. The molecular dynamics method and the embedded atom potential were used to simulate the bonding properties in nano-scale under the assumption of the single crystal structure of face centred cubic and the motion of each atom in the system dominated by Newton's laws of motion. The total energy of the simulated system includes the molecular potential energy and embedded atom potential. The aluminium alloy is divided into upper and lower blocks while the former moves downward to the latter maintained steady during simulation. Simulation comprises three stages: compressing, holding and stretching. In the first two simulation stages, simulation results depict that in stretching stage the multilayer slips occur along (111) surfaces, and the final plastic deformation leads to strain hardening and the fracture of ductile shear. read less

Abstract: Through the classical molecular dynamics, the melting properties of the the tungsten and aluminum mixed clusters are explored. By using simulated annealing the structure of the mixed cluster is obtained. It is found that the Al atoms tend to locate on the surface of the clusters. The melting behaviors of the clusters are analyzed in detail by the atom-resolved Lindemann index. The curves of Lindemann index show that the melting process of the can experience three steps which is not common in the melting process of the nanoclusters. read less

Abstract: In bimetallic clusters, the component and position of doping atoms can influence on structures and properties. It may induce irregular physical and chemical properties by tuning the doping atoms. In this study, (PdAg)309 clusters with different contents and positions of doping Ag atoms have been investigated by molecular dynamics simulation based on the embedded atom method. It is found that the structure of (PdAg)309 clusters transformed from truncated octahedron to icosahedron, and melting in heating processes from 200 K to 1500 K. Results indicated that the temperature of structural transformation and melting are strongly related to the number and position of doping Ag atoms. The melting point decreased with the increased Ag atoms, but there is irregular by doping Ag atoms at different position. The structural transformed temperature from truncated octahedron to icosahedron decreased with the increasing Ag atoms and the outer doping position. This means that the effect of doping Ag atoms can be used to tune the special structures of bimetallic clusters. read less

Abstract: In bimetallic cluster, research on the frozen structure with the changing concentration plays an important role in exploring new structural materials. This paper studies the freezing processes of (AgCu)309 clusters with different Ag concentrations. The results indicated that the structural transformation was strongly related to concentration. It was found that the frozen structures were changed form icosahedron, hcp and fcc-hcp with the change of Ag concentration. The frozen structures were formed icosahedral for the clusters with Ag concentration at 10%, 20%, 30%, and the pure Ag309. For the clusters with Ag content at 40%, 50%, 60%, 70%, and 80%, the frozen structures were formed defect icosahedral. It was also found that the frozen structure have hcp character for the pure Cu309 cluster. Meanwhile, the frozen structure of (AgCu)309 with 90% Ag concentration was formed fcc-hcp structure. The segregation effects of the Ag-Cu are the key reason for the structural transformation. read less

Abstract: Atomistic simulations add a new and fundamental dimension to structural design by integrating development, optimization, and life-time predictions of materials with the well established macroscopic simulations based on classical theories. This integration has now become possible due to the availability of advanced computational approaches and atomistic simulation software environments such as MedeA combined with ready access to massive compute power. The current capabilities will be illustrated by specific examples including the quantitative prediction of structural and elastic properties as a function of temperature for materials ranging from metallic alloys to thermoset polymers; the precipitation of new phases either for precipitation hardening or as undesired phenomenon leading to embrittlement, for example in Ni-Cr alloys; the effect of alloying elements and impurities on microstructures due to modifications in the strength of grain boundaries; the stress-strain behavior of amorphous boron; the bonding strength of interfaces; and the prediction of transport coefficients including diffusion and thermal conductivity. This contribution will conclude with an analysis of the issues related to the integration of these quantitative atomistic capabilities in the structural design process and it will outline a path forward. read less

Abstract: Atomic segregation in bimetallic clusters can influence the surface nucleation and also the structures of clusters. It is important to study the effect of atomic segregation on the structure. In this study, initial cooling temperatures were used to tune the atomic segregation ability. Molecular dynamics simulation with an embedded atom method was used to study the relationship between the structure and atomic segregation. It was found that the higher the initial cooling temperature, the more obvious the Ag atomic segregation. When the clusters cooled down from 800 K and 600 K, the clusters formed a mixed icosahedron due to the weak atomic segregation ability. When the initial cooling temperature is higher than 1200 K, all the Ag atoms segregated to the surface layer, the clusters formed a Pd–Ag-core–shell chemical ordering. For 1200 K initial temperature, the structure is decahedral. The clusters formed an fcc structure when the clusters cooled down from 2000 K, 1800 K, and 1500 K. When the clusters cooled down from 860 K and 900 K, not all the Ag atoms segregated to the surface layer, the clusters formed a core–shell chemical ordering with a mixed Pd–Ag core. But the structure of 860 K is twinned of icosahedron and decahedron and that of 900 K is decahedral. This means that the chemical orderings and structures were influenced by the atomic segregation. read less

Abstract: In order to make clear the mechanism of the directional coarsening (rafting) of γ′ phases in Ni-base superalloys under uni-axial tensile strain, molecular dynamics (MD) analysis was applied to investigate effects of alloying elements on diffusion characteristics around the interface between the γ phase and the γ′ phase. In this study, a simple interface structure model corresponding to the γ/γ′ interface, which consisted of Ni as γ and Ni3Al as γ′ structure, was used to analyze the diffusion properties of Ni and Al atoms under tensile strain. The strain-induced anisotropic diffusion of Al atoms perpendicular to the interface between the Ni(001) layer and the Ni3Al(001) layer was observed in the MD simulation, suggesting that the strain-induced anisotropic diffusion of Al atoms in γ′ phase is one of the dominant factors of the kinetics of the rafting during creep damage. The effect of alloying elements in the Ni-base superalloy on the strain-induced anisotropic diffusion of Al atoms was also analyzed. Both the atomic radius and the binding energy with Al and Ni of the alloying element are the dominant factors that change the strain-induced diffusion of Al atoms in the Ni-base super-alloy.Copyright © 2013 by ASME read less

Abstract: 本文采用分子动力学方法研究了Ag-Cu共晶合金的结晶过程。依靠体系内能变化、公共近邻分析和原子可视化技术对Ag-Cu共晶合金的结构演变进行了分析。模拟结果表明，在10%~40%Cu范围内，随着Cu含量的增加，合金形成非晶结构的临界冷却速度减小，玻璃化转变温度增加。公共近邻分析的结果表明，在非晶晶化前的阶段，表征非晶结构的1551、1541和1431键对占据主要部分，但也存在少量的bcc和fcc晶核团簇。其中，表征正二十面体团簇的1551键对，随着Cu含量的增加而增加，表征缺陷二十面体团簇的1541和1431键对则相应减少，这表明，随着Cu含量的增加非晶体系的稳定性增加。与此同时，表征晶体相结构的键对随Cu含量的增加都有所减少，其中，bcc晶核的数量减少较小，但fcc晶核的数量却有较大的下降，从而使非晶晶化的形核核心减少，这也从结构上解释了非晶合金的玻璃化转变温度随成分增加而增加的现象。在非晶晶化以后，虽然体系以fcc结构为主，但仍存在一些无序的非晶态结构，这些结构存在于共晶两相界面，随Cu含量的增加而增加。利用可视化技术，我们可以发现，随着Cu含量的增加，Ag-Cu合金的共晶组织存在从固溶体形、网状共晶到层状共晶的演变。 The crystallization of Ag-Cu alloys was studied by molecular dynamics method (MD) with embedded atom potential (EAM). The structural developments of Ag-Cu alloys were analyzed based on the variations of internal energy, common neighbor analysis (CNA), and atomic visualization technique. The simulation results showed that with the increase in Cu composition, the critical cooling rate of amorphous formation decreased and the glass-transition temperature of amorphous increased under the same heating rate. The results of CNA showed that the amorphous structure is main, and a few crystal clusters (bcc and fcc) are in it. Among the amorphous bond pairs, the pairs 1551 increases with the increase in Cu composition, which is the character of regular icosahedrons cluster. Meanwhile, the pairs 1541 and 1431, which are the character of defect icosahedrons clusters, reduce correspondingly. These show that the stability of amorphous increases. With the increase in Cu composition, the pairs 1441, 1661 and 1421 are all reduced, the 1441 and 1661 decrease little and the 1421 decreases great, which implies that the nuclei reduce during the crystallization of amorphous and the glass-transition temperature increases. After crystallization, the fcc structure is dominant but there are a few defect icosahedrons clusters between the eutectic boundaries. Moreover, the eutectic structure of Ag-Cu alloys can be transformed from solid solution, net-like into the lamellar morphologies with composition during the solidification and crystallization. read less

Abstract: On the basis of DFT results for a set of representative bulk, surface, and cluster systems, we determined a new embedded-atom potential for Pd/Au alloys. This embedded-atom approach accurately reproduces DFT properties of such alloy systems. We applied this potential in Monte Carlo simulations to study the effects of temperature and composition on the structure and bonding of Pd/Au alloy nanoparticles up to 5 nm in diameter. We characterized the structure of those particles by evaluating the gold concentration at the surface and in the interior, the coordination numbers of atoms, the nature of Pd entities at the surface, and the number of suggested active sites for vinyl-acetate formation. read less

Abstract: 文章采用分子动力学方法，以金属银为对象，模拟了面心立方金属的凝固和非晶化过程，借助于体系的内能变化、径向分布函数(RDF)、公共近邻分析(CNA)、原子可视化技术对凝固和晶化形核过程中的结构演变细节进行了描述。模拟结果表明，液态金属中就存在11% bcc结构的短程有序集团，随着液态金属不断过冷，bcc集团逐渐增加，在凝固前达到最高的31%，在非晶形成时，bcc集团大约有24%。当最近邻原子距离相同时，bcc结构单位原子所占体积大于fcc结构，有利于在液态金属和非晶中存在。这些bcc结构作为结晶时的非自发形核的核心，与周围的无序原子一起迅速转变为fcc结构和少量的hcp结构。在过冷液体和非晶系统中并没有发现fcc和hcp的结构，fcc结构为凝固和非晶晶化的很短周期中迅速转变而成。 The melting and solidification of Ag was simulated by molecular dynamics method. The structural transfor- mation of Ag during the metal solidification and amorphous crystallization was analyzed based on the variations of the internal energy, radial distribution function (RDF), common neighbor analysis (CNA), and atomic visualization technique. The simulation results showed that the embryonic crystals similar to body-centered cubic (bcc-like) structure (about 11%) already exist in the liquid metal, the content of bcc increases with cooling and it is up to 31% near the solidifying. About 24% bcc structure has in the amorphous structure. The volume per atom of bcc is larger than that of fcc, which is beneficial in the cooling liquid metal and amorphous structure. In the nucleation process, bcc-like embryonic crystals can be used as the nucleus with disorder atoms near the bcc to transform fcc crystal. There is no fcc and hcp structure in the cooling melt and amorphous structure. They are formed in the solidification directly. read less

Abstract: Some findings in heterogeneous nucleation that the structural features of a growing crystal are usually inherited from the heterogeneous nucleus, although attracting more and more attention, are not yet well understood. Here we report numerical simulations of copper nucleation on bending graphene (BG) to explore the microscopic details of how the curved surface influences the freezing structure of the liquid metal. The simulation result clearly shows that copper atoms become layered at the solid-liquid interface in a "C"-shaped pattern resembling the BG. This kind of shape control decays with increasing distance from the wall and the outmost layers transform into twin crystal composed of two fcc wedges. It is found that the final structures have striking correlations with the curvature radius, central angle and arc length of the BG. Our study would provide an opportunity for comprehensive and satisfactory understanding of the heterogeneous nucleation on curved surfaces. read less

Abstract: The structural transitions of Ag965 clusters during two different quenching processes (Q1:1.0×1014 K/s, Q2: 1.0×1012 K/s) were studied using molecular dynamics simulations. This work gives the structure properties including the variations with temperature of pair-correlation function, bond-angle distribution function, bond pairs and bond orientational order parameters in both rapid quenching processes. Our results indicated that the liquid Ag965 was frozen into amorphous structure at 100 K under the quenching condition Q1. While the liquid Ag965 transformed to hexagonal close-packed (hcp) phase at the temperature 100 K under the quenching condition Q2.These instructions give you basic guidelines for preparing papers for conference proceedings. read less

Abstract: Blistering and delamination are the primary failure mechanisms during the processing of depleted uranium (DU) hohlraums. These hohlraums consist of a sputter-deposited DU layer sandwiched between two sputter-deposited layers of gold; a final thick gold layer is electrodeposited on the exterior. The hohlraum is deposited on a copper-coated aluminum mandrel; the Al and Cu are removed with chemical etching after the gold and DU layers are deposited. After the mandrel is removed, blistering and delamination are observed on the interiors of some hohlraums, particularly at the radius region. It is hypothesized that blisters are caused by pinholes in the copper and gold layers; etchant leaking through these holes reaches the DU layer and causes it to oxidize, resulting in a blister. Depending on the residual stress in the deposited layers, blistering can initiate larger-scale delamination at layer interfaces. Scanning electron microscopy indicates that inhomogeneities in the machined aluminum mandrel are replicated in the sputter-deposited copper layer. Furthermore, the Cu layer exhibits columnar growth with pinholes that likely allow etchant to come in contact with the gold layer. Any inhomogeneities or pinholes in this initial gold layer then become nucleation sites for blistering. Using a focused ion beam system to etch through the gold layer and extract a cross-sectional sample for transmission electron microscopy, amorphous, intermixed layers at the gold/DU interfaces are observed. Nanometer-sized bubbles in the sputtered and electrodeposited gold layers are also present. Characterization of the morphology and composition of the deposited layers is the first step in determining modifications to processing parameters, with the goal of attaining a significant improvement in hohlraum yield. read less

Abstract: Molecular dynamics simulation has been performed to study the splitting of the second peak in pair correlation functions of quasi-two-dimensional disordered film. A quasi-two-dimensional inhomogeneous structural model, which contains both crystal-like and disordered regions, supports the hypothesis that the splitting of the second peak is result of a statistical average of crystal-like and disordered structural regions in the system, not just the amorphous structure. The second-peak splitting can be viewed as a prototype of the crystal-like peak exhibiting distorted and vestigial features. read less

Abstract: This paper studies the influence of Ag atomic segregation on the structural evolutions of the mixed (PdAg)309clusters during the heating processes by using molecular dynamics with a general embedded atom method. The results show that the Ag atomic segregation makes the cluster exhibit a segregate-melting stage in which the energy does not monotonic increase with the increase of temperature. In this stage, the cluster first transforms to form a disorder structure from the initial icosahedron and then a decahedron. By comparing with the cases in the pure Pd309, Ag309, and core-shell (PdAg)309, it is found that the icosahedral-decahedral transformation is induced by the Ag atomic segregation. read less

Abstract: Screw dislocations in bcc metals display non-planar cores at zero temperature which result in high lattice friction and thermally-activated strain rate behavior. In bcc W, electronic structure molecular statics calculations reveal a compact, non-degenerate core with an associated Peierls stress between 1.7 and 2.8 GPa. However, a full picture of the dynamic behavior of dislocations can only be gained by using more efficient atomistic simulations based on semiempirical interatomic potentials. In this paper we assess the suitability of five different potentials in terms of static properties relevant to screw dislocations in pure W. Moreover, we perform molecular dynamics simulations of stress-assisted glide using all five potentials to study the dynamic behavior of screw dislocations under shear stress. Dislocations are seen to display thermally-activated motion in most of the applied stress range, with a gradual transition to a viscous damping regime at high stresses. We find that one potential predicts a core transformation from compact to dissociated at finite temperature that affects the energetics of kink-pair production and impacts the mechanism of motion. We conclude that a modified embedded-atom potential achieves the best compromise in terms of static and dynamic screw dislocation properties, although at an expense of about ten-fold compared to central potentials. read less

Abstract: Research on the influence of alloy concentration and distribution on bimetallic cluster plays a key role in exploring new structural material. This paper studies the melting process of icosahedral bimetallic cluster (PdPt)147 with different Pt concentrations and different atomic distributions by using molecular dynamics with an embedded atom method. The results indicate that the mixed Pd–Pt cluster shows an irregular phenomenon between 580 and 630 K, i.e. the atomic energy decreases with the increase of temperature. This is because the surface energy of Pd is lower than that of Pt; the decreased energy due to Pd atomic segregation is larger than the increased energy due to heating during the segregation process. In addition, the temperature of Pd atomic segregation is strongly related to Pt concentration. This leads to that Pd atoms prefer to remain on the surface even after the cluster melted. read less

Abstract: Main concepts of Hydrogen permeability (HP) mechanism for the pure crystal metals are already stated. There are well-founded theoretical models and numerous experimental researches. As far as disordered systems (in which Hydrogen solubility is much more, than in crystal samples) are concerned, such works appear to be comparatively recent and rare. Particularly, they are devoted to Hydrogen interaction of with amorphous structures. Deficiency of similar researches is caused by thermo-temporal instability of amorphous materials structure and properties. read less

Abstract: Hydrogen and iron diffusion coefficients in molten zirconium have been calculated using a molecular dynamics (МD) model. The molecular dynamics method using micro-canonical (NVT) ensemble was used to analyze iron and zirconium diffusion coefficient dependence on electric field intensity and the presence of hydrogen in the zirconium melt. Results obtained are compared to the literature data on impurity removal in plasma-arc zirconium melting in the presence of hydrogen as well as in electron-beam and vacuum-arc melting. The limiting stage of iron removal from the melt is established. The contribution of the electric field to iron removal is estimated. We carried out systematization of the DHMe data for Zr, Nb, and Ta. An Arrhenius equation analysis for DHMe and its extrapolation to the premelting zone taking into account the Gorsky effect was carried out too. The analysis enabled the estimation of DHMe for the temperature interval where experiment encounters difficulties. read less

Abstract: Molecular dynamics (MD) simulations were performed for a Zr2Ni alloy by referring to crystallographic features of a metastable Zr2Ni phase. Simulation method was identical to our previous studies named plastic crystal model (PCM), which includes crystallographic operations for an intermetallic compound in terms of the random rotations of hypothetical clusters around their center of gravity and subsequent annealing at a low temperature. On the basis of MD-PCM, the present study considers an additional refinement named united atom scheme (UAS) on the motions of atoms in the hypothetical clusters. In MD-PCM-UAS, Dreiding potential was assigned for atomic bonds in a cluster whereas Generalized Embedded Atom Method potential for the other atomic pairs. The simulation results by MD-PCM-UAS yield a liquid-like structure. However, annealing did not cause subsequent structural relaxation, which differs from the results by MD-PCM and conventional MD simulations. Further simulations based on MD-PCM-UAS were performed for a nanostructure comprising clusters and glue atoms, leading to the best fit with the experimental data. read less

Abstract: The melting curve is an important thermodynamic property in studies of solid-liquid phase transitions. It can be calculated via molecular dynamics simulations. We simulated the melting process of pure Al with three methods, the heat-until-it-melts (HUM) method, the two-phase method and the hysteresis method. The results calculated via HUM method is approximately 20% higher than experiment data while the results calculated via two-phase method and hysteresis method are in good agreement with experiment data. read less

Abstract: A simple empirical embedded-atom potential that includes a long range force is developed for FCC metals. The potential parameters of this model are determined by fitting lattice constant, three elastic constants, cohesive energy, and vacancy formation energy using an optimisation technique (Arora, 2007). Parameters for Cu, Ag, Au, Al, Ni, Pd and Pt have been obtained. The obtained parameters are used to calculate bulk modulus, divacancy formation energy, and melting point. The predicted values are in good agreement with experimental results. The predicted total energy as a function of lattice parameter is also in good agreement with the equation of state of Rose et al. read less

Abstract: This Letter studies the size-dependent freezing of Co, Co–Ni, and Co–Cu clusters by using molecular dynamics with embedded atom method. Size effect occurs in these three types of clusters. The clusters with large sizes always freeze to form their bulk-like structures. However, the frozen structures for small sizes are generally related to their compositions. The icosahedral clusters are formed for Co clusters (for ⩽ 3.2 nm diameter) and also for Co–Ni clusters but at a larger size range (for ⩽ 4.08 nm ). Upon the Co–Cu clusters, decahedral structure is obtained for small size (for 2.47 nm). The released energy induced the structural transformation plays a key role in the frozen structures. These results indicate that the preformed clusters with special structures can be tuned by controlling their compositions and sizes. read less

Abstract: Amorphous alloys Ni64Zr36 and Ni36Zr64 structure and hydrogen mobility are researched by the molecular dynamics method. The analysis of structure factors and partial distribution functions of atoms revealed hydrogen affects the short order parameters of the disordered systems. Diffusion coefficients of hydrogen are shown to depend on its concentration. read less

Abstract: The surface segregation of Pt atoms in liquid bimetallic alloys confined in carbon nanotube cavities was studied using molecular dynamics simulations. Considerable enrichment in the Pt-atom surface density was found to occur in Pt alloys, when the complementary metal has surface energy higher than Pt and simultaneously metal-wall interaction strength lower than that of Pt with the confining wall. The results suggest that solidification of liquid binary alloys in nanochannels could produce core-shell nanorods with the shell enriched in one of the components for catalytic and other applications. read less

Abstract: Three-dimensional atom probe tomography (APT) is successfully used to analyze the near-apex regions of an atomic force microscope (AFM) tip. Atom scale material structure and chemistry from APT analysis for standard silicon AFM tips and silicon AFM tips coated with a thin film of Cu is presented. Comparison of the thin film data with that observed using transmission electron microscopy indicates that APT can be reliably used to investigate the material structure and chemistry of the apex of an AFM tip at near atomic scales. read less

Abstract: Atom probe tomography is a most advanced microscopic analysis technique based on the principle of field ion microscopy. Atoms are individually desorbed from the specimen, identified and localized. From the measured data, the three-dimensional atomic structure of nanometer-sized specimens is reconstructed numerically. Significant improvements in the total volume of analysis, in the range of materials that can be investigated, and in specimen preparation have recently broadened the range of possible applications of this method decisively. In this chapter, the physical basis of this exciting nanoanalytical tool is presented in some detail, in order that the reader can appreciate the advantages and limitations of the method. The design of actual instruments is described, and the physics of field desorption and volume reconstruction algorithms are discussed. Application of the method to relevant problems of material physics in nanotechnology is illustrated with case studies. Keywords: atom probe tomography; field evaporation; 3D-volume reconstruction; chemical mapping; thin-film reaction; magnetic sensors; interfaces; grain boundaries read less

Abstract: The molecular dynamics (MD) method for analysis of the electric field intensity affecting iron impurities removal from tantalum in the presence of hydrogen is used. The radial distribution functions and diffusivities of hydrogen and iron atoms in a tantalum melt at 3400K under an electric field and without are obtained. An electric field imposed on liquid tantalum with an iron impurity increases the tantalum diffusivity more considerably than for iron atoms. Hydrogen introduction into the MD - cell without an electric field gives a much lower increase of the tantalum diffusivity, but a more considerable increase for iron. Simultaneous imposing an electric field and hydrogen introduction into the MD – cell keeps the tantalum and iron diffusivities at the level of the effect of an electric field. Thus the imposition of an electric field is a main parameter of the increase of the iron diffusivity. read less

Abstract: In this study a nanometer-sized metallic glass (nano-MG) Ti50Cu50 was generated with constrained atoms at both ends and was extended until fracture under a tensile load by molecular dynamics simulation using the general embedded-atom model (GEAM) potential. Totally different mechanical behavior was observed in the nano-MG, such as strain hardening and necking, both of which have been discovered in a few real and simulated MGs and can be related to the generation of shear transformation zones (STZs). A dramatic drop in Young's modulus was found due to the surface effect. Such effect results from the large fraction of surface atoms which have a different surrounding configuration from bulk atoms. At fracture the nano-MG breaks by atomic separation as reported in metal nanowires. The fracture strain is as large as about 120%, indicating that nano-MGs are intrinsically ductile. read less

Abstract: Ti-based metallic glasses (MGs) due to their relative low densities exhibit ultrahigh specific characteristics. In this article the glass-forming behavior and atomic structure of Ti50Cu50 MG were investigated through molecular dynamics simulation (MDS) using the general embedded-atom method (GEAM) potential. As observed experimentally, simulated Ti50Cu50 alloy undergoes three states on quenching: (i) equilibrium liquid; (ii) supercooled liquid and (iii) glassy solid. The atomic configuration of the glass was analysed based on the radial distribution function (RDF) and Voronoi tessellation (VT). It was found that there exist a variety of polyhedral units in Ti50Cu50 MG, where distorted icosohedral and bcc clusters are dominant. read less

Abstract: Using molecular-dynamics simulation we simulate nanoindentation into the three principal surfaces—the (100), (110), and (111) surface—of Cu and Al. In the elastic regime, the simulation data agree fairly well with the linear elastic theory of indentation into an elastically anisotropic substrate. With increasing indentation depth, the effect of pressure hardening becomes visible. When the critical stress for dislocation nucleation is reached, even the elastically isotropic Al shows a strong dependence of the force-displacement curves on the surface orientation. After the load drop, when plasticity has set in, the influence of the surface orientation is lost, and the contact pressure (hardness) becomes independent of the surface orientation. read less

Abstract: Ultrafast laser irradiation of solids may ablate material off the surface. We study this process for thin films using molecular-dynamics simulation and thermodynamic analysis. Both metals and Lennard-Jones (LJ) materials are studied. We find that despite the large difference in thermodynamical properties between these two classes of materials--e.g., for aluminum versus LJ the ratio T{sub c}/T{sub tr} of critical to triple-point temperature differs by more than a factor of 4--the values of the ablation threshold energy E{sub abl} normalized to the cohesion energy, {epsilon}{sub abl}=E{sub abl}/E{sub coh}, are surprisingly universal: all are near 0.3 with {+-}30% scattering. The difference in the ratio T{sub c}/T{sub tr} means that for metals the melting threshold {epsilon}{sub m} is low, {epsilon}{sub m} {epsilon}{sub abl}. This thermodynamical consideration gives a simple explanation for the difference between metals and LJ. It explains why despite the universality in {epsilon}{sub abl}, metals thermomechanically ablate always from the liquid state. This is opposite to LJ materials, which (near threshold) ablate from the solid state. Furthermore, we find that immediately below the ablation threshold, the formation of large voids (cavitation) in the irradiated material leads to a strong temporary expansion on amore » very slow time scale. This feature is easily distinguished from the acoustic oscillations governing the material response at smaller intensities, on the one hand, and the ablation occurring at larger intensities, on the other hand. This finding allows us to explain the puzzle of huge surface excursions found in experiments at near-threshold laser irradiation.« less read less

Abstract: Oxidation kinetics of Ni-Al (100) alloy surface is investigated at low temperatures (300-600 K) and at different gas pressures using molecular dynamics (MD) simulations with dynamic charge transfer between atoms. Monte Carlo simulations employing the bond order simulation model are used to generate the surface segregated minimum energy initial alloy configurations for use in the MD simulations. In the simulated temperature-pressure-composition regime for Ni-Al alloys, we find that the oxide growth curves follow a logarithmic law beyond an initial transient regime. The oxidation rates for Ni-Al alloys were found to decrease with increasing Ni composition. Structure and dynamical correlations in the metal/oxide/gas environments are used to gain insights into the evolution and morphology of the growing oxide film. Oxidation of Ni-Al alloys is characterized by the absence of Ni-O bond formation. Oxide films formed on the various simulated metal surfaces are amorphous in nature and have a limiting thickness ranging from {approx}1.7 nm for pure Al to 1.1 nm for 15% Ni-Al surfaces. Oxide scale analysis indicates significant charge transfer as well as variation in the morphology and structure of the oxide film formed on pure Al and 5% Ni-Al alloy. For oxide scales thicker than 1 nm, the oxidemore » structure in case of pure Al exhibits a mixed tetrahedral (AlO{sub 4}{approx}37%) and octahedral (AlO{sub 6}{approx}19%) environment, whereas the oxide scale on Ni-Al alloy surface is almost entirely composed of tetrahedral environment (AlO{sub 4}{approx}60%) with very little AlO{sub 6} ( 100 ps) occurs in case of 5% Ni-Al alloy. The oxide growth on both pure Al and Ni-Al alloy surfaces occurs by inward anion and outward cation diffusions. The cation diffusion in both the cases is similar, whereas the anion diffusion in case of 5% Ni-Al is 25% lower than pure Al, thereby resulting in reduced self-limiting thickness of oxide scale on the alloy surface. The simulation findings agree well with previously reported experimental observations of oxidation on Ni-Al alloy surface.« less read less

Abstract: The deposition behavior of an Fe–Cu magnetic metallic multilayer system was investigated at the atomic level using molecular dynamics simulation. It was found that a mixture confined to a single atomic layer at the Cu(001) surface was formed at room temperature. The Fe–Cu system shows mixing characteristics such as layer coverage function and mixing length, that are significantly different from those of the Fe–Al metallic system. The different intermixing behavior could be successfully explained in terms of cohesive energy and atomic size mismatch. read less

Abstract: Zr-based metallic glasses have superior characteristics such as mechanical strength, corrosion resistance and precision casting ability. From positron lifetime measurements, a negative temperature dependence of the mean positron lifetime above room temperature was reported and ascribed to the presence of shallow traps. However, the trapping sites remain unknown under the present circumstances. To get a further understanding of the experimental positron lifetime value, the positron density distribution and the lifetime in the Zr-based metallic glass Zr55Cu30Ni5Al10 have been calculated. The calculation shows that the positrons are annihilated inhomogeneously and most positrons are annihilated preferentially around Cu/Al. These results indicate that the positron lifetime not exactly reflects the total free volume. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) read less

Abstract: A new method to estimate the stability of supercooled liquid based on the temperature dependence of the free volume fraction, which is obtained by a molecular dynamics simulation with no empirical data was proposed. The molecular dynamics simulations for some Zr-based metallic glasses, Zr55Cu30Ni5Al10 ,Z r 67Ni33 and Zr67Cu33 in Zr-Cu-Ni-Al system were performed. The features of the first peaks in calculated radial density functions well correspond to the experimental results by XRD and EXAFS spectroscope. The free volume fractions at 0 K in the quenched amorphous, Fquenched, and in ‘‘fully relaxed’’ supercooled liquid states, Frelaxed, are evaluated by the fitting of the temperature vs freevolume-fraction curve obtained by a molecular dynamics simulation. The calculated normalized free volume fraction Nfree, defined as Fquenched=Frelaxed, shows similar tendency with other experimental Trg. criterion. The temperature dependence of the free volume fraction in a supercooled liquid state governs the quenched free volume in the amorphous phase, which shows that the local structures in supercooled liquid or liquid phases are especially important for the glass formation. [doi:10.2320/matertrans.MF200608] read less

Abstract: This study develops a way of determining the interatomic potential of Zr-Ni using an embedded atom method for binary systems that can reproduce the material properties of its amorphous states. In order to ensure the robustness of the developed interatomic potential, the potential energies and lattice constants of Zr crystals, Ni crystals, and Zr-Ni binary crystals that involve a wide range of local atomic environments are employed for fitting. The elastic properties of some such crystals are also employed. In addition, in order to reproduce Zr-Ni amorphous properties, the radial distribution function of Zr70Ni30 amorphous structures and the defect formation energies of Zr-Ni structures are employed. By fitting to a portion of the material properties that requires relatively little computation time, optimization using genetic algorithms is carried out as a first step. As a result, several potential parameter sets are generated. The final potential parameter set, which can reproduce all the material properties used for fitting, is selected from them. The developed potential can reproduce the material properties used for fitting which involve the radial distribution function of the Zr70Ni30 amorphous structure. [doi:10.2320/matertrans.MF200602] read less

Abstract: Using molecular-dynamics computer simulation, we study the materials processes in ultrathin metal films induced by ultrafast laser irradiation. We investigate four different metals (Al, Cu, Ti, W), which vary widely in their cohesive energy, melting temperature, bulk modulus, and crystal structure. Despite these variations, we find that the same materials processes are induced in these films: With increasing laser fluence, the film melts, voids are formed, the film tears (spallation), and finally fragments to form a multitude of clusters. When the energy transfer starting the process is scaled to the cohesive energy of the material, the thresholds of these processes adopt similar – but not identical – values. read less

Abstract: The evolution of the diffraction profiles during the fast thermoelastic deformation and structural transformations induced in a thin Ni film by short pulse laser irradiation is investigated in molecular dynamics simulations. Fast disappearance of the diffraction peaks characteristic for the initial crystal structure is related to the homogeneous nucleation and growth of liquid regions inside the overheated crystal. Transient thermoelastic deformation of the film prior to melting is reflected in shifts and splittings of the diffraction peaks, providing an opportunity for experimental probing of the ultrafast deformations. read less

Abstract: A method for making interatomic potentials is proposed and is applied to Cu-Zr-Hf-Ni- Al bulk-metallic-glass systems. The method consists of three steps. Firstly, potential function form is determined so that bonding nature can be described. Secondly, materials properties used for fitting are selected so that the potential has enough robustness. Here, it is noted that materials properties must be added in accordance with the purpose of the study. Finally, potential parameters have been optimized using global-search procedure. Developed potential well reproduces material properties of them. read less

Abstract: Detailed studies of the structure of magnetic nanoclusters are crucial for understanding their magnetic properties. We have investigated the structure of CoxAg1−x nanoparticles by means of molecular dynamics simulations utilizing the embedded atom method. Starting from a completely random distribution of Co and Ag atoms, the clusters were heated up to 1300K and subsequently cooled down. The size of the resulting particles was 2.8nm (864 atoms). A clear segregation of the Ag atoms on the surface of the Co core was obtained. read less

Abstract: We have investigated the effect of lattice fluctuations on the magnetic properties of nanoparticles of Fe and Co. Atomic structures were simulated using a molecular-dynamic approach, with the system slowly cooled into the ordered phase. The magnetic properties were then simulated using an atomistic approach using a classical spin Hamiltonian taking into account the long-range nature and atomic separation dependence of the exchange. The magnetic properties are found to be affected by both the particle shape and the lattice fluctuations. For a perfectly ordered lattice we find that a spherical particle has a larger magnetization for a given temperature than a cube containing the same number of atoms. We have also studied the effect of lattice fluctuations. This involves a comparison of M(T) for two cases, firstly, a nanoparticle with a fixed lattice corresponding to the low-temperature annealed state (T=20K), and secondly a nanoparticle with a lattice structure equilibrated at the temperature T, the latter ... read less

Abstract: Diffusion in nanostructures has many challenging features even if the role of structural defects (dislocations, phaseor grain-boundaries) can be neglected. This can be the case for diffusion in amorphous materials, in epitaxial, highly ideal thin films or multilayers, in dissolution of thin films or in kinetics of surface segregation, where diffusion along short circuits can be ignored and “only” fundamental difficulties, related to nanoscale effects, raise. Different examples for diffusion in such materials will be given with special emphasis on the validity limit of the continuum approach especially if the diffusion coefficient depends strongly on composition (nonlinearity). It was shown recently by us that even for ideal systems (like Cu/Ni) this non-linearity leads to linear shift of an originally sharp interface (instead of the parabolic behaviour) and to sharpening of an initially diffuse interface. The sharpening takes place, even if the effects of stresses (built in mismatch, thermal and diffusional stresses) are taken into account. Furthermore deviations from the parabolic law are present in phase separating systems as well, but here the interplay of the strong composition dependence and the phase separation tendency results in a more complex behaviour. These deviations are typical nanoeffects i.e. they diminish with time; in the examples investigated here after dissolution of several hundred atomic planes the parabolic law already fulfills. read less

Abstract: In this paper, we evaluate the single films stress of various component layers in spin-valve or magnetic tunnel junction multilayers and the magnetic thickness loss due to interlayer atomic mixing in the magnetic thin films deposited by sub-milli-Torr magnetron sputtering with that from ion beam sputtering. read less

Abstract: A comprehensive knowledge about how crystallographic features are developed at the early stage of a phase transformation in an anisotropic misfit system (e.g., bcc/fcc) is limited, which impedes the deep understanding and quantitative modeling of microstructure evolution. In this work, we simulated coherency loss process during the early growth of a Cr precipitate (bcc) in a Cu matrix (fcc) by the combination of Monte Carlo and molecular dynamics. The simulation results reveal the fine details in the evolution of the crystallographic features, including the orientation relationship (OR) between two phases, the precipitate morphology, and interfacial dislocation structure at the early stage of the precipitate growth. Generation of a dominant set of dislocations is the governing event during the evolution process. The selection of the dominant dislocations was rationalized based on the minimization of interfacial energy in the major facet containing a single set of these dislocations. The initial OR developed between the coherent precipitate and the matrix is near the Nishiyama-Wassermann OR, and it changes discretely in response to the early generation of the dominant dislocations, towards the ideal OR corresponding to the O-line condition, near the Kurdjumov-Sachs OR. This tendency reflects the experimental observations in a Cu-Cr system, and provides helpful insight into the evolution of crystallographic features in physical reality. read less

Abstract: In recent advanced semiconductor products, elements of thickness and width are frequently in nanoscale. As the size of elements reduces to nanometer, a ratio of surface to volume increases. Surface properties, such as surface stresses and surface elasticity, influence on the distributions of bulk stresses near the surface. In the present study, the singular stress at the corner in an anisotropic two-dimensional multi-wedge joint consisting of three kinds of material is analyzed using molecular statics (MS) method and the anisotropic elasticity theory with a boundary condition considering interface properties. The joints are composed of Ni, Au and Cu. Several joints with different wedge angles at the interface are analyzed. Distributions of atomic stress calculated by the MS method are compared with those by the theory. In the results of the MS analysis, stress jumps exist at the interface in the circumferential direction of the stress distribution around the stress singular point. It is supposed that these are attributed to the influence of interface stresses. Then, interface stresses and elastic properties at the interfaces are investigated using the MS method. The obtained interface properties are used in the stress analysis using the Stroh’s formalism. Finally, the theoretical analysis considering the interface properties can be expressed the jumps of stress at the interfaces. read less

Abstract: During the last couple of decades, treatment of microstructure in materials science has been shifted from the diagnostic to design paradigm. Design of microstructure is inherently complex problems due to non linear spatial and temporal interaction of composition and parameters leading to the target properties. In most of the cases, different properties are reciprocally correlated i.e., improvement of one lead to the degradation of other. Also, the design of microstructure is a multiscale problem, as the knowledge of phenomena at range of scales from electronic to mesoscale is required for precise compositionmicrostructure-property determination. In the view of above, present chapter provides the introduction to computationally driven microstructure engineering in the framework of constitutive length scale in microstructure design. The important issues pertaining to design such as phase stability and interfaces has been explained. Additionally, the bird-eye view of various computational techniques in order of length scale has been introduced, with an aim to present the picture of combination of various techniques for solving microstructural design problems under various scenarios. read less

Abstract: The mechanism of the directional coarsening (rafting) of γ' phases of Ni-base super-alloy under uniaxial strain at high temperatures was investigated by molecular dynamics (MD) analysis of stacked thin-film multi-layer structures. The strain-induced anisotropic diffusion of constituent atoms perpendicular to the interface was observed clearly in Ni(001)/Ni 3 Al(001) and Ni(001)/Cu(001) interfaces. In contrast, no strain-induced anisotropic diffusion occurred in the interface structure consisted of the face-centered cubic (FCC) metals with (111) crystallographic orientation. The estimated strain-induced anisotropic diffusion was validated by experiment using the Ni/Cu stacked thin films structures. read less

Abstract: 1 緒 言 多くの金属間化合物は,フレンケルペアの導入だけで 容易に固相アモルファス化 (Solid-State Amorphization, SSA) することが知られている.1)MeV電子照射による固 相アモルファス化に関する系統的な実験研究 1)および理 論研究 2)によって,この現象が材料において普遍的におこ る一般的な現象であることが明らかとなっている. Okamotoらは,カウツマンパラドックスを考慮し,固相ア モルファス化を理想ガラス転移温度 (Ideal glass transition temperature, TK) 以下における機械的融解 (Mechanical Melting) 現象であると考え,Generalized Lindemann Melting (GLM) Criterionを提唱した.2) 近年,Bulk Metallic Glass (BMG) が開発され,その工 学的応用と同時にガラス転移 (Glass-to-Liquid transition) の研究が進展している.現実に観察されるガラス転移は 一次の相転移であるが,二次で発現すれば理想ガラス (Ideal glass) 状態を経由すると考えられる.固相アモル ファス化すなわち結晶-ガラス転移 (Crystal-to-Glass transition) の場合も,二次の相転移で発現すれば理想ガ ラスが形成される.ガラス転移・固相アモルファス化は 一次相転移であるが,いずれも二次的に発現すれば理想 ガラス状態を経るという共通点を考えると,この両者に は大きな相関が存在する.固相アモルファス化現象の解 明は,ガラス転移・理想ガラスの根源を明らかにするこ とにつながり,結果としてバルク金属ガラスの開発およ び材料科学に大きく貢献する.そこで本研究では,固相 アモルファス化現象の中でも,実験的に最も二次に近い 形で相転移を誘起できると考えられる高圧電子顕微鏡MeV電子照射法 2)に注目し,MeV電子照射誘起固相ア モルファス化現象の未解明点である,B2化合物における 固相アモルファス化発現とマルテンサイト変態の相関関 係について調べた. 2 実験方法・計算方法 照射に用いた TiNiおよび Ti (Ni, Fe)合金インゴット は,純金属を所定の組成になるよう秤量し,アーク溶解 法により作製した.照射用試料は,このインゴットある B2化合物の電子照射誘起固相アモルファス化現象 永 瀬 丈 嗣 佐々木 淳 志 滝 澤 和 也 安 田 弘 行 寺 井 智 之 福 田 隆 掛 下 知 行 譯 田 真 人 渋 谷 陽 二 保 田 英 洋 森 博太郎 渋 谷 陽 二 MeV Electron Irradiation Induced Solid-State Amorphization (SSA) in B2 Intermetallic Compounds read less

Abstract: Grain boundary (GB) sliding is an important deformation mode in polycrystals, and it has been extensively investigated, for example, there are many studies on influences of the atomic geometry in the GB region. However, it is important to investigate GB sliding from the electronic structure of GB for deeper understandings of the sliding mechanisms. In the present work, we investigated the GBs sliding in pure and segregated bicrystals with classical molecular dynamics (MD) simulations and first-principles calculations. It is accepted that the sliding rate is affected by the GB energy. However, there was no correlation between the sliding rate and the GB energy in either the pure or the segregated bicrystals. First-principles calculations revealed that the sliding rate calculated by the MD simulations increases with decreasing minimum charge density at the bond critical point in the GB. This held in both the pure and segregated bicrystals. It seems that the sliding rate depends on atomic movement at the minimum charge density sites. read less

Abstract: Generally, it is difficult to develop practical interatomic potentials which can be used in classical molecular dynamics calculations, since a method for development had not been established. In order to solve such problems, we had proposed a framework to develop interatomic potentials. However, the framework had not been widely used due to the difficulty of coding a process to optimize potential-parameters, which was involved in the framework. In this work, a practical software to optimize potential-parameters for solid systems, was developed based on real-coded genetic algorithm (GA). Fitness (=optimization-function) in GA for potential-parameter optimization could be calculated in practical time, and a concept of the corresponded module in GA-code was proposed. The developed software has an extensible structure: a user can define a new crystal-structure, or a new potential function to use in potential-parameter optimization by defining a corresponded class for that in separated header files. Convenient interfacial class to calculate equilibrium material properties of crystals and that to calculate material properties of user-inputted atomic-structures were prepared to define optimization-function. Several sample input files and header files were available for easy starting. As a result of distribution, several effective interatomic potentials were developed by using the software. As a sample of the development of interatomic potential, Tersoff-type potential for SiO2 systems were shown. read less

Abstract: Reaction pathway analysis was carried out for homogeneous dislocation nucleation in perfect crystal Mo. The reaction sampling method employed was based on the Nudged Elastic Band algorithm and other extended schemes. Results obtained were compared with corresponding results for Cu and Si. The stress range for activation energies less than 5 eV is found to be considerably higher for Mo than those for Cu as well as Si. Stresses in excess of 12 GPa make homogeneous dislocation nucleation in Mo an unrealistic transition. The results also show the dislocation cores under this stress range to be diffused, with shear displacement of particles being considerably less than the Burgers vector. Depending on the applied stress, displacement of extra slip-plane atoms can be considerable in Mo. This is in contrast to Cu, in which dislocation nucleation is essentially a two-plane phenomenon. read less

Abstract: Molecular dynamics simulations based on plastic crystal model were performed for the Fe69.0Nb10.3B20.7 and Zr57.1Ni28.6Al14.3 alloys to investigate their forming ability and features of the noncrystalline structures. The Fe69.0Nb10.3B20.7 and Zr57.1Ni28.6Al14.3 alloys were created from C6Cr23 and Ti4Ni2O structures, respectively, by assuming the presence of hypothetical clusters in the crystalline structures under a treatment that the clusters and glue atoms are regarded as the components. The analysis with total pair-distribution function, g(r), revealed that these alloys have high tendencies to form noncrystalline structures while their partial g(r) profiles for pairs among components showed the presence of medium range order. Cluster packed structures from bcc derivative compounds cause high forming ability of noncrystalline structure and the medium range order in the noncrystalline structure. read less

Abstract: In this paper, the contribution of magnetoelastic-anisotropy effect on perpendicular-magnetic anisotropy of Co/Pt superlattice thin films is studied considering the strain-dependent elastic constants. We measured the effective-magnetic-anisotropy energy changing the Pt-layer thickness (2 to 20 A) keeping the Co-layer thickness unchanged (4 A). The effective-magnetic-anisotropy energy changed depending on the Pt-layer thickness and showed a maximum near dPt=12 A. This result is explained by calculating the magnetoelastic-anisotropy energy considering the change of the elastic constants caused by the lattice misfit. We measured anisotropic elastic constants of Co/Pt superlattice thin films by resonance-ultrasound-spectroscopy method coupled with laser-Doppler interferometry and the picosecond laser-ultrasound method. The measured elastic constants showed elastic anisotropy between the in-plane and out-of-plane directions. They agreed with those predicted from the strain-dependent elastic-constant model. The elastic constants show correlations with the perpendicular-magnetic anisotropy. read less

Abstract: As the individual layer of Ti is reduced in thickness in a Ti/Nb multilayered thin film structure, the Ti layer can undergo a change in phase stability from hcp to bcc . Due to its high spatial resolution, atom probe tomography (APT) is ideally suited to characterize the compositional variations in such thin film structures. A series of hcp Ti / bcc Nb and bcc Ti / bcc Nb multilayers have been sputtered deposited and prepared as APT specimens using a Focus Ion Beam (FIB) milling procedure. The APT results have shown a substantial interdiffusion of Nb into the bcc Ti layers to a pseudo-equilibrium concentration of approximately Ti-20at%Nb while maintaining a compositionally modulated interface. In contrast, the hcp Ti layers indicated little Nb interdiffusion within the layers. Thermodynamic volumetric free energy modeling has indicated that this unexpected Nb interdiffusion facilitated the bcc phase stability. The coupling of APT results to the pseudomorphic bcc Ti phase demonstrates the capability APT has in quantifying the compositional characteristics in these types of multilayered nanocomposite systems. read less

Abstract: Solute clusters are one of the important mechanisms of irradiation embrittlement of ferritic steels. It is of great significance to study the stability of solute clusters in ferritic steels and their effects on the mechanical properties of the materials. Molecular dynamics was used to study the binding energy, defect energy, and interaction energy of 2 nm-diameter Cu-Ni clusters in the ferritic lattice, which have six categories of Cu-Ni clusters, such as the pure Cu cluster, the core–shell structural cluster with one layer to four layers of Ni atoms and the pure Ni cluster. It was found that Cu-Ni clusters have lower energy advantages than pure Ni clusters. Through shear strain simulation of the three clusters, the structure of 2 nm diameter clusters does not undergo phase transformation. The number of slip systems and the length of dislocation lines in the cluster system are positively correlated with the magnitude of the critical stress of material plastic deformation. read less

Abstract: High-entropy alloys (HEAs), composed of multiple constituent elements with concentrations ranging from 5% to 35%, have been considered ideal solid solution of multi-principal elements. However, recent experimental and computational studies have demonstrated that complex enthalpic interactions among constituents lead to a wide variety of local chemical ordering (LCO) at lower temperatures. HEAs containing Cu typically decompose by forming of Cu-rich phases during annealing, thus affecting mechanical properties. In this study, CuNiCoFe HEA was chosen as a model with a tendency for Cu segregation at low temperatures. The formation of LCO and its impact on the deformation behaviors in the single-crystalline CuNiCoFe HEA were studied via molecular dynamics simulations. Our results demonstrate that CuNiCoFe HEA decomposes by Cu clustering, in agreement with prior experimental and computational studies, owing to insufficient configuration entropy to compete against the mixing enthalpy at lower temperatures. A softening in ultimate stress in the LCO models was observed compared to the random solid solution models. The softening is due to the lower unstable stacking fault energy, which determines the nucleation event of dislocations, thereby rationalizing the dislocation nucleation in the Cu-rich regions and the softening of the overall ultimate strength in the LCO models. Additionally, the inhomogeneous FCC–BCC transformation is closely associated with concentration inhomogeneity. CuNiCoFe HEA with LCO can be regarded as composites, consisting of clusters with different properties. Consequently, concentration inhomogeneity induced by LCO profoundly impacts the mechanical properties and deformation behaviors of the HEA. This study provides insights into the effect of LCO on the mechanical properties of CuNiCoFe HEAs, which is crucial for developing HEAs with tailored properties for specific applications. read less

Abstract: Understanding the relationship among elemental compositions, nanolamellar microstructures, and mechanical properties enables the rational design of high-entropy alloys (HEAs). Here, we construct nanolamellar AlxCoCuFeNi HEAs with alternating high– and low–Al concentration layers and explore their mechanical properties using a combination of molecular dynamic simulation and density functional theory calculation. Our results show that the HEAs with nanolamellar structures exhibit ideal plastic behavior during uniaxial tensile loading, a feature not observed in homogeneous HEAs. This remarkable ideal plasticity is attributed to the unique deformation mechanisms of phase transformation coupled with dislocation nucleation and propagation in the high–Al concentration layers and the confinement and slip-blocking effect of the low–Al concentration layers. Unexpectedly, this ideal plasticity is fully reversible upon unloading, leading to a remarkable shape memory effect. Our work highlights the importance of nanolamellar structures in controlling the mechanical and functional properties of HEAs and presents a fascinating route for the design of HEAs for both functional and structural applications. read less

Abstract: In this study, we systematically investigate the influence of stress states, relative locations, and orientations of crack—γ/γ′ phase interfaces on the deformation and crack propagation behaviors of the Ni-based superalloy through molecular dynamics simulations. The stress state with high stress triaxiality will impede the plastic deformation process of the system, thereby promoting brittle crack propagation within the system. But the stress state of low stress triaxiality results in obvious plastic deformation and plastic crack propagation behaviors of the system. The deformation system with cracks located in both the γ and γ′ phase exhibits the slowest growth rate, regardless of applied stress states. Additionally, the deformation process demonstrates prominent plastic behavior. For the deformation system with cracks perpendicular to the γ/γ′ phase interface, the γ/γ′ phase interface will hinder the crack propagation. Our research provides interesting observations on deformation and crack propagation behaviors at an atomic level and at a nano-scale which are important for understanding deformation and fracture behaviors at a macroscopic scale for the Ni-based superalloy. read less

Abstract: The field of machine learning-based interatomic potentials (ML-IAPs) has seen increasing development in recent years. In this work, we compare three widely used ML-IAPs–the moment tensor potential (MTP), the spectral neighbor analysis potential (SNAP), and the tabulated Gaussian approximation potential (tabGAP)with a conventional non-ML-IAP, the embedded atom method (EAM) potential. We evaluated these potentials on the basis of their accuracy and efficiency in determining basic structural parameters and Peierls stress under equivalent conditions. Three tungsten (W)-based alloys (Mo-W, Nb-W, and Ta-W) are considered, and their lattice parameter, formation energy, elastic tensor, and Peierls stress of edge dislocation are calculated. Compared with DFT results, MTP demonstrates the highest accuracy in predicting the lattice parameter and the best computational efficiency among the three ML-IAPs, while tabGAP accurately predicts two independent elastic constants, C 11 and C 12. Despite being the slowest, SNAP shows the highest accuracy in predicting the third independent elastic constant C 44 and its Peierls stress value is comparable to that based on MTP. read less

Abstract: For the widespread adoption of polymer electrolyte membrane fuel cells, it is compelling to investigate the influence of the Pt nanoparticle shapes on the electrocatalytic activity. In this study, a catalyst layer was modeled by incorporating four types of Pt nanoparticles: tetrahedron, cube, octahedron, and truncated octahedron, to investigate the relationship between the shapes of the nanoparticles and their impact on the oxygen transport properties using molecular dynamics simulations. The results of our study reveal that the free volume, which has a substantial impact on the oxygen transport properties, exhibited higher values in the sequence of the tetrahedron, cube, octahedron, and truncated octahedron model. The difference in free volume following the formation of less dense ionomers was also related to the surface adsorption of Pt nanoparticles. Consequently, this led to an improved facilitation of oxygen transport. To clarify the dependence of the oxygen transport on the shape of the Pt nanoparticles in detail, we analyzed the structural properties of different Pt shapes by dividing the Pt nanoparticle regions into corners, edges, and facets. Examination of the structural properties showed that the structure of the ionomer depended not only on the shape of the Pt nanoparticles but also on the number of corners and edges in the upper and side regions of the Pt nanoparticles. read less

Abstract: The thermophoresis of nanoparticles suspended in gas is investigated in the transition regime by molecular dynamics simulations. It is found that there exists significant discrepancy between the simulation results and the theoretical predictions for the thermophoretic force, which is attributed to the adsorption of gas molecules on nanoparticles and the gas–particle non-rigid body collisions. By using the effective particle radius, the simulation results and Talbot et al.'s equation could agree with each other in the transition regime. In addition, the effect of the finite system size of the molecular dynamics simulations is non-negligible, and the simulation results modified by effective particle radius can coincide with Phillips' equation quite well. Therefore, for particles of a few nanometers, the non-rigid body collision effect and the adsorption of gas molecules and the effective radius of the nanoparticle under strong gas–particle coupling should be taken into account in the theoretical model. The investigation presented in this paper provides guidance for the application of nanoparticles in aerosol science. read less

Abstract: Up till now, there have been extremely contradictory opinions and inadequate results concerning surface segregation in binary platinum–palladium (Pt–Pd) nanoparticles, including the problems regarding segregating components, as well as the size and temperature dependences of segregation. Taking into account such a situation, we investigated the surface segregation in Pt–Pd nanoparticles by combining atomistic (molecular dynamics) and thermodynamic simulations. For molecular dynamics experiments, the well-known program LAMMPS and the embedded atom method were employed. In the course of the atomistic simulations, two different sets of parameterizations for the Pt–Pt, Pd–Pd, and Pt–Pd interatomic interaction potentials were used. The thermodynamic simulation was based on solving the Butler equation by employing several successive approximations. The results obtained via atomistic simulation and thermodynamic simulation on the basis of the Butler equation were compared with each other, as well as with predictions that were based on the Langmuir–McLean equation and some experimental data. Both simulation methods (atomistic and thermodynamic) predicted the surface segregation of Pd, which diminishes with the nanoparticle size and with increasing temperature. Our simulation results do not confirm the predictions of some authors on surface segregation inversion, i.e., the reversal from the surface segregation of Pd to the surface segregation of Pt when diminishing the nanoparticle size. read less

Abstract: Terrestrial exoplanets are of great interest for being simultaneously similar to and different from Earth. Their compositions are likely comparable to those of solar-terrestrial objects, but their internal pressures and temperatures can vary significantly with their masses/sizes. The most abundant non-volatile elements are O, Mg, Si, Fe, Al, and Ca, and there has been much recent progress in understanding the nature of magnesium silicates up to and beyond ~3 TPa. However, a critical element, Fe, has yet to be systematically included in materials discovery studies of potential terrestrial planet-forming phases at ultra-high pressures. Here, using the adaptive genetic algorithm (AGA) crystal structure prediction method, we predict several unreported stable crystalline phases in the binary Fe-Mg and ternary Fe-Mg-O systems up to pressures of 3 TPa. The analysis of the local packing motifs of the low-enthalpy Fe-Mg-O phases reveals that the Fe-Mg-O system favors a BCC motif under ultra-high pressures regardless of chemical composition. Besides, oxygen enrichment is conducive to lowering the enthalpies of the Fe-Mg-O phases. Our results extend the current knowledge of structural information of the Fe-Mg-O system to exoplanet pressures. read less

Abstract: Physical catalysts often have multiple sites where reactions can take place. One prominent example is single-atom alloys, where the reactive dopant atoms can preferentially locate in the bulk or at different sites on the surface of the nanoparticle. However, ab initio modeling of catalysts usually only considers one site of the catalyst, neglecting the effects of multiple sites. Here, nanoparticles of copper doped with single-atom rhodium or palladium are modeled for the dehydrogenation of propane. Single-atom alloy nanoparticles are simulated at 400–600 K, using machine learning potentials trained on density functional theory calculations, and then the occupation of different single-atom active sites is identified using a similarity kernel. Further, the turnover frequency for all possible sites is calculated for propane dehydrogenation to propene through microkinetic modeling using density functional theory calculations. The total turnover frequencies of the whole nanoparticle are then described from both the population and the individual turnover frequency of each site. Under operating conditions, rhodium as a dopant is found to almost exclusively occupy (111) surface sites while palladium as a dopant occupies a greater variety of facets. Undercoordinated dopant surface sites are found to tend to be more reactive for propane dehydrogenation compared to the (111) surface. It is found that considering the dynamics of the single-atom alloy nanoparticle has a profound effect on the calculated catalytic activity of single-atom alloys by several orders of magnitude. read less

Abstract: TiVZrTa high-entropy alloys (HEAs) have been experimentally proven to exhibit excellent irradiation tolerance. In this work, defect energies and evolution were studied to reveal the underlying mechanisms of the excellent irradiation tolerance in TiVZrTa HEA via molecular statics calculations and molecular dynamics simulations. The atomic size mismatch of TiVZrTa is ∼6%, suggesting a larger lattice distortion compared to most face-centered cubic and body-centered cubic M/HEAs. Compared to pure Ta and V, smaller vacancy formation and migration energies with large energy spreads lead to higher equilibrium vacancy concentration and faster vacancy diffusion via low-energy migration paths. Vacancies in TiVZrTa have weaker abilities to form large vacancy clusters and prefer to form small clusters, indicating excellent resistance to radiation swelling. The formation energies of different types of dumbbells in TiVZrTa show significant differences and have large energy spreads. The binding abilities of interstitials in TiVZrTa are weaker compared to that in pure Ta and V. In TiVZrTa, fast vacancy diffusion and slow interstitial diffusion result in closer mobilities of vacancies and interstitials, significantly promoting point defect recombination. We further studied the effects of short-range ordered structures (SROs) on defect diffusion and evolution. SROs in TiVZrTa can effectively lead to higher fractions of defect recombination and fewer surviving defects. Our findings provide a comprehensive understanding of the underlying mechanisms of the high irradiation tolerance in body-centered cubic HEAs with large lattice distortion and suggest SROs are beneficial microstructures for enhancing irradiation tolerance. read less

Abstract: The purpose of this study is to examine the microstructural properties of Ni pure metallic glass and to uncover how the system reacts when subjected to mechanical pressure during tensile testing. Molecular dynamics simulations, in conjunction with the embedded-atom approach, were used to carry out the investigation. The local structure of the Ni-monatomic metallic glass was examined by analyzing structure parameters such as the radial distribution function and Voronoi tessellation. The results show that the distorted icosahedra <0,1,10,2> and <0,2,8,4> are the most significant structures in the Ni system. Furthermore, the study reveals that mechanical testing has an impact on the local structures. In fact, the tensile testing decreased the significance of short-range order by reducing the fraction of icosahedra and icosahedra-like structures. read less

Abstract: A solid-state bonding technology, particularly Cu-Cu bonding, has attracted attention to overcome crucial drawbacks of conventional Sn-based solders in the latest three-dimensional integrated circuits. In this study, the effects of bonding parameters on its bonding behavior were investigated by an atomistic-scale simulation method, molecular dynamics (MD) simulation. MD simulations with various bonding temperatures and interfacial crystalline structures were performed focusing on interfacial densification behavior. The results indicated that the interfacial densification rapidly occurred with increasing the bonding temperature. It was also suggested that the combination of crystal orientations of the bonding surfaces constituting the interface would be important for its bondability. read less

Abstract: Die-bonding technology using a sintering reaction of Cu nanoparticles at low temperatures is an attractive and important joint method for power devices operating at higher temperatures. We report the adhesive strengths of mechanical pressure-assisted sintered Cu (sCu) onto eight different metal surfaces (Cu, Ag, Au, Pd, Pt, Ni, NiPx, and NiBx). The die-shear strengths (τb) of sCu on the metal platings under sintering temperatures (260-300 °C) in N2 were measured. As the sintering temperatures increased, the τb on the Cu, Ag, Ni, NiPx, and NiBx increased, whereas those on the Au, Pd, and Pt decreased. The MD simulations imply that the diffused couples (Au, Pd, Pt) with high a ratio of intrinsic diffusion coefficients can deteriorate the shear strengths and the self- and non-diffusion couples (Cu, Ni, Ag) can keep the high shear strengths. read less

Abstract: The structural stability of tight-pitched (18 nm and below) damascene interconnects for back-end-of-line (BEOL) technologies are analyzed using force-field-based molecular dynamics simulations and finite-element modeling. At these pitches surface energy-dominated effects come into the picture, which lead to structural instability. The candidate metals analyzed are beyond copper (Cu) interconnect metals-ruthenium (Ru), cobalt (Co), and tungsten (W); and Cu is analyzed for reference. Cohesive traction and normal bonding energy are calculated using force-field- based molecular dynamics simulations and then fed as input to a finite-element analysis (FEA) tool, where their dependence on the physical dimensions of the interconnect lines is studied. The parameters studied for the BEOL structures are sidewall angle, aspect ratio, the internal stress of the metal, and modulus of elasticity of the dielectric material around the metal to understand the sensitivity of these parameters to the structural stability of the interconnects. We observe that a lower aspect ratio and higher modulus of elasticity of the dielectric results in stable structures whereas, intrinsic stress of the metal and side wall angle have a minor impact on the overall stability. The stability is analyzed at the seed-layer deposition step and based on this study, Co is the most stable alternate metal amongst Ru, Co, and W. read less

Abstract: The atomic vibrations of a solid surface can play a significant role in the reactions of surface-bound molecules, as well as their adsorption and desorption. Relevant phonon modes can involve the collective motion of atoms over a wide array of length scales. In this paper, we demonstrate how the generalized Langevin equation can be utilized to describe these collective motions weighted by their coupling to individual sites. Our approach builds upon the generalized Langevin oscillator (GLO) model originally developed by Tully. We extend the GLO by deriving parameters from atomistic simulation data. We apply this approach to study the memory kernel of a model platinum surface and demonstrate that the memory kernel has a bimodal form due to coupling to both low-energy acoustic modes and high-energy modes near the Debye frequency. The same bimodal form was observed across a wide variety of solids of different elemental compositions, surface structures, and solvation states. By studying how these dominant modes depend on the simulation size, we argue that the acoustic modes are frozen in the limit of macroscopic lattices. By simulating periodically replicated slabs of various sizes, we quantify the influence of phonon confinement effects in the memory kernel and their concomitant effect on simulated sticking coefficients. read less

Abstract: A series of atomistic simulations are adopted to explore the influences of relative density, grain size, and temperature on the tensile characteristics of nanoporous tungsten (NPW). Results illustrate that the dominant mechanism of deformation for monocrystalline NPW is the combination of twin boundaries (TBs) migration and 1/2 〈111〉 dislocation movement. The relative density, which has a positive relationship with stiffness and strength, significantly affects the mechanical properties of NPW. With relative density growing from 0.30 to 0.60, Young’s modulus, UTS, and yield strength of monocrystalline NPW increase from 18.55, 0.65, and 0.45 GPa to 93.78, 2.93, and 2.59 GPa, respectively. Young’s modulus and relative density have a quadratic relationship, meaning that the dominant deformation is the bending deformation of ligaments during the elastic stage. The scaling law for yield strength reveals that the axial yielding of ligaments dominates the yielding behavior of NPW. The relationship between mean grain size (5.00 ∼ 17.07 nm) and strength follows the reverse Hall-Petch relation. Besides, the effect of temperature on mechanical characteristics is discussed. With the increase of temperature from 10 K to 1500 K, Young’s modulus of monocrystalline NPW and nanocrystalline NPW (d = 5.00, 10.99, and 17.07 nm) decrease from 69.24, 51.73, 61.08, and 63.75 GPa to 48.98, 34.77, 44.65, and 49.05 GPa. The findings systematically reveal the mechanical properties of NPW under tension and provide guidance for its application. read less

Abstract: The processes of deposition and crystallization of high-entropy AlCoCuFeNi alloy thin film on a substrate of silicon (100) are studied by classical molecular dynamics simulation. Total simulation time reaches 50 ns. The embedded atom model is used to describe the interaction among Al–Co–Cu–Ni–Fe. Interaction between the atoms of Al, Co, Cu, Fe, Ni, and the Si substrate is described using the Lennard–Jones potential, and the interaction between the silicon atoms was modeled using the Stillinger–Weber potential. It is found that at the first stage of deposition small clusters are formed and the process of crystallization starts after ~ 5 ns of simulation, at the characteristic sizes of clusters of about 2 nm. At the end of the simulation, after the 50 ns of modeling, the simulated film contains a face-centered cubic phase, a body-centered cubic phase, a hexagonal close-packed phase, and an amorphous phase. An analysis of the radial distribution of atoms makes it possible to determine the distances between the nearest neighbors and estimate the lattice parameters of these phases. read less

Abstract: It is a macro-micro model study for defect initiation, growth and crack propagation of metallic truss structure under high engine temperature and pressure conditions during the reentry atmosphere. Till now, the multi-scale simulation methods for these processes are still unclear. We explore the deformation and failure processes from macroscale to nanoscale using the Gas-Kinetic Unified Algorithm (GKUA) and all-atomic, molecular dynamic (MD) simulation method. The behaviors of the dislocations, defect evolution and crack propagation until failure for Aluminum-Magnesium (Al-Mg) alloy are considered with the different temperature background and strain fields. The results of distributions of temperature and strain field in the aerodynamic environment obtained by molecular dynamics simulations are in good agreement with those obtained from the macroscopic Boltzmann method. Compared to the tensile loading, the alloy structure is more sensitive to compression loading. The polycrystalline Al-Mg alloy has higher yield strength with a larger grain size. It is due to the translation of plastic deformation mode from grain boundary (GB) sliding to dislocation slip and the accumulation of dislocation line. Our findings have paved a new way to analyze and predict the metallic structural failure by micro-scale analysis under the aerodynamic thermal extreme environment of the reentry spacecraft on service expiration. read less

Abstract: Wire bonding is the most popular first-level interconnection technology used in traditional electronic packaging but still needs fundamental research. This work investigated the formation and breakage mechanism of wire-bonding microwelds in typical wire-bonding material pairs by analyzing the loading force curve, total energy curve, atomic morphology, and von Mises stress. The longer the range of the high attractive force of the atoms of a material, the earlier the real contact between the wire and substrate occurs, which means the bonding is completed earlier. The higher stiffness of the material strengthens the weld and causes damage during the bonding process. Graphical abstract read less

Abstract: ABSTRACT Nanocrystalline alloy design with the synergistic contribution of ‘thermodynamic’ and ‘kinetic’ stabilisation mechanism leads to much superior microstructural stabilisation at elevated temperatures. Ternary Cu98.5W1Zr0.5 (at. %) alloy, synthesised by mechanical milling under cryogenic temperature followed by consolidation through hot pressing at 550°C, has been examined to access the potential of their concurrence. A meager drop in hardness (∼0.5 GPa) confirms the stability of the alloy up to 800°C. The effect of alloy addition has been studied in terms of microstructure alteration, measured by X-ray diffraction, transmission electron microscopy, and Molecular dynamics (MD) simulation. In addition, the shear punch test (SPT) has been employed to assess the mechanical property of the consolidated alloy. Results suggest that the current approach provides a framework en route to designing bulk nanostructured alloys adapting the bottom-up method. read less

Abstract: With the objective of understanding microscopic principles governing thermal energy flow in nanojunctions, we study phononic heat transport through metal-molecule-metal junctions using classical molecular dynamics (MD) simulations. Considering a single-molecule gold-alkanedithiol-gold junction, we first focus on aspects of method development and compare two techniques for calculating thermal conductance: (i) The Reverse Nonequilibrium MD (RNEMD) method, where heat is inputted and extracted at a constant rate from opposite metals. In this case, the thermal conductance is calculated from the nonequilibrium temperature profile that is created at the junction. (ii) The Approach-to-Equilibrium MD (AEMD) method, with the thermal conductance of the junction obtained from the equilibration dynamics of the metals. In both methods, simulations of alkane chains of a growing size display an approximate length-independence of the thermal conductance, with calculated values matching computational and experimental studies. The RNEMD and AEMD methods offer different insights, and we discuss their benefits and shortcomings. Assessing the potential application of molecular junctions as thermal diodes, alkane junctions are made spatially asymmetric by modifying their contact regions with the bulk, either by using distinct endgroups or by replacing one of the Au contacts with Ag. Anharmonicity is built into the system within the molecular force-field. We find that, while the temperature profile strongly varies (compared with the gold-alkanedithiol-gold junctions) due to these structural modifications, the thermal diode effect is inconsequential in these systems-unless one goes to very large thermal biases. This finding suggests that one should seek molecules with considerable internal anharmonic effects for developing nonlinear thermal devices. read less

Abstract: Titanium alloys have become the material of choice for marine parts manufacturing due to their high specific strength and excellent resistance to seawater corrosion. However, it is still challenging for a single titanium alloy to meet the comprehensive specifications of a structural component. In this study, we have applied a molecular dynamics approach to simulate the aging phase transformation, K-TIG welding process, and mechanical properties of the TC4-TA17 (Ti6Al4V-Ti4Al2V) alloy. The results show that during the aging phase transformation process, changes in the structure of the titanium alloys are mainly manifested in the precipitation of a new phase from the sub-stable β-phase, and after the state stabilization, the α-phase content reaches 45%. Moreover, during the melting and diffusion process of TC4-TA17, aluminum atoms near the interface diffuse, followed by titanium atoms, while relatively few vanadium atoms are involved in the diffusion. Finally, the results of tensile simulations of the TC4-TA17 alloy after welding showed that stress values can reach up to 9.07 GPa and that the mechanical properties of the alloy in the weld zone are better than those of the single alloys under the same conditions. This study will provide theoretical support for the optimization of process parameters for TC4-TA17 alloy welding. read less

Abstract: In this paper, the thermal diffusion behavior of Fe/Cu/Ni multilayer coatings was investigated by molecular dynamics. The results show that the Fe, Cu, and Ni elements can diffuse each other at 1250 K. Meanwhile, the intrinsic diffusion coefficients and interdiffusion coefficients of the Fe, Cu, and Ni were calculated. Besides, the diffusion mechanism for high melting-point elements of Fe and Ni at 1250 K was analyzed in the paper. According to the simulation result, the Fe and Ni lattices were disturbed by the active Cu particles. Fe and Ni particles at higher energies may move out of their original positions and migrate into the Cu lattice randomly. Thus, the Fe and Ni elements were involved in the thermal diffusion. This can be confirmed by the decrease of the peak and the disappearance of the secondary peak in the radial distribution function curves. However, the position of the curve peaks did not change. Thus, the lattice structure was still maintained during the whole diffusion process. The thermal diffusion of the three elements was carried out by particle substitution at the lattice positions. It was a solid phase diffusion process. Furthermore, there was a clear particle diffusion asymmetry at the original interface of the element. It was consistent with the diffusion asymmetry of diffusion-couple experiments. The primary reason for this diffusion asymmetry was the difference in the interaction potential of the three elements. This asymmetry was ultimately reflected in the intrinsic diffusion coefficient and the interdiffusion coefficient of each element. For the Fe–Cu–Ni ternary system, the largest diffusion coefficient was copper and the smallest was iron at 1250 K. read less

Abstract: Multiwalled carbon nanotubes (MWCNTs) are decorated with Pt nanoparticles by a “layer-by-layer” approach using poly (sodium 4-styrene sulfonate) (PSS) and poly (diallyl dimethylammonium chloride) (PDDA). Transmission electron microscopy (TEM) images and Energy Dispersive X-ray (EDX) analysis of the samples confirm Pt deposition on surfaces of CNTs. Dispersibility and dispersion stability of MWCNTs in the solvents are enhanced when MWCNTs are coated with Pt nanoparticles. Mg AZ91 composites reinforced with MWCNTs are then produced by a melt stirring process. Compression tests of the composites show that adding 0.05% wt Pt-coated MWCNTs in AZ91 improves the composite’s mechanical properties compared to the pure AZ91 and pristine MWCNT/AZ91. Fracture surface analysis of the composite using a scanning electron microscope (SEM) shows individual pulled out MWCNTs in the case of the Pt-coated MWCNT/AZ91 composites. This finding can be attributed to the uniform dispersion of Pt-coated MWCNTs in Mg due to the improved wettability of Pt-coated MWCNTs in Mg melts. The study of the pull-out behaviour of pristine and Pt-coated CNTs from an Mg matrix using molecular dynamics simulation supports this interpretation. read less

Abstract: High entropy alloys (HEAs) are emerging as a novel class of superior catalysts for diverse chemical conversions. The activity of a catalyst is intimately related to the composition and atomic structure at its surface. In this work, we used embedded atom (EAM) potential based Monte Carlo – Molecular Dynamics simulations to study surface segregation in the equimolar AuAgCuPdPt HEA, that was recently shown to be an efficient catalyst for CO2 electrochemical reduction. Firstly, EAM potentials were extensively validated against experimental segregation data for several different binary and ternary compositions. Subsequently, simulations on the HEA were carried out for four different surface orientations, spherical and cubical nanoparticles, to obtain detailed structural and concentration profiles normal to the surface. In all cases, Ag atoms were found to preferentially segregate to the surface while the subsurface layer mainly consisted of Au atoms. No Pt atoms were found on the surface layer for all systems. A detailed analysis neighborhood of each surface site revealed that the atoms formed a finite number of chemically unique clusters. The percentage of chemically unique sites were larger for elements with lower concentration at the surface. Together with the physical diversity surrounding each site, the enrichment of one or more element(s) at the surface also increased its number of unique catalytically active sites. Results from our work suggest that HEAs are prone to surface segregation and such effects must be taken into consideration while modeling the surface chemistry of these materials. read less

Abstract: Ab-initio density functional theory (DFT) calculations on the elastic coefficients at no-load equilibrium, (001) surface energy and stability limit under the [001] static tension are implemented for eight fcc, four bcc and four hcp metals. The elastic coefficients and surface energy are compared with DFT database of Materials Project, Zhou’s EAM potential, and experimental data available. The stability limit is evaluated with the eigenvalues of the elastic stiffness matrix, B ij = ∆ σ i / ∆ ε j . Here ∆ σ i and ∆ ε j are the change in the stress and strain in Voigt notation. Most elements show instability for the eigenvalues of η Born (= B 11 − B 12 ) or η spinodal of the 3x3 partial matrix of B ij . Pt and W show instability for shear component B 44 while Fe does for B 66 . Then the separation energy of bimetal interface for 120 combinations of these 16 metal elements are estimated from the energy deference of adhered/separated interface in a supercell of stacked unit lattices. read less

Abstract: Multi-principal element alloys (MPEAs) continue to garner interest due to their remarkable mechanical properties, especially at elevated temperatures. Here, we examine a representative nanocrystalline refractory MPEA and identify a crossover from a Hall-Petch to inverse-Hall-Petch relation. While the considered MPEA predominantly assumes a single-phase BCC lattice, the presence of grain boundaries imparts amorphous phase distributions that increase with decreasing grain size (i.e., increasing grain boundary volume fraction). Using molecular dynamics simulations, we find that the yield strength of the MPEA increases with decreasing average grain size, but below a critical grain size < 23.2 nm the yield strength decreases. This change in the deformation behavior is driven by the transition from dislocation slip to grain-boundary slip as the predominant mechanism. Our results reveal that the change from Hall-Petch to inverse-Hall-Petch regime is correlated to dislocation stacking at the grain boundary when dislocation density reaches a maximum. read less

Abstract: Heat treatment is a necessary means to obtain desired properties for most of the materials. Thus, the grain boundary (GB) phenomena observed in experiments actually reflect the GB behaviors at relatively high temperature to some extent. In this work, 405 different GBs were systematically constructed for body-centered cubic (BCC) metals and the grain boundary energies (GBEs) of these GBs were calculated with molecular dynamics for W at 2400 K and β-Ti at 1300 K and by means of molecular statics for Mo and W at 0 K. It was found that high temperature may result in the GB complexion transitions for some GBs, such as the Σ11{332}{332} of W. Moreover, the relationships between GBEs and sin(θ) can be described by the functions of the same type for different GB sets having the same misorientation axis, where θ is the angle between the misorientation axis and the GB plane. Generally, the GBs tend to have lower GBE when sin(θ) is equal to 0. However, the GB sets with the <110> misorientation axis have the lowest GBE when sin(θ) is close to 1. Another discovery is that the local hexagonal-close packed α phase is more likely to form at the GBs with the lattice misorientations of 38.9°/<110>, 50.5°/<110>, 59.0°/<110> and 60.0°/<111> for β-Ti at 1300 K. read less

Abstract: An effective and reliable Finnis–Sinclair (FS) type potential is developed for large-scale molecular dynamics (MD) simulations of plasticity and phase transition of magnesium (Mg) single crystals under high-pressure shock loading. The shock-wave profiles exhibit a split elastic–inelastic wave in the [0001]HCP shock orientation and a three-wave structure in the [10-10]HCP and [-12-10]HCP directions, namely, an elastic precursor, a followed plastic front, and a phase-transition front. The shock Hugoniot of the particle velocity (U p) vs the shock velocity (U s) of Mg single crystals in three shock directions under low shock strength reveals apparent anisotropy, which vanishes with increasing shock strength. For the [0001]HCP shock direction, the amorphization caused by strong atomic strain plays an important role in the phase transition and allows for the phase transition from an isotropic stressed state to the product phase. The reorientation in the shock directions [10-10]HCP and [-12-10]HCP, as the primary plasticity deformation, leads to the compressed hexagonal close-packed (HCP) phase and reduces the phase-transition threshold pressure. The phase-transition pathway in the shock direction [0001]HCP includes a preferential contraction strain along the [0001]HCP direction, a tension along [-12-10]HCP direction, an effective contraction and shear along the [10-10]HCP direction. For the [10-10]HCP and [-12-10]HCP shock directions, the phase-transition pathway consists of two steps: a reorientation and the subsequent transition from the reorientation hexagonal close-packed phase (RHCP) to the body-centered cubic (BCC). The orientation relationships between HCP and BCC are (0001)HCP ⟨-12-10⟩HCP // {110}BCC ⟨001⟩BCC. Due to different slipping directions during the phase transition, three variants of the product phase are observed in the shocked samples, accompanied by three kinds of typical coherent twin-grain boundaries between the variants. The results indicate that the highly concentrated shear stress leads to the crystal lattice instability in the elastic precursor, and the plasticity or the phase transition relaxed the shear stress. read less

Abstract: High entropy alloy has offered significant attention in various material science applications, due to its excellent material features. In this investigation, the mechanical characteristics of Ni2FeCrCuAl High Entropy Alloy (HEA) have been examined under variable temperature and strain rates to analyze its influence over the material features of high entropy alloy through Molecular Dynamics (MD) simulation and it is stated that the formation of various point defects and dislocations are the major cause for the augmentation of tensile deformation which impacts the tensile behavior of high entropy alloy. Moreover, the Radial Distribution Function (RDF) has been examined throughout tensile deformation, to investigate the impact of applied stress over the de-bonding of various atoms and it is found that the strain rate has a greater beneficial impact over the material feature trailed by the temperature outcome, owed to its superior impact on the formation of point defects and shear strain during tensile characterization. read less

Abstract: The grain boundary energies (GBEs) of symmetric tilt grain boundaries (STGBs) and asymmetric tilt grain boundaries (ATGBs) for W at 0 and 2400 K and β-Ti at 1300 K were calculated by means of molecular statics and dynamics simulations to investigate the effects of temperature on GBE and the relationships between GBEs and grain boundary (GB) planes. Generally, the variation trends of GBE with the tilt angle are similar for the three cases, when the tilt axis is specified. It is of course that these similarities result from their similar GB microstructures in most cases. However, the variation trends of GBE with tilt angle are somewhat different between β-Ti at 1300 K and W at 2400 K for STGBs with <100> and <110> tilt axes. This difference mainly stems from the following two reasons: firstly, the GB microstructures of W at 2400 K and β-Ti at 1300 K are different for some STGBs; secondly, the atoms at the STGB of β-Ti at 1300 K tend to evolve into the local ω- or α-like structures distributed at the STGB for some STGBs with <110> tilt axis, which makes the corresponding STGBs more stable, thereby decreasing the GBEs. Furthermore, a geometric parameter θ, the angle between the misorientation axis and the GB plane, was defined to explore the relationships between GBEs and GB planes. It was found that the relationships between GBEs and GB planes can be described by some simple functions of sin(θ) for the GBs with definite lattice misorientation, which can well explain and predict the preferred GB planes for the GBs having the same lattice misorientation. Our calculations not only extend the investigation of GBs to higher temperature, but also deepen the understanding on the temperature contributions to the microstructure evolution at GBs and on the relationships between GBEs and possible geometric parameters. read less

Abstract: Based on molecular dynamics simulations, the creep behaviors of nanocrystalline Ni before and after the segregation of Mo atoms at grain boundaries are comparatively investigated with the influences of external stress, grain size, temperature, and the concentration of Mo atoms taken into consideration. The results show that the creep strain rate of nanocrystalline Ni decreases significantly after the segregation of Mo atoms at grain boundaries due to the increase of the activation energy. The creep mechanisms corresponding to low, medium, and high stress states are respectively diffusion, grain boundary slip and dislocation activities based on the analysis of stress exponent and grain size exponent for both pure Ni and segregated Ni-Mo samples. Importantly, the influence of external stress and grain size on the creep strain rate of segregated Ni-Mo samples agrees well with the classical Bird-Dorn-Mukherjee model. The results also show that segregation has little effect on the creep process dominated by lattice diffusion. However, it can effectively reduce the strain rate of the creep deformation dominated by grain boundary behaviors and dislocation activities, where the creep rate decreases when increasing the concentration of Mo atoms at grain boundaries within a certain range. read less

Abstract: Supratransmission waves are stable objects that can exist in different discrete environments. In this paper, we consider the interaction of such waves with single edge dislocations of various configurations in a crystal with A3B stoichiometry. The model was a Pt3Al crystal, the potential obtained by the embedded atom method was used to describe the interaction of its atoms. Quantitative characteristics of the wave were obtained before and after the interaction. It is found that the degree of energy dissipation by dislocations depends on the mutual orientation of the wave front and the extra plane of the dislocation. Numerical estimates are made for four different configurations. The results obtained can be useful in studying the propagation of soliton-type waves in defect crystals of various compositions. read less

Abstract: High-entropy alloys (HEAs) have stimulated great interest due to their remarkable mechanical and irradiation performance. Experiments suggest that delayed defect evolution in HEAs, compared to conventional metals and dilute alloys, is the main reason for their improved irradiation resistance. However, the mechanism responsible for the observation remains elusive. Here we show that the potential energy landscape of defects under the influence of random arrangement of different species is the reason for the delayed defect evolution. We arrive at the conclusion by investigating the diffusion of defects and defect clusters under three cases: the averaged-atom model, random model, and the model with local short-range ordering. Our results suggest that, compared to the average model, the chemical fluctuation inherent in HEAs can suppress interstitial motion more than vacancy motion. The effects are more pronounced when SRO develops. For defect clusters, the chemical disorder can reduce their jump frequencies significantly and enhance correlation effects, leading to suppressed defect motion. Notably, we find that with SRO, such defect motion can be entirely trapped in local regions. This work demonstrates that chemical fluctuations and SRO are the main reason responsible for the suppressed defect evolution in HEAs, which dictates a promising way to improve the irradiation performance of HEAs through manipulating its chemical disorder states, such as local ordering. read less

Abstract: An analytical bond-order potential (BOP) of Fe–Bi has been constructed and has been validated to have a better performance than the Fe–Bi potentials already published in the literature. Molecular dynamics simulations based on this BOP has been then conducted to investigate the ground-state properties of Bi, structural stability of the Fe–Bi binary system, and the effect of Bi on mechanical properties of BCC Fe. It is found that the present BOP could accurately predict the ground-state A7 structure of Bi and its structural parameters, and that a uniform amorphous structure of Fe100−x Bi x could be formed when Bi is located in the composition range of 26 ⩽ x < 70. In addition, simulations also reveal that the addition of a very small percentage of Bi would cause a considerable decrease of tensile strength and critical strain of BCC Fe upon uniaxial tensile loading. The obtained results are in nice agreement with similar experimental observations in the literature. read less

Abstract: In this study, an experimental and atomic-scale simulationbased investigation has been performed to investigate the possible stability of the nano-crystalline structure and thereby considerable strength at a higher temperature in the case of synthesised Cu alloy along with logical understanding. Dispersions of high melting BCC metal (1 at % W) in nanostructured Cu is achieved using conveniently scalable cryomilling followed by hot pressing (at 550 °C). The thermal stability (till 800 °C) of grain size in synthesised nano-crystalline Cu alloy has been examined through X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). The nano-sized W particles dispersed at the Cu matrix grain boundaries, restricting grain growth by Zener pinning even after annealing was carried out at 800 °C. The hot-pressed pellets of nanocrystalline Cu99W1 alloy with a nearly uniform distribution of W particles have exhibited higher hardness than pure Cu and increased in strength and strain to failure up to 10% and 46%, respectively. The improvement in mechanical properties is further rationalised by the Molecular Dynamics (MD) based simulation findings. ARTICLE HISTORY Received 6 December 2020 Accepted 26 September 2021 read less

Abstract: The mechanism of annealing-induced amorphization of metallic glass is investigated in this study via molecular dynamics simulation. Spherical nucleuses of Cu–Ni–Al alloy with a face-centered cubic structure are embedded to simulate nanograins in Cu–Ni–Al amorphous alloy; subsequently, the material is annealed at different temperatures. The results show that the critical radius for nucleation at temperatures above the glass transition temperature (Tg) affected the behavior, grain growth, and annihilation of nanograins in the Cu–Ni–Al amorphous alloy during annealing. When the temperature increased, the critical radius for nucleation increased as well. This causes the small nanograins to annihilate quickly and the large nanograins to develop rapidly. When the annealing temperature is higher than Tg, part of the crystal nuclei, which is smaller than the critical radius, can be eliminated. The crystallinity of the metallic glass decreased, and the minimum crystallinity is attained after a period of annealing simulation. Subsequently, as the residual effective nanograins began developing, the crystallinity of the amorphous metal increased again. Therefore, the annealing duration time is critical to the crystallinity of the amorphous alloy after annealing. read less

Abstract: W-Cu laminated composites are critical materials used to construct nuclear fusion reactors, and it is very important to obtain direct alloying between W and Cu at the W/Cu interfaces of the composites. Our previous experimental studies showed that it is possible to overcome the immiscibility between W and Cu and obtain direct alloying when the alloying temperature is close to the melting point of Cu. Because the W-Cu interatomic potentials published thus far cannot accurately reproduce the alloying behaviors of immiscible W and Cu, an interatomic potential suitable for the W-Cu system has been constructed in the present study. Based on this potential, direct alloying between W and Cu at high temperature has been verified, and the corresponding diffusion mechanism has been studied, through molecular dynamics (MD) simulations. The results indicate that the formation of an amorphous Cu layer at the W/Cu interface plays a critical role in alloying because it allows Cu atoms to diffuse into W. The simulation results for direct alloying between W and Cu can be verified by experimental results and transmission electron microscopy observations. This indicates that the constructed W-Cu potential can correctly model the high-temperature performance of the W-Cu system and the diffusion mechanism of direct alloying between W and Cu. read less

Abstract: Concurrent atomistic‐to‐continuum methods commonly employ either a dynamic or a quasi‐static continuum model. Both of these approaches have advantages and drawbacks which render their applicability problem‐specific. We present a new hybrid static–dynamic continuum model, which combines the advantages of both approaches while potentially removing the drawbacks. This numerical approach is an innovative superposition of a dynamic and a quasi‐static subproblem and, thus, is limited to linear elastic continua. We apply the approach to a prototypical representative of the concurrent atomistic‐to‐continuum methods and namely, the coupled atomistic and discrete dislocation method. We present three numerical examples to compare the performance of the dynamic, quasi‐static and hybrid continuum model. read less

Abstract: Experiments have shown that the ultrahigh strength of nanolayered metallic composites originates from their high-density interfaces of special characteristics. Hence, the modulation of interface structures becomes an effective route to enhance the mechanical performance of the nanolayered composites. One of the general ways to tune the interfacial feature is to introduce interlayers of several nanometers among constituent layers, such as amorphous (disordered) and crystalline (ordered) interlayers. Here, the deformation of a Cu/Ni layered composite with Ag interlayers of different thicknesses was simulated by molecular dynamics simulations. Our simulations show that the yield stress of 25 nm Cu/25 nm Ni nanolayered composites with Ag interlayers can be significantly enhanced, i.e., it can be 56.4% higher than that of their counterparts without interlayers. We also found that the yield strength of the new composites can be maximized by selecting an appropriate thickness for the Ag interlayer. The optimum interlayer thickness is 2.1 nm in tension and 4.2 nm for compression. It is revealed that the extra strength results from the alleviation of stress concentration by stimulating abundant interfacial dislocations at the Cu–Ag and Ag–Ni interfaces. These findings show that the introduction of additional interlayers is a new route to design stronger nanolayered metallic composites. read less

Abstract: The development of classical interatomic potential for iron is a quite demanding task with a long history background. A new interatomic potential for simulation of iron was created with a focus on description of crystal defects properties. In contrast with previous studies, here the potential development was based on force-matching method that requires only ab initio data as reference values. To verify our model, we studied various features of body-centered-cubic iron including the properties of point defects (vacancy and self-interstitial atom), the Peierls energy barrier for dislocations (screw and mix types), and the formation energies of planar defects (surfaces, grain boundaries, and stacking fault). The verification also implies thorough comparison of a potential with 11 other interatomic potentials reported in literature. This potential correctly reproduces the largest number of iron characteristics which ensures its advantage and wider applicability range compared to the other considered classical potentials. Here application of the model is illustrated by estimation of self-diffusion coefficients and the calculation of fcc lattice properties at high temperature. read less

Abstract: Reducing sulfur poisoning is significant for maintaining the catalytic efficiency and durability of heterogeneous catalysts. We screened PdAu nanoclusters with specific Pd : Au ratios based on Monte Carlo simulations and then carried out density functional calculations to reveal how to reduce sulfur poisoning via alloying. Among various nanoclusters, the core-shell structure Pd13Au42 (Pd@Au) exhibits a low adsorption energy of SO2 (-0.67 eV), comparable with O2 (-0.45 eV) and lower than CO (-1.25 eV), thus avoiding sulfur poisoning during the CO catalytic oxidation. Fundamentally, the weak adsorption of SO2 originates from the negative d-band center of the shell and delocalized charge distribution near the Fermi level, due to the appropriate charge transfer from the core to shell. Core-shell nanoclusters with a different core (Ni, Cu, Ag, Pt) and a Pd@Au slab model were further constructed to validate and extend the results. These findings provide insights into designing core-shell catalysts to suppress sulfur poisoning while optimizing catalytic behaviors. read less

Abstract: To estimate the thermal properties of plumbene under different temperatures, a plumbene sheet is developed by multi-scale modeling. Plumbene sheets of variable sample sizes are also investigated to obtain a broader view of thermal properties of the material for useful application in several engineering fields. Melting point, specific heat at constant volume and pressure, heat of fusion and the coefficient of linear and surface expansion within a temperature range of 318–398 K are also determined. Present applied techniques will guide experimental approaches for designing of plumbene sheets with specific thermophysical properties for targeted applications. read less

Abstract: Phase diagrams supported by density functional theory methods can be crucial for designing high-entropy alloys (HEAs). We present phase and property analysis of refractory quinary (MoW)_xZr_y(TaTi)_1-x-y HEAs from combined Calculation of Phase Diagram (CALPHAD) and density-functional theory results, supplemented by molecular dynamics (MD) simulations. Our analysis indicates a Mo-W-rich region of this quinary system has a stable single-phase body-centered-cubic (bcc). The (MoW)₈₅Zr_7.5(TaTi)_7.5 was down-selected based on temperature-dependent CALPHAD phase diagram analysis and MD predicted elastic behavior that reveals twinning-assisted pseudoelastic behavior in this refractory HEA. While mostly unexplored in bcc crystals, twinning is a fundamental deformation mechanism that competes against dislocation slip in crystalline solids. This alloy shows identical cyclic deformation characteristics during uniaxial <100> loading, i.e., the pseudoelasticity is isotropic in loading direction. Additionally, a temperature increase from 77 to 1,500 K enhances the elastic strain recovery in load-unload cycles, offering possibly control to tune the pseudoelastic behavior. read less

Abstract: In this paper, molecular dynamics (MD) simulation is utilized for the investigation of impact of heating rates on Au and Cu nanoparticles alloying process. Aggregation of contacted nanoparticles experiences three stages due to the contacting, while the alloying process can be distinguished into five regimes because of the contacting and melting. Different heating rates result in different contact temperatures. The decrease of the potential energy can be observed when the temperature reaches the melting temperature. When the temperature reaches the melting point, shrinkage ratio and relative gyration radius have drastic changes during the alloying process. It is shown that heating rates have an apparent effect on the shrinkage ratio and the relative gyration radius during the fusing process, and the shrinkage ratio and the relative gyration radius of Au and Cu alloying systems with lower heating rates have relative larger increasing ratio and decreasing ratio, respectively. read less

Abstract: We prepared two types of perfluorosulfonic acid (PFSA) ionomers with Aquivion (short side chain) and Nafion (long side chain) on a Pt surface and varied their water contents (2.92 ≤ λ ≤ 13.83) to calculate the solubility and permeability of O2 in hydrated PFSA ionomers on a Pt surface using full atomistic molecular dynamics (MD) simulations. The solubility and permeability of O2 molecules in hydrated Nafion ionomers were greater than those of O2 molecules in hydrated Aquivion ionomers at the same water content, indicating that the permeation of O2 molecules in the ionomers is affected not only by the diffusion coefficient of O2 but also by the solubility of O2. Notably, O2 molecules are more densely distributed in regions where water and hydronium ions have a lower density in hydrated Pt/PFSA ionomers. Radial distribution function (RDF) analysis was performed to investigate where O2 molecules preferentially dissolve in PFSA ionomers on a Pt surface. The results showed that O2 molecules preferentially dissolved between hydrophilic and hydrophobic regions in a hydrated ionomer. The RDF analysis was performed to provide details of the O2 location in hydrated PFSA ionomers on a Pt surface to evaluate the influence of O2 solubility in ionomers with side chains of different lengths. The coordination number of C(center)–O(O2) and O(side chain)–O(O2) pairs in hydrated Nafion ionomers was higher than that of the same pairs in hydrated Aquivion ionomers with the same water content. Our investigation provides detailed information about the properties of O2 molecules in different PFSA ionomers on a Pt surface and with various water contents, potentially enabling the design of better-performing PFSA ionomers for use in polymer electrolyte membrane fuel cells. read less

Abstract: In order to enrich the understanding of the relationship between 1D and 3D Ag nanomaterials in welding deformation, molecular dynamics simulations were performed to study a common structure of welded joints in Ag nanowire (NW) connectors on Ag substrates. The effects of the overlapping length, welding temperature and NW diameter on welding strength, dislocation and atomic strain were investigated, with the aim of understanding welding deformations of welded joints. With the increase in the overlapping length, welding temperature and NW diameter, the welding strength increases while the increment decreases. Dislocations can be reduced by increasing the overlapping lengths, NW diameters and annealing time. Moreover, the welded joint performance in shear strength could be improved by performing thermal annealing or decreasing NW diameters. The coordination number, residual stress and energy variation have also been analyzed to explain the above phenomenon. This work can provide guidance for the welding of nanomaterials with different dimensions. read less

Abstract: Bu calisma, farkli boyutlarda CuTi (B11) kristal nanotellerinin [001] yonundeki esneklik-kirilma mekanizmasini ve deformasyonunu gozlemek icin ayrintili bir analiz sunmaktadir. Germe orani ve boyut gibi degiskenlerin nanotelin mekanik ozellikleri uzerine etkileri etkilesmelerin gomulu atom potansiyeli ile tanimlandigi molekuler dinamik benzetimleri ile incelenmistir. Uygulanan dis degiskenlerin CuTi nanotellerinin elastik ve plastik deformasyonlari uzerindeki etkileri iki temel baslik altinda ozetlenmistir. Nanotelin elastik tepkisinin yuksek germe orani ve kucuk boyut ile arttigi gozlenmistir. Elastisite Modulunun germe orani ile de karakterize edilebilmesine ragmen nanotel boyutu istenen dayaniklilik mekanizmasini belirlemede daha etkin role sahiptir. Diger yandan, dusuk germe orani ve kucuk boyutun CuTi nanotellerin kirilma dayanimini ve esnekligini azalttigi izlenmistir. read less

Abstract: Ag-Cu-Au ternary alloys are promising solder materials for wire bonding. Limited experimental studies on Ag-Cu-Au materials can be found due to the high cost of gold. In this study, face-centered-cubic Cu(100), Cu(111), and Cu(110) substrates wetted by molten Ag45Cu42Au13 were investigated via molecular dynamics (MD). As demonstrated by melting simulation results, the Ag45Cu42Au13 alloy has a lower melting temperature compared to the eutectic alloy, Ag60Cu40. MD methods were also used to investigate the dissolutive characteristics of Ag45Cu42Au13/Cu wetting. Density profiles and contact angles show an increase in wettability in the Ag45Cu42Au13/Cu(100) wetting system. For molten Ag60Cu40 and Ag45Cu42Au13 the spreading behavior on Cu(100) shows a promoted tendency, which contrasts with both Cu(111) and Cu(110). Solid-liquid adhesion is indicative of the comparative spreading degrees. The contact angles and PMF analysis of wetting behaviors on rough and smooth Cu substrates illustrate that solid-liquid adhesion in Wenzel states is stronger than in Cassie wetting states. read less

Abstract: The study of nano-scale surface phenomena is essential in understanding the physical processes that aid in technologically relevant applications, such as catalysis, material growth, and failure nuc... read less

Abstract: In order to understand the role of chemical short-range order on deformation mechanisms in FCC compositionally complex alloys, a random model alloy (Co30-Fe16.67-Ni36.67-Ti16.67) is annealed at various temperatures using Hybrid Molecular-dynamics/Monte-Carlo simulations. The simulations produce significant chemical short-range order (CSRO) that increases with decreasing annealing temperature. Annealing tends to homogenize regions of high enthalpy due to: (1) chemical species redistributing into more compact configurations, and (2) pairs of atoms forming chemical bonds that lower the overall energy of the system; the composition explored here shows significant amount of ordering in Ti-Fe pairs with respect to random distributions as described by pairwise (EAM) potentials due to Johnson and Zhou. An energy topology approach is used to assess the local strengthening behavior in random solid solutions and annealed systems, where an interesting interplay is observed between misfit components and chemical short-range order affecting the overall critical resolved shear stress. The role of short-range order on the critical yield stress is quantified and compared with current solid solution models. Finally, we propose and validate an extension to the Labusch-Varvenne class of high-concentration solid-solution analytic models that incorporates the effects of chemical short range order. read less

Abstract: Particle coalescence has wide applications in nature and industry. In this study, molecular dynamics (MDs) simulations were employed to examine the sintering of Cu and Au nanoparticles, as well as ... read less

Abstract: As a new ferroelastic state, strain glass has attracted a lot of recent attentions and, most importantly, strain glass transitions (SGTs) could underpin many phenomena that have puzzled the physics community for decades, including the quasi-linear superelasticity and Invar and Elinvar anomalies. However, there has been a lack of fundamental understanding at the atomistic level beyond the phenomenological Landau theory. In this paper, we propose a way to obtain quantitatively the continuous strain/stress fields distribution caused by point defects through molecular statics calculations by incorporating a Gaussian probability distribution function. By using the quantitative strain/stress fields distribution to inform phase field simulations, we reproduce quantitatively the experimentally observed critical defect concentrations separating the normal martensitic phase transition from SGTs at different temperatures and critical temperatures for spontaneous strain glass to martensitic transition at different defect concentrations. Based on percolation theory, we demonstrate how the strain network created by point defects with a critical concentration regulates the nucleation and growth of martensitic domains, suppresses autocatalysis by strain frustration, and changes the sharp first-order martensitic transformation into a continuous SGT. A general temperature- and defect-concentration-dependent percolation criterion is formulated for accurate prediction of SGT, which could enable high throughput computations for systematic search of new strain glass systems using simply molecular static calculations. read less

Abstract: Molecular dynamics (MD) simulations of metal-metallic glass (Al-Cu50Zr50) multilayer during nanoindentation is carried out to investigate the load-displacement response, mechanical properties and deformation mechanisms. The indentation study is carried out at temperatures in the range of cryogenic to room temperature (10 K-300 K). The indenter speeds are varied between 0.5-5 Å/ps to study the effect of loading rate. The interaction between Al-Cu-Zr atoms are defined by EAM (Embedded Atom Method) potential. A sample size of 200 Å × 200 Å × 200 Å (in x y z-direction) comprising of 538538 atoms is used for nanoindentation. P P S boundary condition (BC) in x y z direction and NVT ensemble are used. We observed a peak load of 117 nN, at a temperature of 10 K with a loading rate of 5 Å/ps. We found that as the loading rate increase, the peak load also increases. As anticipated, the increase in temperature decreases the strength of the multilayer. The atomic displacement vector plots reveal that MG act as hurdles to the movement of dislocations nucleated at the interface. read less

Abstract: According to experimental observations, the temperature dependence of self-diffusion coefficient in most body-centered cubic metals (bcc) exhibits non-Arrhenius behavior. The origin of this behavio ... read less

Abstract: This article analyzes the structure and properties of the CuPt7 crystal for the possible existence of non-linear high-amplitude oscillations of the light alloy component. The densities of the phonon states of a crystal are calculated and analyzed for its different structures. It is demonstrated that the phonon system of a crystal has a narrow forbidden zone. The dependences of the possibility of the existence of high-amplitude oscillations on the initial conditions are revealed. It is shown that such oscillations are possible with the initial deviation of the Cu atom from the equilibrium position by more than 0.95 Å for FCC CuPt7. For such oscillators, their lifetime, the amount of stored energy, and other characteristics were obtained, which make it possible to characterize nonlinear high-amplitude lattice vibrations. read less

Abstract: Noble metal nanostructure synthesis via seed-mediated route is a widely adopted strategy for a plethora of nanocrystal systems. Ag@Au core-shell nanostructures are radiolytically grown in real-time using in situ liquid-cell (scanning) transmission electron microscopy (LCTEM). Here we employ a capping agent, dimethyl-amine (DMA) and a coordinating complex, potassium iodide (KI) in an organic solvent (methanol) in order to: 1) slow down the reaction kinetics to observe mechanistic insights into the overgrowth pro-cess, 2) shift the growth regime from galvanic-replacement mode to direct synthesis mode resulting in the conventional synthesis of Ag@Au core-shell structures. A theoretical approach based on classical simulations complements our experiments providing further insight on the growth modes. In particular, we focus on the shape evolution and chemical ordering, as currently there is an insufficient understanding regarding mixed composition phases at interfaces of alloys even with well-known miscibilities. Furthermore, the comparison of theoretical and experimental data reveals that the final morphology of these nanoalloys is not simply a function of crystallinity of the underlying seed structure but instead is readily modified by extrinsic parameters such as additives, capping agent and modulation of surface energies of exposed crystal surfaces by the encapsulating solvent. The impact of these additional parameters is systematically investigated using an empirical approach in light of ab-initio simulations. read less

Abstract: The processes of melting and solidification of AlCoCuFeNi nanoparticle of about 10 nm is studied by molecular dynamics simulation at three different cooling rates (1∙1011 K/s, 1∙1012 K/s, and 1∙1013 K/s) using the embedded atom model (EAM) potential. The melting and crystallization of the nanoparticle are characterized by studying the temperature dependence of the potential energy. The adaptive common neighbor analysis (CNA) is performed and the radial distribution function (RDF) is calculated to determine the structure and lattice parameters of phases of the AlCoCuFeNi nanoparticle. It is shown that the final structure of the investigated nanoparticle changes from amorphous to crystalline with decreasing of the rate of cooling, and the temperature hysteresis takes place during the melting and crystallization of AlCoCuFeNi HEA nanoparticle. read less

Abstract: An approach to measuring electrical contact resistance as a direct function of the true contact size at the nanoscale is presented. The approach involves conductive atomic force microscopy (C-AFM) measurements performed on a sample system comprising atomically flat interfaces (up to several hundreds of nanometers in lateral size) formed between gold islands and a highly oriented pyrolytic graphite (HOPG) substrate. The method overcomes issues associated with traditional C-AFM such that conduction can be correlated with a measurable true, conductive contact area. Proof-of-principle experiments performed on gold islands of varying size point toward an increasing contribution of the island-HOPG junction to the measured total resistance with decreasing island size. Atomistic simulations complement and elucidate experimental results, revealing the maximum island size below which the electrical contact resistance at the island-HOPG junction can be feasibly extracted from the measured total resistance. read less

Abstract: Molecular dynamics simulations were employed to investigate the aggregation of monocrystal and polycrystal nanoparticles. The lattice structure, displacement vector, potential energy, shrinkage ratio, relative gyration radius and mean square displacement of the two systems are compared. The results indicate that the aggregation of polycrystal nanoparticles is more drastic than that of monocrystal nanoparticles. Besides, the polycrystal nanoparticles are found contacted and melted at lower-temperature than that of monocrystal nanoparticles. The reason for all these phenomena is that there is additional surface energy in the grain boundary of polycrystal nanoparticles. read less

Abstract: The study of nanocrystalline(NC) polycrystals is a hot topic, and the study of nanomaterial properties by molecular dynamics has become the first choice for many researchers. The purpose of this paper is to simulate the tensile tests of single and polycrystalline tantalum by molecular dynamics(MD) to obtain its mechanical properties. Firstly, the Ravelo-EAM potential was used to conduct tensile tests on tantalum in the <100> direction. Secondly, it can be seen that the elastic modulus E100 decreases with the temperature gradually increases from 1 K to 1500 K according to the simulation results. Finally, the Hall-Petch(H-P) effect based on grain size is verified from the tensile test of polycrystalline tantalum. read less

Abstract: Metallic glasses are amorphous solids produced by rapid cooling, with disordered atomic structure and lacking long-range order. This structural disorder makes them to show mechanical properties different from those observed in a crystalline solid. Metallic glasses are ideally isotropic, but they can become anisotropic during the manufacturing process or as a result of non-homogeneous plastic deformation, creep, etc. Experimental studies showed remnant anisotropy in amorphous PdSiCuP under a homogeneous deformation regime at temperatures close to the glass transition (Tg). In this thesis we studied the remnant anisotropy induced by shear in two amorphous systems by using molecular dynamics simulation. The Cu13Ni34Pd53 system was chosen to approach the PdCuSiP compositions, while the Zr46Cu46Al8 system is an excellent metallic glass former. Amorphous systems were obtained by fast cooling from the liquid and subsequent thermal annealing. Both systems were then sheared in the [100] direction at a deformation speed of 10^10 s^(-1), and returned to its original form at the same speed. Shearing simulations were performed in isothermal-isobaric conditions at different temperatures. The resulting states were examined using the directional pair distribution function (d-PDF), calculated from the distribution function of interatomic distances in planes perpendicular to the selected axis. Remnant anisotropy was detected in both systems after a deformation-recovery cycle. Cu13Ni34Pd53 displays remnant anisotropy below the glass transition at 0 and 300 K after the shear process, and again at 700 K after the full deformation-recovery cycle. The intensity of anisotropy in the d-PDF of Zr46Cu46Al8 is lower than in Cu13Ni34Pd53, possibly due to the different atomic radii. However, it is found in all the studied temperature range, with decreasing intensity with temperature. Anisotropy is concentrated in the [110] planes and [1(-1)0], corresponding to the planes of maximum and minimum shear respectively. This result indicates that remnant anisotropy is highly directional and can go unnoticed if not searched for. It also states that the shear process is essentially symmetrical with respect to the plane [110]. The shear and recovery process induces the creation of free volume, falling into the category of rejuvenation processes discussed in the literature. Counterintuitively, the process of free volume creation is accompanied by a reduction of the distance of the first maximum of the pair distribution function g (r), indicating a decrease of the most likely distance (mode) between first neighbors. However, this decrease in mode is accompanied by an increase of the width of the first peak, increasing the standard deviation of the distribution of distances between first neighbors. This explains the apparent contradiction between the increase in the volume and reduction of the most probable distance between first neighbors. Simultaneously, the second coordination sphere is more homogenous after the deformation and recovery process. As a result, medium range order is increased. The discrepancy between the results obtained between Cu13Ni34Pd53 and Zr46Cu46Al8 may be due to the fact that the potential used in the simulations of Zr46Cu46Al8 is more representative of the behavior of metallic glasses. As a result, we hypothesize that the presence of remnant anisotropy after mechanical deformation could be a general feature in metallic glasses. The computational cost of this study was very high, since it was necessary to simulate million-atom systems to avoid that results were dependent on the size of the simulation box. Los vidrios metálicos son sólidos amorfos producidos por enfriamiento rápido, con estructura atómica desordenada y sin orden de largo alcance. Este desorden estructural les proporciona propiedades mecánicas diferentes de las observadas en un sólido cristalino. Idealmente los vidrios metálicos son isotrópicos pero pueden convertirse en anisotrópicos durante el proceso de fabricación o como consecuencia de una deformación plástica no homogénea, fluencia, etc. Estudios experimentales mostraron anisotropía remanente en PdSiCuP amorfo bajo un régimen de deformación homogéneo a temperaturas cercanas a la transición vítrea (Tg). En esta tesis estudiamos la anisotropía remanente inducida por cizalla en dos sistemas amorfos mediante simulación por dinámica molecular. El sistema Cu13Ni34Pd53 se eligió intentando aproximarse a las composiciones PdCuSiP. Por otra parte, el sistema Zr46Cu46Al8 es un excelente formador de vidrios metálicos. Tras la obtención del sistema amorfo por enfriamiento rápido y equilibrado térmico ambos modelos fueron sometidos a deformación por cizalladura en la dirección [100], a una velocidad de deformación de 10^10 s^(-1), y retornados a su forma original con la misma velocidad, en condiciones isotérmicas-isobáricas a diferentes temperaturas. Los estados obtenidos fueron examinados utilizando la función de distribución par direccional (dPDF), calculada a partir de la distribución de distancias interatómicas en los planos perpendiculares al eje escogido. En ambos sistemas se detectó anisotropía remanente después de un ciclo deformación- recuperación. En Cu13Ni34Pd53 se observa anisotropía remanente por debajo de la transición vítrea en 0 y 300 K al final del proceso de cizalla, y nuevamente en el estado deformado a 700 K. En Zr46Cu46Al8, la intensidad de la anisotropía determinada en las d-PDF es menor que en Cu13Ni34Pd53, posiblemente debido a los radios atómicos, pero se observa en todo el rango de temperaturas estudiadas con intensidad decreciente con la temperatura. La anisotropía se concentra en los planos [110] y [1(-1)0], correspondientes a los planos de máxima y mínima cizalla respectivamente. Este resultado indica que la anisotropía remanente es altamente direccional y puede pasar desapercibida si no se busca específicamente. Asimismo, se observa que el proceso de cizalla es esencialmente simétrico con respecto al plano [1(-1)0]. El proceso de cizalla y recuperación induce la creación de volumen libre, entrando dentro de la categoría de los procesos de rejuvenecimiento analizados en la literatura. Contraintuitivamente, el proceso de creación de volumen libre va acompañando de una reducción de la distancia del primer máximo de la función de distribución de pares g(r), indicando una disminución de la distancia más probable (moda) entre primeros vecinos. Sin embargo, esta disminución de la moda va acompañada de un aumento de la anchura del primer pico, aumentando la desviación estándard de la distribución de distancias entre primeros vecinos. Esto explica la aparente contradicción entre el aumento del volumen y la reducción de la distancia más probable entre primeros vecinos. Simultáneamente, la segunda esfera de coordinación es más homogénea después del proceso de deformación y recuperación. En consecuencia, se produce un aumento del orden a media distancia. La discrepancia entre los resultados obtenidos entre Cu13Ni34Pd53 y Zr46Cu46Al8 puede deberse a que el potencial en Zr46Cu46Al8 es más representativo del comportamiento de los vidrios metálicos. Consecuentemente, la presencia de anisotropía remanente después de deformación mecánica podría ser una característica general en los vidrios metálicos. El coste computacional de este estudio ha sido muy alto, puesto que ha sido necesario simular sistemas con muchos átomos para evitar que los resultados obtenidos fuesen dependientes del tamaño de la caja de simulación utilizadas read less

Abstract: We describe an investigation of microstructure in pure nickel (Ni) after cathodic charging in aqueous electrolyte. Intergranular corrosion occurs on the sample surface and takes the form of long trenches along coherent twin boundaries (CTBs): sites that have often been considered especially resistant to corrosion. Integrating electron backscatter diffraction and x-ray computed tomography, we show that the trenches are formed by the growth and eventual overlap of isolated conical cavities, which, in turn, are aligned with <110>-type directions within CTB planes. The <110>-type orientation is consistent with cavity initiation at high-energy, symmetric incoherent twin boundary facets within CTB planes. Our observations show that, far from being always corrosion resistant, CTBs are especially susceptible to intergranular corrosion under some conditions. read less

Abstract: The validation of embedded atom models (EAM) for modelling nanoalloys requires to verify both a faithful description of the individual phases and a convincing scheme for the mixed interactions. In this work, we present a systematic benchmarking of two widely adopted EAM parameterizations, i.e. by Foiles [S. M. Foiles et al. Phys. Rev. B 33, 7983 (1986)] and by Zhou [X. W. Zhou et al. Phys. Rev. B, 69, 144113 (2004)] with density functional theory calculations for the description of processes at Ag@Au nanoalloys surfaces and nanoclusters. read less

Abstract: The crystallization behavior occurred in the heat affected zone in the process of bulk metallic glasses preparation with selective laser melting has been studied with molecular dynamics simulation. Cyclical heating conditions are applied with three different temperature variation rates in the simulation cases, while the heating rate is ten times of the cooling rate in each case. The results reveal that the newly formed crystal cluster has more Ni atoms than the parent phase. When the cooling rate is about one third of the critical cooling rate for the formation of metallic glass, the crystallization behaviors happen in the first cooling process, which is controlled alternately by growth mechanism and nucleation mechanism. In the following heating process, the numbers of both crystal clusters and crystal atoms first increase and then decrease. When the cooling rate is about two fifths of the critical cooling rate, the crystallization behaviors happen in the second cooling process. When the cooling rate increases to about two thirds of the critical cooling rate, massive crystallization behavior is not observed, showing that the crystallization occurred in the HAZ in the process of preparing BMGs with SLM can be suppressed with a critical temperature variation rate. read less

Abstract: Deformation twinning has been frequently observed in body-centered cubic (BCC) high entropy alloys (HEAs), however, the underlying mechanism remains elusive. We perform molecular dynamics simulations on a representative BCC HEA nanopillar under high-symmetry compression, describe atomic details of deformation twinning, and propose a mechanism of twin nucleation from the surface. We find that twinned regions are formed by partial dislocations and that chemical heterogeneity can reduce local fault energy and promote stacking faults and twins. These results help to understand the propensity for stacking fault formation and twinning in HEAs and may guide the design of novel HEAs through control of active twinning mechanisms. read less

Abstract: The atomic level study of NiTi alloy at high temperature is very important to understand the mechanism of NiTi fabrication, in partial the process during the hot working. In the atomic investigation using molecular dynamics simulation, the use of the interatomic potential greatly affects the results. Therefore, the suitability of the interatomic potential applied in some specific condition has to be examined. In our previous work, we have tested the performance of EAM and MEAM potential to reproduce the lattice constant of NiTi alloy. Our previous results have shown that the MEAM potential work better than the EAM potential. In this research, we further investigate the performance of EAM and MEAM type potential to describe the melting behavior of nickel, titanium, and NiTi alloy. We find from the current result that the accuracy of the MEAM potential is better than EAM potential in high temperature MD simulations. read less

Abstract: We present our investigation of the current state of the art for the transferability of molecular dynamics (MD) interatomic potentials for high temperature simulations of material processes in terms of elastic constants. With the current advancement of computer power, nanoscale computational models such as MD have the potential to accelerate optimization and development of high temperature material processes provided a robust and transferable interatomic potential. Temperature dependency of elastic constants, despite the low temperature elastic constants, is not commonly used as one of the target material properties to develop interatomic potentials for metals; thus, it is a reliable index to determine the transferability of interatomic potentials for high temperature simulations. We consider all five independent elastic constants and their temperature dependency as an index for our evaluations of available interatomic potentials for titanium (Ti), zirconium (Zr), and magnesium (Mg) as representative metals with a relatively complex crystal structure (hcp). The calculated elastic constants and their deviation from their corresponding experimental values are presented. We provide a through discussion on the transferability of each potential and summarize with the most suitable potentials for high temperature material process simulations for each considered material. read less

Abstract: High Entropy Alloys (HEAs) represent an important class of structural materials because of their high strength, ductility, and thermal stability due to the solid solution nature of the multi-component metallic system. Understanding the mechanical response of isolated phases (FCC and BCC) of a dual-phase HEA is integral to understanding mechanical properties of these special alloys in bulk. We investigate the compressive response of single crystalline cylinders with diameters between 400 nm and 2 µm excised from individual grains within FCC and BCC phases of Al0.7CoCrFeNi HEA at 295 K, 143 K and 40 K. Micro-compression experiments were conducted in an in-situ SEM equipped with a custom-constructed cryogenic setup; FCC samples had a [324] crystallographic orientation, and those extracted from the BCC phase had a [001] orientation. We observed a "smaller is stronger" size effect in the yield strength as a function of pillar diameter, D, of both alloy phases for all temperatures, τ_y ∝D^m with a power law exponent, m, decreasing from -0.68 at 295K to -0.47 at 143K to -0.38 at 40K for FCC phase, and remaining constant at ~-0.33 for all temperatures for the BCC phase. We also observed reduced work hardening rates and more extensive strain bursts during deformation at lower temperatures in all samples. All deformed FCC samples contained multiple parallel slip offsets for all pillar sizes and temperatures; compressed BCC pillars had wavy slip traces, which are evidence of multiple intersecting slip systems. Transmission Electron Microscopy (TEM) microstructural analysis of the compressed FCC samples reveals parallel slip lines and distorted slip planes, while compressed BCC samples contained entangled dislocation networks, as well as several twinned regions in samples deformed at 40 K. Molecular dynamics (MD) simulations of representative FCC and BCC HEA compressions reveal that deformation in FCC HEAs is dominated by nucleation and propagation of partial dislocations along parallel slip planes but by partial dislocation/twinning in the BCC HEA at all temperatures. Simulations also predict a decrease in stacking fault energy with increased alloying. For example, a reduction in the stable stacking fault energy of the FCC HEA up to 55% with respect to pure constituents is observed. This reduction in stable stacking fault energy may drive the observed deformation mechanisms. We also discuss theories of low-temperature strengthening in HEAs, compare them to our experimental data and assess how they manifest in the observed temperature-dependent size effect. read less

Abstract: A commonly used strategy to enhance the mass activity of Pt-based catalysts involves the synthesis of Au nanoparticles (NPs) with a monolayer-thick Pt-skin layer. The synergistic effect of Au and Pt results in a higher catalytic activity and better Pt utilization. However, the stability of the Pt-skin layer is questionable as our recent equilibrium Monte Carlo simulations predict that eventually the surface Pt is replaced by Au. The role of Au during destabilization of Pt-skin in vacuum and solution is investigated with the help of molecular dynamics. Different starting Au–Pt arrangements are studied mimicking various NP synthesis approaches. Beyond a critical number of atoms in a Pt cluster, the ideal Pt monolayer rapidly transforms to a three-dimensional (3D) Pt cluster. This is supported by our model predicting transition from the Pt monolayer to Volmer–Weber growth in the Au–Pt system. At room temperature, Pt atoms move into the subsurface layer at second timescales mainly via the exchange mechanism i... read less

Abstract: An Ag–Ni semi-empirical potential was developed to simulate the segregation of Ni solutes at Ag grain boundaries (GBs). The potential combines a new Ag potential fitted to correctly reproduce the stable and unstable stacking fault energies in this metal and the existing Ni potential from Mendelev et al (2012 Phil. Mag. 92 4454–69). The Ag–Ni cross potential functions were fitted to ab initio data on the liquid structure of the Ag80Ni20 alloy to properly incorporate the Ag–Ni interaction at small atomic separations, and to the Ni segregation energies at different sites within a high-energy Σ9 (221) symmetric tilt GB. By deploying this potential with hybrid Monte Carlo/molecular dynamics simulations, it was found that heterogeneous segregation and clustering of Ni atoms at GBs and twin boundary defects occur at low Ni concentrations, 1 and 2 at%. This behavior is profoundly different from the homogeneous interfacial dispersion generally observed for the Cu segregation in Ag. A GB transformation to amorphous intergranular films was found to prevail at higher Ni concentrations (10 at%). The developed potential opens new opportunities for studying the selective segregation behavior of Ni solutes in interface-hardened Ag metals and its effect on plasticity. read less