Abstract: Grain boundaries often exhibit ordered atomic structures. Increasing amounts of evidence have been provided by transmission electron microscopy and atomistic computer simulations that different stable and metastable grain boundary structures can occur. Meanwhile, theories to treat them thermodynamically as grain boundary phases have been developed. Whereas atomic structures were identified at particular grain boundaries for particular materials, it remains an open question if these structures and their thermodynamic excess properties are material specific or generalizable to, e.g., all fcc metals. In order to elucidate that question, we use atomistic simulations with classical interatomic potentials to investigate a range of high-angle [111] symmetric tilt grain boundaries in Ni, Cu, Pd, Ag, Au, Al, and Pb. We could indeed find two families of grain boundary phases in all of the investigated grain boundaries, which cover most of the standard fcc materials. Where possible, we compared the atomic structures to atomic-resolution electron microscopy images and found that the structures match. This poses the question if the grain boundary phases are simply the result of sphere-packing geometry or if material-specific bonding physics play a role. We tested this using simple model pair potentials and found that medium-ranged interactions are required to reproduce the atomic structures, while the more realistic material models mostly affect the grain boundary (free) energy. In addition to the structural investigation, we also report the thermodynamic excess properties of the grain boundaries, explore how they influence the thermodynamic stability of the grain boundary phases, and detail the commonalities and differences between the materials. read less

Abstract: The mechanical behavior of materials operating under high temperatures is strongly influenced by creep mechanisms such as dislocation climb, which is controlled by the diffusion of vacancies. However, atomistic simulations of these mechanisms have traditionally been impractical due to the long time scales required. To overcome these time scale challenges, we use Parallel Trajectory Splicing (ParSplice), an accelerated molecular dynamics method, to simulate dislocation climb in nickel. We focus on modeling the activity of a vacancy near a jog on an edge dislocation in order to observe vacancy pipe diffusion and vacancy absorption at the jog. From rigorously constructed trajectories encompassing more than 2000 vacancy absorption events over a simulation time of more than $4\phantom{\rule{0.222222em}{0ex}}\ensuremath{\mu}\mathrm{s}$ at 900 K, a comprehensive sampling of available atomistic mechanisms is collated and analyzed further with molecular statics calculations. We estimate average rates for pipe diffusion and vacancy absorption into the jog using data from the dynamic and static calculations, finding very good agreement. Our results strongly suggest that the dominant mechanism for vacancy absorption by jogs is via biased diffusion to the dislocation core followed by fast pipe diffusion to the jog. 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: ABSTRACT Understanding the evolution of dislocations and twinning at the crack front is critical for designing micro-mechanical systems with improved performance. In this work, the dislocation evolution at the crack front in thin pre-cracked FCC single crystals is correlated with the associated fracture toughness, which is shown to be dependent on material specific properties such as stable stacking fault energy and crystal orientation using atomistic simulations. For materials with high value, sessile dislocations form at the crack front causing increased localised plastic flow stress that leads to low fracture toughness. Whereas the fracture process in materials with low value is governed by the motion of glissile dislocations and stress-induced twinning leads to high fracture toughness. For this case, twinning occurs at high stress levels followed by un-twinning due to stress relaxation at crack front by twinning. The crystal orientation influences the type of dislocations emitted (screw/edge) from the crack front which governs the mode of crack propagation. The Mode-III crack propagation by the emission of screw type dislocations causes significant decrease in the fracture toughness compared to Mode I crack propagation which is caused by simultaneous emission of edge type dislocations on the two symmetrically inclined slip planes at the crack front. For certain pre-cracked crystal orientations, twinning is seen during the early stages of plastic deformation in materials with high value. However, un-twinning is not observed in crystal orientation-based twinning at the crack front. read less

Abstract: The importance of taking into account directional solidification of grains formed during 3D printing is determined by a substantial influence of their crystallographic orientation on the mechanical properties of a loaded material. This issue is studied in the present study using molecular dynamics simulations. The compression of an FCC single crystal of aluminum bronze was performed along the <111> axis. A Ni single crystal, which is characterized by higher stacking fault energy (SFE) than aluminum bronze, was also considered. It was found that the first dislocations started to move earlier in the material with lower SFE, in which the slip of two Shockley partials was observed. In the case of the material with higher SFE, the slip of a full dislocation occurred via successive splitting of its segments into partial dislocations. Regardless of the SFE value, the deformation was primarily occurred by means of the formation of dislocation complexes involved stair-rod dislocations and partial dislocations on adjacent slip planes. Hardening and softening segments of the calculated stress–strain curve were shown to correspond to the periods of hindering of dislocations at dislocation pileups and dislocation movement between them. The simulation results well agree with the experimental findings. read less

Abstract: We present a continuum model to determine the dislocation structure and energy of low angle grain boundaries in three dimensions. The equilibrium dislocation structure is obtained by minimizing the grain boundary energy that is associated with the constituent dislocations subject to the constraint of Frank's formula. The orientation-dependent continuous distributions of dislocation lines on grain boundaries are described conveniently using the dislocation density potential functions, whose contour lines on the grain boundaries represent the dislocations. The energy of a grain boundary is the total energy of the constituent dislocations derived from discrete dislocation dynamics model, incorporating both the dislocation line energy and reactions of dislocations. The constrained energy minimization problem is solved by the augmented Lagrangian method and projection method. Comparisons with atomistic simulation results show that our continuum model is able to give excellent predictions of the energy and dislocation densities of both planar and curved low angle grain boundaries. read less

Abstract: The shear-coupled grain boundary (GB) migration in bicrystal Ni with metallic dopant segregation was investigated by the molecular dynamics simulations. Different from the approximately linear relation of the GB migration of pure bicrystal Ni with the nominal shear strain, the curve of doped bicrystal Ni can be divided into three stages. The threshold strain, saturated strain, and saturated GB migration displacement can be used to characterize them. They are considerably affected by the Cr concentration in GB, temperature, and dopant type. The higher the dopant concentration is or the lower the temperature is, the greater the resistance to GB migration is. Cu dopant induces the greatest resistance, Cr and Fe dopants have great effect on the GB migration, but Co has almost no influence. All these hindering effects can be explained from the variation of the number of pinning points induced by the dopant atoms in GB. read less

Abstract: High-strength metal alloys achieve their performance via careful control of precipitates and solutes. The nucleation, growth, and kinetics of precipitation, and the resulting mechanical properties, are inherently atomic scale phenomena, particularly during early-stage nucleation and growth. Atomistic modeling using interatomic potentials is a desirable tool for understanding the detailed phenomena involved in precipitation and strengthening, which requires length and timescales far larger than those accessible by first-principles methods. Current interatomic potentials for alloys are not, however, sufficiently accurate for such studies. Here a family of neural-network potentials (NNPs) for the Al-Cu system are presented as a first example of a machine learning potential that can achieve near-first-principles accuracy for many different metallurgically important aspects of this alloy. High-fidelity predictions of intermetallic compounds, elastic constants, dilute solid-solution energetics, precipitate-matrix interfaces, generalized stacking fault energies and surfaces for slip in matrix and precipitates, antisite defect energies, and other quantities, are shown. The NNPs also captures the subtle entropically induced transition between ${\ensuremath{\theta}}^{\ensuremath{'}}$ and $\ensuremath{\theta}$ at temperatures around 600 K. Many comparisons are made with the state-of-the-art angular-dependent potential for Al-Cu, demonstrating the significant quantitative benefit of a machine learning approach. A preliminary kinetic Monte Carlo study shows the NNP to predict the emergence of GP zones in Al-4at%Cu at $T=300$ K in agreement with experiments. These studies show that the NNP has significant transferability to defects and properties outside the structures used to train the NNP but also shows some errors highlighting that the use of any interatomic potential requires careful validation in application to specific metallurgical problems of interest. read less

Abstract: Perturbative treatments of the lattice dynamics are widely successful for many crystalline materials; however, their applicability is limited for strongly anharmonic systems, metastable crystal structures and liquids. The full dynamics of these systems can, however, be accessed via molecular dynamics (MD) simulations using correlation functions, which includes dynamical structure factors providing a direct bridge to experiment. To simplify the analysis of correlation functions, here the dynasor package is presented as a flexible and efficient tool that enables the calculation of static and dynamical structure factors, current correlation functions as well as their partial counterparts from MD trajectories. The dynasor code can handle input from several major open source MD packages and thanks to its C/Python structure can be readily extended to support additional codes. The utility of dynasor is demonstrated via examples for both solid and liquid single and multi‐component systems. In particular, the possibility to extract the full temperature dependence of phonon frequencies and lifetimes is emphasized. read less

Abstract: Computer simulation shows that an increase of the volume V due to point defects in a simple metallic crystal (Al) and high entropy alloy (Fe20Ni20Cr20Co20Cu20) leads to a linear decrease of the shear modulus G. This diaelastic effect can be characterized by a single dimensionless parameter K = dln G/dln V. For dumbbell interstitials in single crystals K ≈ −30 while for vacancies the absolute K-value is smaller by an order of magnitude. In the polycrystalline state, K ≈ −20 but its the absolute value remains anyway 5–6 times larger than that for vacancies. The physical origin of this difference comes from the fact that dumbbell interstitials constitute elastic dipoles with highly mobile atoms in their nuclei and that is why produce much larger shear softening compared to vacancies. For simulated Al and high entropy alloy in the glassy state, K equals to −18 and −12, respectively. By the absolute magnitude, these values are by several times larger compared to the case of vacancies in the polycrystalline state of these materials. An analysis of the experimental data on isothermal relaxations of G as a function of V for six Zr-based metallic glasses tested at different temperatures shows that K is time independent and equals to ≈−43, similar to interstitials in single-crystals. It is concluded that K constitutes a important simple kinetic parameter indicating the origin of relaxations induced by point(-like) defects in the crystalline and glassy states. read less

Abstract: Radiation-induced defects are the significant cause of radiation damage in austenitic stainless steels. In this work, the interactions between a screw dislocation and stacking fault tetrahedrons (SFTs) are studied in Fe–10Ni–20Cr (a model alloy of austenitic stainless steel) and Ni, using molecular dynamics simulations. Four interaction processes were primarily found, including (1) partial absorption, (2) sheared, (3) bypassing, and (4) restored through double cross-slip. In Fe–10Ni–20Cr alloys, there is almost no correlation between the critical resolved shear stress (CRSS) value and SFT size, but the intersection position largely impacts the results. And the CRSS value decreases with temperature increasing in Fe–10Ni–20Cr. The occurrence of cross-slip has a significant effect on the interactions, and there exist greater CRSS value in the interactions of cross-slipping. Besides, the occurrence of the cross-slip is linked to the stacking fault energy and the distribution of solute atoms of the alloys. read less

Abstract: ABSTRACT A number of recent Molecular Dynamics (MD) simulations have demonstrated that screw dislocations in face centred cubic (fcc) metals can achieve stable steady state motion above the lowest shear wave speed ( ) which is parallel to their direction of motion (often referred to as transonic motion). This is in direct contrast to classical continuum analyses which predict a divergence in the elastic energy of the host material at a crystal geometry dependent ‘critical’ velocity . Within this work, we first demonstrate through analytic analyses that the elastic energy of the host material diverges at a dislocation velocity ( ) which is greater than , i.e. . We argue that it is this latter derived velocity ( ) which separates ‘subsonic’ and ‘supersonic’ regimes of dislocation motion in the analytic solution. In addition to our analyses, we also present a comprehensive suite of MD simulation results of steady state screw dislocation motion for a range of stresses and several cubic metals at both cryogenic and room temperatures. At room temperature, both our independent MD simulations and the earlier works find stable screw dislocation motion only below our derived . Nonetheless, in real-world polycrystalline materials cannot be interpreted as a hard limit for subsonic dislocation motion. In fact, at very low temperatures our MD simulations of Cu at 10 Kelvin confirm a recent claim in the literature that true ‘supersonic’ screw dislocations with dislocation velocities are possible at very low temperatures. read less

Abstract: An approach to the study of the mechanisms of shear deformation in the bulk of face centered cubic (FCC) single crystals based on molecular dynamics simulation is proposed. Similar shear patterns obtained experimentally, and in simulations, allow consideration of the effect of crystallographic and geometric factors on deformation mechanisms. Deformation of <001> single-crystal samples in the form of tetragonal prisms with {110} and {100} lateral faces and different height-to-width ratios was studied. The simulation showed that the sample vertices are the preferential sites for shear initiation. It was found that the formation of deformation domains and interaction of shear planes are caused by the geometry of shear planes in the bulk of the single crystal, i.e., by their location in relation to basic stress concentrators and by their orientations relative to the lateral faces. The deformation patterns obtained in the simulations were in good agreement with those observed in the experiments. The fractions of sliding dislocations and dislocation barriers were determined for different materials, taking into account the crystallographic and geometric factors. read less

Abstract: K-means clustering was carried out to identify the atomic structure of nanocrystalline aluminum. For this purpose, per-atom physical quantities were calculated by means of molecular dynamics simulations, such as the potential energy, stress components, and atomic volume. Statistical analysis revealed that potential energy, atomic volume and von Mises stress were relevant parameters to distinguish between fcc atoms and grain boundary atoms. These three parameters were employed with the K-means algorithm to establish two clusters, one corresponding to fcc atoms and another to GB atoms. When comparing the K-means classification performance with that of CNA, an F-1 score of 0.969 and a Matthews correlation coefficient of 0.859 were achieved. This approach differs from other traditional methods in that the quantities employed here do not require input settings such as the number of nearest neighbor nor a cut-off value. Therefore, K-means clustering could be eventually used to inspect the atomic structure in more complex systems. read less

Abstract: In this paper, a concurrent multiscale simulation strategy coupling atomistic and continuum models was proposed to investigate the three-dimensional contact responses of aluminum single crystal under both dry and lubricated conditions. The Hertz contact is performed by using both the multiscale and full molecular dynamics (MD) simulations for validation. From the contact area, kinetic energy and stress continuity aspects, the multiscale model shows good accuracy. It can also save at least five times the computational time compared with the full MD simulations for the same domain size. Furthermore, the results of lubricated contact show that the lubricant molecules could effectively cover the contact surfaces; thereby separating the aluminum surfaces and bearing the support loads. Moreover, the surface topography could be protected by the thin film formed by the lubricant molecules. It has been found that the contact area decreases obviously with increasing the magnitude of load under both dry and lubricated contacts. Besides, a decrease in contact area is also seen when the number of lubricant molecules increases. The present study has confirmed that the dimension of lubricated contacts could be greatly expanded during the simulation using the proposed multiscale method without sacrificing too much computational time and accuracy. read less

Abstract: Faceted grain boundaries can migrate in interesting and unexpected ways. For example, faceted {\Sigma}11 tilt grain boundaries were observed to exhibit mobility values that could be strongly dependent on the direction of migration. In order to understand whether this directionally-anisotropic mobility is a general phenomenon and to isolate mechanistic explanations for this behavior, molecular dynamics simulations of bicrystals evolved under an artificial driving force are used to study interface migration for a range of boundary plane inclination angles and temperatures in multiple face centered cubic metals (Al, Ni, and Cu). We find that directionally-anisotropic mobility is active in a large fraction of these boundaries in Ni and Cu and should therefore impact the coarsening of polycrystalline materials. On the other hand, no such anisotropy is observed in any of the Al boundaries, showing that this behavior is material-dependent. Migration of the faceted boundaries is accomplished through transformation events at facet nodes and incommensurate boundary plane facets, which are termed shuffling modes. Three major shuffling modes have been identified, namely Shockley shuffling, slip plane shuffling, and disordered shuffling. A shift from the first two ordered modes to the third disordered mode is found to be responsible for reducing or removing directionally-anisotropic mobility, especially at the highest temperatures studied. read less

Abstract: ABSTRACT Uniaxial deformation was performed using molecular dynamics to estimate the mechanical properties of nanocrystalline aluminium. It was observed that the stacking faults and sliding of the grain boundaries affected the mechanical properties. In addition, accumulation of atoms near grain boundaries during deformation hardened the nanocrystalline material as the grain diameter increased (reverse Hall-Petch relation). Further, the effects of strain rate and temperature were investigated with various mean grain diameters. Investigation showed that mechanical properties were independent of tested strain rates (109–1010 s−1) and that the nanocrystalline material softened with increasing temperature. The elastic modulus was then compared to experimental results from literature at room temperature. The change in crystalline structure was observed with respect to percent strain and various mean grain diameters of nanocrystalline aluminium. It was observed that stacking faults increased with decreased mean grain diameter, which led to reduced mechanical properties. read less

Abstract: ABSTRACT The purpose of this study is to explore the hyperelastic effect on the energy release rate of a crack extension at the nanoscale. A molecular statics computation has been carried out to characterise the atomistic nature of fracturing. The concept of the J-integral is employed to measure the energy release rate for Ni and Si single crystals having a hyperelastic nature. The obtained J-integral is compared to the energy release rates predicted using the two-specimen method and the finite-element method. The results show that the effect of the highly-localized nonlinear zone in front of the crack tip potentially induces a breakdown of the linear elastic fracture mechanics model. read less

Abstract: This study unveils C3N, a new material that serves as an excellent reinforcement to enhance the mechanical properties of aluminum using a molecular dynamics simulation method. Results show that the C3N nanosheets greatly improve the mechanical properties of aluminum-based nanocomposites. With only 1.3 wt% C3N, the Young's modulus, fracture strength, and fracture strain increased by 27, 70, and 51 percent, respectively. A comparison between the reinforcement of graphene and C3N in an aluminum (Al) matrix shows that in terms of the mechanical properties, the graphene–aluminum composite is weaker than the C3N–aluminum composite in the tensile tests, but slightly stronger in the energy adsorption tests. Our findings show that the mechanical properties are highly dependent on the strain rate and temperature. The effects of various imperfections, such as the vacancy, crack, and void defects, on the mechanical properties were also studied. Results show that in the presence of void defects, the structure exhibited higher mechanical properties than when there were other defects. This phenomenon was found to be related to the decrease in the effective load transfer from aluminum to C3N. Furthermore, by increasing the weight percent of the nanosheets up to 5%, the energy absorption rate increased by 25% compared to the pure aluminum. When C3N was placed on top of the aluminum surface, the silicon nanoparticles were associated with a 35% energy adsorption by the nanocomposite. The results of this paper could be used to help understand and overcome some limitations in the fabrication of metallic nanocomposites with 2D material reinforcement. read less

Abstract: A mixed radial, angular three-body distribution function g3(rBC, θABC) is introduced, which allows the local atomic order to be more easily characterized in a single graph than with conventional correlation functions. It can be defined to be proportional to the probability of finding an atom C at a distance rBC from atom B while making an angle θABC with atoms A and B, under the condition that atom A is the nearest neighbor of B. As such, our correlation function contains, for example, the likelihood of angles formed between the nearest and the next-nearest-neighbor bonds. To demonstrate its use and usefulness, a visual library for many one-component crystals is produced first and then employed to characterize the local order in a diverse body of elemental condensed-matter systems. Case studies include the analysis of a grain boundary, several liquids (argon, copper, and antimony), and polyamorphism in crystalline and amorphous silicon including that obtained in a tribological interface. read less

Abstract: This work investigates the deformation and dynamic property of the pre-existing void or helium (He) bubble in aluminum (Al) under compression and tension with molecular dynamics simulations, where both the uniaxial and triaxial loadings at a high strain rate are considered. Under compression, the void completely collapses by plastic deformation and after that a recrystallization process is found. The He bubble undergoes a finite collapse because of its internal pressure. Solidification of the He bubble is also observed at a sufficiently high pressure. Moreover, stacking fault tetrahedrons (SFTs) are found under the triaxial compression, instead of the shear dislocation loops under the uniaxial compression. Under tension, the volume of the void or He bubble is nearly proportional to the strain before instability and begins to grow sharply after instability. The instability volumes of the void or He bubble and the corresponding strains approximately satisfy a linear reduction distribution. When the pre-existing void collapses completely, new void nucleation will appear under tension. Interestingly, the triaxial tension produces a large void from the vertex of SFTs, while uniaxial tension produces many small voids at the intersection of the stacking faults. As for the He bubble, it will first be elongated along the tension direction under uniaxial tension. With the increase of tension strain, the void or He bubble grows into an irregular polyhedron and tends to be isotropic for both uniaxial and triaxial tension. In the meantime, the He atoms will deposit on the boundary eventually because the attraction between He and Al is included. In addition, the difference of temperature and pressure between the He bubble and Al matrix is discussed, and an empirical model is proposed to describe the He bubble pressure during the loading process. read less

Abstract: Nanoindentation simulations are performed for a Ni(111) bi-crystal, in which the grain boundary is coated by a graphene layer. We study both a weak and a strong interface, realized by a 30∘ and a 60∘ twist boundary, respectively, and compare our results for the composite also with those of an elemental Ni bi-crystal. We find hardening of the elemental Ni when a strong, i.e., low-energy, grain boundary is introduced, and softening for a weak grain boundary. For the strong grain boundary, the interface barrier strength felt by dislocations upon passing the interface is responsible for the hardening; for the weak grain boundary, confinement of the dislocations results in the weakening. For the Ni-graphene composite, we find in all cases a weakening influence that is caused by the graphene blocking the passage of dislocations and absorbing them. In addition, interface failure occurs when the indenter reaches the graphene, again weakening the composite structure. read less

Abstract: ABSTRACT We propose a dislocation adsorption-based mechanism for void growth in metals, wherein a void grows as dislocations from the bulk annihilate at its surface. The basic process is governed by glide and cross-slip of dislocations at the surface of a void. Using molecular dynamics simulations we show that when dislocations are present around a void, growth occurs more quickly and at much lower stresses than when the crystal is initially dislocation-free. Finally, we show that adsorption-mediated growth predicts an exponential dependence on the hydrostatic stress, consistent with the well-known Rice-Tracey equation. GRAPHICAL ABSTRACT IMPACT STATEMENT Void growth during ductile rupture is mediated by dislocation adsorption, not dislocation emission as previously theorized. The mechanism is consistent with Rice-Tracey scaling and observations of voids near cell walls. read less

Abstract: In order to study the effects of kink-like defects in twin boundaries on deformation mechanisms and interaction between dislocations and defects in twin boundaries under localized load, nanotwinned Cu with two defective twin (TDT) boundaries is compared with the nanotwinned Cu with two perfect twin (TPT) boundaries, and nanotwinned Cu with single defective twin (SDT) boundary and single perfect twin boundary by simulating spherical nanoindentations using molecular mechanics. The indenter force-depth and hardness-contact strain responses were analyzed. Results show that the existence of intrinsic defects in twin boundary could reduce the critical load and critical hardness of nanotwinned material. A quantitative parameter was first proposed to evaluate the degree of surface atom accumulation around the indenter during nanoindentation, and it can be inferred that the surface morphology in TDT changes more frequently than the surface morphologies in TPT and SDT. The atomistic configurations of incipient plastic structures of three different models were also analyzed. We found that the intrinsic defects in twin boundary will affect the incipient plastic structures. The formation of twinning partial slip on the defective twin boundary happens before the contact of the dislocation and twin boundary. The kink-like defects could introduce Frank partial dislocation to the twin boundary during interaction between dislocation and twin boundary, which was not detected on the perfect twin boundary. In addition, the area of twinning partial slips on the upper twin boundary in the incipient plastic structures in SDT and TDT are larger than the twinning partial slip area in TPT, which results in the reduction of the critical hardness in SDT and TDT. The kink-like defects could also block the expansion of twinning partial slip on the twin boundary. Furthermore, we investigated the dislocation transmission processes in three different models. It is found that the dislocation transmission event could be delayed in model containing single defective twin boundary, while the transmission process could be advanced in model containing two consecutive defective twin boundaries. The quantitative analysis of dislocation length was also implemented. Result shows that the main emitted dislocation during nanoindentation is Shockley partial, and the dislocation nucleation in SDT and TDT is earlier than the dislocation nucleation in TPT due to the existence of defects. It is inferred that the intrinsic defects on twin boundaries could enhance the interaction between dislocations and twin boundaries, and could strongly change the structure evolution and promote the dislocation nucleation and emission. These findings about kink-like defects in twin boundaries show that the inherent kink-like defects play a crucial role in the deformation mechanisms and it should be taken into consideration in future investigations. Single defective twin boundary structure is recommended to delay the transmission and block the expansion of twin boundary migration. Some of the results are in good agreement with experiments. read less

Abstract: Faceted grain boundaries, where grain boundary area is increased in the name of producing low-energy segments, can exhibit new and unexpected migration trends. For example, several faceted Ʃ3 boundaries have demonstrated anti-thermal and thermally damped mobility. Ʃ11 ⟨110⟩ tilt boundaries represent another promising but relatively unexplored set of interfaces, with a (113) low-energy plane that can lead to faceting. In this study, molecular dynamics simulations are used to study grain boundary migration of an asymmetric Ʃ11 ⟨110⟩ grain boundary in two face centered cubic metals. Mobility of this boundary in Cu is strongly dependent on the direction of the applied driving force. The mobility anisotropy generally becomes smaller, but does not disappear completely, as temperature is increased. In contrast, the same boundary in Al demonstrates similar mobilities in either direction, illustrating that the anisotropic mobility phenomenon is material-dependent. Finally, relationships between stacking fault energy, facet junction defect content, and boundary crystallography are uncovered that may inform future studies of faceted grain boundaries. read less

Abstract: The total energy of an atomistic dislocation includes contributions from the inelastic/large-distortion ‘core’ region. Capturing this inelastic ‘core’ energy is important, especially for dislocations with a curvature in the 10–100 nm scale. Current implementations of discrete dislocation dynamics (DDD) mesoscale simulations either approximate or neglect the core energy and so do not provide consistency with fully-atomistic studies. Using established interatomic potentials for FCC metals, the total dislocation energy is computed directly in atomistic simulations of straight dislocations and a core energy at any desired cut-off core radius is obtained as a function of dislocation character. A proper introduction of the atomistic core energy into the ParaDiS DDD code that uses a non-singular theory (Cai et al 2006 J. Mech. Phys. Solids 54 561–87) is then presented. The resulting atomistically-informed ParaDiS DDD is used to simulate the periodic bow-out of edge and screw dislocations in near-elastically-isotropic aluminum at various length and stress, with comparisons to fully-atomistic simulations. Generally good agreement is obtained between DDD and atomistics, with the best agreement achieved using a non-singular regularization parameter in the range of a = 5 – 10b. The analysis is then extended to compute the core energy of the Shockley partial dislocations that arise in the dissociation of perfect dislocations in fcc metals. read less

Abstract: The micropillar compression test is a novel experiment to study the mechanical properties of materials at small length scales of micro and nano. The results of the micropillar compression experiments show that the strength of the material depends on the pillar diameter, which is commonly termed as size effects. In the current work, first, the experimental observations and theoretical models of size effects during micropillar compression tests are reviewed in the case of crystalline metals. In the next step, the recent computer simulations using molecular dynamics are reviewed as a powerful tool to investigate the micropillar compression experiment and its governing mechanisms of size effects. read less

Abstract: In this study, molecular dynamics simulation of the ultrasonic welding of aluminum alloys is presented. Ultrasonic metal welding is counted as a solid state consolidation of the mating parts, in which the growing interface temperature is far below the melting point of the material and the energy consumption is relatively low compared to the common welding processes such as arc welding. The reciprocating motion of the sonotrode on the mating parts combined with the application of external pressure in the process of ultrasonic welding is the source of temperature rise and strong plastic deformations at the mating interface. This contribution is devoted to study the interactive effect of the process parameters on the interface temperature, deformation process of the mating parts and the diffusion behavior of the interface atoms. read less

Abstract: The embrittling or strengthening effect of solute atoms at grain boundaries (GBs), commonly known as the embrittling potency, is an essential thermodynamic property for characterizing the effects of solute segregation on GB fracture. One of the more technologically relevant material systems related to embrittlement is the Ni–S system where S has a deleterious effect on fracture behavior in polycrystalline Ni. In this work, we develop a Ni–S embedded-atom method (EAM) interatomic potential that accounts for the embrittling behavior of S at Ni GBs. Results using this new interatomic potential are then compared to previous density functional theory studies and a reactive force-field potential via a layer-by-layer segregation analysis. Our potential shows strong agreement with existing literature and performs well in predicting properties that are not included in the fitting database. Finally, we calculate embrittling potencies and segregation energies for six [100] symmetric-tilt GBs using the new EAM potential. We observe that embrittling potency is dependent on GB structure, indicating that specific GBs are more susceptible to sulfur-induced embrittlement. read less

Abstract: The influences of indenter shape on dislocation actives and stress distributions during nanoindentation were studied by using molecular dynamics (MD) simulation. The load-displacement curves, indentation-induced stress fields, and dislocation activities were analyzed by using rectangular, spherical, and Berkovich indenters on single crystal nickel. For the rectangular and spherical indenters, the load-displacement curves have a linear dependence, but the elastic stage produced by the spherical indenter does not last longer than that produced by the rectangular indenter. For a Berkovich indenter, there is almost no linear elastic regime, and an amorphous region appears directly below the indenter tip, which is related to the extremely singular stress field around the indenter tip. In three indenters cases, the prismatic dislocation loops are observed on the {111} planes, and there is a sudden increase in stress near the indenter for the Berkovich indenter. The stress distributions are smooth with no sudden irregularities at low-indentation depths; and the stress increases and a sudden irregularity appears with the increasing indentation depths for the rectangular and spherical indenters. Moreover, the rectangular indenter has the most complex dislocation activities and the spherical indenter is next, while very few dislocations occur in the Berkovich indenter case. read less

Abstract: We implement non-equilibrium Green's function (NEGF) calculations to investigate thermal transport across graphene/metal interfaces with interlayer van der Waals interactions to understand the factors influencing thermal conductance across the interface. It is found that interfaces with a smaller interfacial lattice mismatch, lighter metal substrate and stronger interfacial bonding strength will show better interfacial thermal transport abilities. Strain induced by the interfacial lattice mismatch in graphene is the key factor for the decrease of interfacial phonon transmission in the main frequency range of metals, which finally results in a decrease of interfacial thermal conductance. A comprehensive interfacial influencing factor is proposed combining the factors of graphene density, metal density and interfacial binding energy to realize the prediction of interfacial thermal conductance across the graphene/metal interface. The results are hoped to promote the understanding of the thermal transport mechanism and design of graphene based 2D/3D materials interfaces. read less

Abstract: Polycrystalline metals have flow stress two to three orders of magnitude lower than the theoretical shear strength estimated by Frenkel model. This significant strength difference is primarily due to the presence of defects, such as dislocations and grain boundaries. However, it was experimentally found that defect-free nanoscale objects (whiskers, nanopillars, etc.) can exhibit strength close to the theoretical limit. With the development of nanotechnology, interest in the study of the theoretical strength of metals and alloys has grown significantly. It is important to find reliable criteria of lattice instability when homogeneous nucleation of defects begins during deformation of an ideal crystal lattice. Note that the Frenkel estimation does not take into account thermal vibrations of atoms and attempts are being made to take into account the effect of temperature on the theoretical strength of defect-free crystals. In this paper, using molecular dynamics simulation, we study shear deformation in the direction of ( )[ ] 111 112 for single crystals of copper and aluminum in the temperature range from 0 to 400 K. Lattice instability was evaluated using two criteria: (i) macroscopic criterion, which is related to the loss of positive definiteness of the stiffness tensor, and (ii) a microscopic criterion related to the formation of a stacking fault, which leads to a drop of the applied shear stress. It was demonstrated that both criteria are consistent at low temperatures, but the macroscopic criterion is less reliable at higher temperatures. read less

Abstract: ABSTRACT Ultra-fine grained copper with nanotwins is found to be both strong and ductile. It is expected that nanocrystalline metals with lamella grains will have strain hardening behaviour. The main unsolved issues on strain hardening behaviour of nanocrystalline metals include the effect of stacking fault energy, grain shape, temperature, strain rate, second phase particles, alloy elements, etc. Strain hardening makes strong nanocrystalline metals ductile. The stacking fault energy effects on the strain hardening behaviour are studied by molecular dynamics simulation to investigate the uniaxial tensile deformation of the layer-grained and equiaxed models for metallic materials at 300 K. The results show that the strain hardening is observed during the plastic deformation of the layer-grained models, while strain softening is found in the equiaxed models. The strain hardening index values of the layer-grained models decrease with the decrease of stacking fault energy, which is attributed to the distinct stacking fault width and dislocation density. Forest dislocations are observed in the layer-grained models due to the high dislocation density. The formation of sessile dislocations, such as Lomer–Cottrell dislocation locks and stair-rod dislocations, causes the strain hardening behaviour. The dislocation density in layer-grained models is higher than that in the equiaxed models. Grain morphology affects dislocation density by influencing the dislocation motion distance in grain interior. read less

Abstract: Mixed-type dislocations are prevalent in metals and play an important role in their plastic deformation. Key characteristics of mixed-type dislocations cannot simply be extrapolated from those of dislocations with pure edge or pure screw characters. However, mixed-type dislocations traditionally received disproportionately less attention in the modeling and simulation community. In this work, we explore core structures of mixed-type dislocations in Al using three continuum approaches, namely, the phase-field dislocation dynamics (PFDD) method, the atomistic phase-field microelasticity (APFM) method, and the concurrent atomistic-continuum (CAC) method. Results are benchmarked against molecular statics. We advance the PFDD and APFM methods in several aspects such that they can better describe the dislocation core structure. In particular, in these two approaches, the gradient energy coefficients for mixed-type dislocations are determined based on those for pure-type ones using a trigonometric interpolation scheme, which is shown to provide better prediction than a linear interpolation scheme. The dependence of the in-slip-plane spatial numerical resolution in PFDD and CAC is also quantified. read less

Abstract: The spall failure phenomenon is investigated at nano-scale in a plate-on-plate impact configuration using large-scale molecular dynamics (MD) high performance computing (HPC) simulations on three multi-billion 0.5 μm thick nanocrystalline aluminum (nc-Al) systems, each with an average grain size in the range of 18–100 nm and each under impact velocities in the range of 0.7–1.5 km s−1. Using a material conserving atom section-averaging process, distributions of several mechanical responses were obtained and the extremely transient spall failure phenomena were located in the interaction zones of the stress release waves released from the impacted and free ends of the atom systems. The spall zones’ thicknesses were estimated along with the spall strengths. For the nc-Al atom systems, the spall strength was observed to increase as the impact velocity increased from 0.7 to 1.5 km s−1, but only showed a slight decrease as the grain size increased from 18 to 100 nm. The spall strengths thus estimated from the stress release waves’ interaction zones were found to be conservative when compared to the traditional estimates obtained from the pullback velocity signatures in the free-end velocity evolutions. The MD results were further analyzed using a crystal analysis algorithm and a twin dislocation identification method to obtain the densities of the atomistic microstructures evolving under the interaction of the stress release waves. High-fidelity large-scale HPC simulation results showed that certain dislocation partials strongly influenced the spall response. The Stair-rod type dislocation partials increased by more than a factor of 10 during the interaction of the stress release waves as the spall failure commenced. read less

Abstract: The paper presents molecular dynamics and -statics simulations of a prototypical mono-atomic metallic system (aluminum) and its defects in the crystalline and glassy states. It is shown that there is a thermodynamic driving force for the association of dumbbell interstitials in the crystalline lattice into clusters consisting of different amounts of defects. Clusters containing seven interstitials constitute perfect icosahedra. Within the general framework of the interstitialcy theory, melting of simple metallic crystals is intrinsically related to a rapid increase of the concentration of dumbbell interstitials, which remain identifiable structural units in the liquid state. Then, the glass produced by rapid melt quenching contains interstitial-type defects. The idea of the present work is to argue that the major structural feature of many metallic glasses—icosahedral ordering—originates from the clustering of interstitial-type defects frozen-in upon melt quenching. Separate defects and their small clusters represent the defect part of the glassy structure. read less

Abstract: ABSTRACT The effects of sub-surface hydrogen and mixed mode loading on dislocation emission in aluminium are studied using a combination of techniques including crack simulations with an empirical interatomic potential, generalised stacking fault energy (GSF) calculations, with empirical interactions and Density Functional Theory, and the model by Rice which links the critical stress intensity factor to the unstable stacking energy. The crack orientation is and the loading is composed of a moderate traction along and a shear along , such that Shockley partials are emitted along the crack plane. The role of the relaxations around the H atoms and of the concentration of H in the glide plane, in the GSF calculation, is revealed by comparing Rice's model to the results of brute force simulations. The enhanced GSF is then calculated ab initio. The conclusion is a large decrease of the critical load to emit a dislocation, due to the displacement transverse to the glide direction. The effect of sub-surface hydrogen is negligible with respect to the mechanical one. read less

Abstract: Molecular dynamics study of the plasticity nucleation mechanisms in a Ni nanocrystalline sample under shear loading in the constrained conditions was carried out. The studied Ni sample consisted of nine grains of the same size with large misorientation angles relative to each other. In one of the directions, grippers were simulated, to which compressive forces and shear with a constant velocity were applied. In two other directions, periodic boundary conditions were used. It is shown that plasticity nucleation occurs in the region of the triple junction. At the same time, in the region of the triple junction, in the zone of which the stacking fault will be formed, tensile stresses are realized along one of the adjacent grain boundaries, and compressive stresses occur along the other. An increase in stresses in the triple junction zone leads to the formation of a stacking fault, which moves to the volume of one of the grains. Another mechanism of plasticity in nanocrystalline nickel is the migration of grain boundaries, which leads to a significant change in grain sizes. read less

Abstract: The ductile versus brittle fracture in crystalline materials depends on the relative values of KIc and KIe as defined by well-known Rice theory, where KIc and KIe are the critical values of stress intensity factor corresponding to cleavage and dislocation emission, respectively. For KIc < KIe, the brittle fracture (or cleavage) takes place in atomically sharp pre-cracked crystal subjected to Mode I loading. For KIe < KIC, the dislocations are emitted from the crack front resulting in ductile fracture. To this end, molecular static simulations are used to explain the crystal orientation dependent fracture behaviour of FCC single crystal and its contradiction with respect to Rice theory based on stress triaxiality at the crack front. The stress triaxiality at crack front changes with crystal orientation due to transformation of stiffness tensor Cijkl. It is shown that high stress triaxiality suppressed the dislocation initiation leading to cleavage failure even for the case when KIe < KIc. read less

Abstract: Significance Many materials of industrial and scientific interest (including metals and ceramics) are polycrystalline. The defect microstructure of these materials has a profound impact on their properties and utility. Microstructure engineering yields materials with greatly enhanced qualities, but the microstructure typically evolves over time via the motion of grain boundaries and triple junctions. One must understand and guide this evolution to produce reliable enhanced materials. Much has been discovered in recent decades regarding the motion of grain boundaries—less so regarding triple junctions. This work presents observations of triple-junction migration from atomistic simulations, explains these observations by extending recent developments in grain boundary theory to triple junctions, and presents a continuum model of triple-junction migration based on this theory. Grain boundary (GB) migration in polycrystalline materials necessarily implies the concurrent motion of triple junctions (TJs), the lines along which three GBs meet. Today, we understand that GB migration occurs through the motion of disconnections in the GB plane (line defects with both step and dislocation character). We present evidence from molecular dynamics grain growth simulations and idealized microstructures that demonstrates that TJ motion and GB migration are coupled through disconnection dynamics. Based on these results, we develop a theory of coupled GB/TJ migration and use it to develop a physically based, disconnection mechanism-specific continuum model of microstructure evolution. The continuum approach provides a means of reducing the complexity of the discrete disconnection picture to extract the features of disconnection dynamics that are important for microstructure evolution. We implement this model in a numerical, continuum simulation and demonstrate that it is capable of reproducing the molecular dynamics (MD) simulation results. read less

Abstract: ABSTRACT In the continuum context, the displacements of atoms induced by a dislocation can be approximated by a continuum disregistry field. In this work, two phase-field (PF)-based approaches and their variants are employed to calculate the disregistry fields of static, extended dislocations of pure edge and pure screw character in two face-centred cubic metals: Au and Al, which have distinct stable stacking fault energy and elastic anisotropy. A new truncated Fourier series form is developed to approximate the generalised stacking fault energy (GSFE) surface, which shows significant improvement over the previously employed Fourier series form. By measuring the intrinsic stacking fault (ISF) width and partial dislocation core size in different ways, the PF-based disregistry fields are quantitatively compared against those predicted by molecular statics. In particular, two new measures for the ISF widths are proposed and shown to overcome drawbacks of the more commonly used standards in the literature. Our calculations also show that continuum formulation of the elastic energy and the GSFE for a homogeneous surface can successfully characterise the core structure. Last, our comparisons highlight the significance of including the gradient energy in the free energy formulation when an accurate description of the dislocation core structure is desired. read less

Abstract: In the community of computational materials science, one of the challenges in hierarchical multiscale modeling is information-passing from one scale to another, especially from the molecular model to the continuum model. A machine-learning-enhanced approach, proposed in this paper, provides an alternative solution. In the developed hierarchical multiscale method, molecular dynamics simulations in the molecular model are conducted first to generate datasets, which represents physical phenomena at the nanoscale. The datasets are then used to train neural networks for failure classification and stress regressions. Finally, the well-trained learning machines are implemented in the continuum model to study the mechanical behaviors of materials at the macroscale. Randomized neural networks are employed due to their computational efficiency. read less

Abstract: A molecular dynamics (MD) simulation was performed on the nanocrystalline (NC) Cu with an edge notch under tensile loadings, with focus on the notch sensitivity. With the increase of notch size, the dominant deformation of material changes from the shear strain, which spreads throughout the entire sample, to a single shear band, which is induced by the stress concentration at the notch root. At the same time, the samples move from notch-insensitivity to notch-sensitivity. These findings offer significant guidelines for the application of NC Cu in engineering. read less

Abstract: HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Thermal fluctuations of dislocations reveal the interplay between their core energy and long-range elasticity Pierre-Antoine Geslin, David Rodney read less

Abstract: A hybrid atomistic-continuum method comprising molecular dynamics combined with a two-temperature model (MD-TTM) is used to investigate the ultra-fast laser shock compression and spallation behavior of pure Al films. The laser material interaction, as predicted using MD-TTM models, suggests laser melting followed by the creation of a compressive shock wave that travels through the metal followed by wave reflections and interactions to initiate spallation failure. MD-TTM simulations investigate the influence of laser parameters by varying the laser fluence values from 0.5 to 13 kJ/m2 and a duration of 150 fs for the [001] orientation. The microstructural response during the various stages that lead to dynamic failure of single crystal Al is studied by characterizing the temporal evolution of the solid-liquid interface, shock wave structure, defect evolution (dislocations and stacking faults), as well as void nucleation and spall failure. The hybrid method is also used to investigate the microstructure evolution during compression and spall failure for the [110] and [111] orientations for the same laser loading conditions. The variations in the spall strengths observed for the variations in strain rates and shock pressures generated suggest that the evolution of microstructure plays an important role in determining the spall strength of the metal. The analysis of defect structures generated suggests that the spall strength is determined by the density of stair-rod partials in the microstructure simulations with the highest spall strength corresponding to the lowest number of stair-rod partials in the metal.A hybrid atomistic-continuum method comprising molecular dynamics combined with a two-temperature model (MD-TTM) is used to investigate the ultra-fast laser shock compression and spallation behavior of pure Al films. The laser material interaction, as predicted using MD-TTM models, suggests laser melting followed by the creation of a compressive shock wave that travels through the metal followed by wave reflections and interactions to initiate spallation failure. MD-TTM simulations investigate the influence of laser parameters by varying the laser fluence values from 0.5 to 13 kJ/m2 and a duration of 150 fs for the [001] orientation. The microstructural response during the various stages that lead to dynamic failure of single crystal Al is studied by characterizing the temporal evolution of the solid-liquid interface, shock wave structure, defect evolution (dislocations and stacking faults), as well as void... read less

Abstract: The technique of molecular statics (MS) simulation was employed to determine the crack growth resistance curve of Cu and Ni single crystals. Copper and Ni single crystal nanoplates with an edge crack subjected to a tensile displacement were simulated. Stress-displacement curves and snapshots of the atomic configuration corresponding to different displacement levels were presented to elucidate the deformation mechanism. It was observed that the edge crack propagated step by step in a brittle manner, and the amount of crack growth at each step was half the lattice parameter. Through an energy consideration, the critical strain energy release rate at the onset of crack propagation and the crack growth resistance were calculated. The crack growth resistance is larger than the critical strain energy release rate because of the crack growth effect. read less

Abstract: Atomistic modeling of fracture is intended to illuminate the complex response of atoms in the very high stressed region just ahead of a sharp crack. Accurate modeling of the atomic scale fracture is crucial for describing the intrinsic nature of a material (intrinsic ductility/brittleness), chemical effects in the crack-tip vicinity, the crack interaction with different defects in solids such as grain boundaries, solutes, precipitates, dislocations, voids, etc. Here, different methods for atomistic modeling of fracture are compared in their ability to obtain quantitatively useful results that are in agreement with the basic principles of linear elastic fracture mechanics (LEFM). We demonstrate that the complicated atomic crack-tip behavior is precisely described in simulations of semi-infinite cracks, where the loading is uniquely controlled by the applied stress intensity factor K. Such ‘K-test’ simulations are shown to be equally applicable in crystalline and amorphous materials, and to be suitable for quantitative evaluation of various critical stress intensity factors, the overall material fracture toughness, and quantitative comparison with theories. We further demonstrate that the simulation of a nanoscale center-crack tension (CCT) specimen often leads to the results that do not satisfy the conditions for application of LEFM. The simulated intrinsic fracture toughness, one of the basic material properties, using CCT test geometry is shown to be dependent on the crack size and far-field loading. In general, this study resolves quantitative differences between several methods for atomistic modeling of fracture and recommends that application of simulations based on nanoscale finite size cracks not be pursued. read less

Abstract: Nanotwinned metals exhibit superior mechanical properties due to unique dislocation–twin boundary interactions. In the present work, we elucidate the microscopic deformation mechanisms and their correlations with the macroscopic mechanical response of nanotwinned Al containing inclined twin boundaries under nanoindentation by means of molecular dynamics simulations. The effect of twin boundary orientation with respect to the indented surface on the nanoindentation is evaluated. Simulation results reveal that dislocation slip, dislocation–twin boundary interaction, and twin boundary migration operate in parallel in the plastic deformation of nanotwinned Al. The inclination angle of twin boundaries with respect to indented surface has a strong influence on the interaction between individual deformation modes, which in turn leads to the anisotropic indentation behavior of nanotwinned Al. read less

Abstract: A set of embedded atom method model interatomic potentials is presented to represent a high-entropy alloy with five components. The set is developed to resemble but not model precisely face-centered cubic (fcc) near-equiatomic mixtures of Fe–Ni–Cr–Co–Cu. The individual components have atomic sizes deviating up to 3%. With the heats of mixing of all binary equiatomic random fcc mixtures being less than 0.7 kJ/mol and the corresponding value for the quinary being −0.0002 kJ/mol, the potentials predict the random equiatomic fcc quinary mixture to be stable with respect to phase separation or ordering and with respect to bcc and hcp random mixtures. The details of lattice distortion, strain, and stress states in this phase are reported. The standard deviation in the individual nearest neighbor bond lengths was found to be in the range of 2%. Most importantly, individual atoms in the alloy were found to be under atomic strains up to 0.5%, corresponding to individual atomic stresses up to several GPa. read less

Abstract: ABSTRACT Adopting the bonded interface technique for wear experiments under vacuum, this paper reports the nature of the localised shear bands that appear at the different deformation zones of the subsurface of aluminium under different sliding conditions. The plastic deformations are mapped under both low load/low sliding velocities as well as high load and high sliding velocities. A monotonic change in local plastic strain as a function of depth at low sliding velocities give way to a discontinuity separating two different zones with differing plastic behaviour for high sliding speed wear test. Besides shear bands, bonded interface also reveals the presence of kinks particularly in the samples subjected to wear test with high sliding velocities. A molecular dynamic simulation of the wear process successfully replicated the experimental observation, thus allowing us to discuss the mechanism of subsurface deformation during the wear process in the absence of any significant oxide layer for aluminium under sliding condition. read less

Abstract: The Hugoniot elastic limit (HEL, or the shock precursor) decay phenomenon was investigated under an uniaxial strain condition, in a plate-on-plate impact configuration, using large-scale molecular dynamics (MD) high performance computing (HPC) simulations on a multi-billion 5000 Å thick nanocrystalline aluminum (nc-Al) system with an average grain size of 1000 Å and at five impact velocities ranging from 0.7 to 1.5 km s−1. The averaged stress and strain distributions were obtained in the shock fronts’ travel direction using a material conserving atom slicing method. The loading paths in terms of the Rayleigh lines experienced by the atom system in the evolving shock fronts exhibited a strong dependency on the shock stress levels. This dependency decreased as the impact velocity increased from 0.7 to 1.5 km s−1. By combining the HELs from MD results with plate impact experimental data, the precursor decay for the nc-Al was predicted from nano-to-macro scale thickness range. The evolving shock fronts were characterized in terms of parameters such as the shock front thickness, shock rise time and strain rate. The MD results were further analyzed using a crystal analysis algorithm and a twin dislocation identification method to obtain the densities of the atomistic defects evolving behind the evolving shock fronts. High-fidelity large-scale HPC simulation results showed that certain dislocation partials strongly influenced the elastic–plastic transition response across the HELs. The twinning dislocations increased by more than a factor of 10 during the transition and remained constant under further shock compression. read less

Abstract: Continuum modeling of finite temperature mechanical behavior of atomic systems requires refined description of atomic motions. In this paper, we identify additional kinematical quantities that are relevant for a more accurate continuum description as the system is subjected to step-wise loading. The presented formalism avoids the necessity for atomic trajectory mapping with deformation, provides the definitions of the kinematic variables and their conjugates in real space, and simplifies local work conjugacy. The total work done on an atom under deformation is decomposed into the work corresponding to changing its equilibrium position and work corresponding to changing its second moment about equilibrium position. Correspondingly, we define two kinematic variables: a deformation gradient tensor and a vibration tensor, and derive their stress conjugates, termed here as static and vibration stresses, respectively. The proposed approach is validated using MD simulation in NVT ensembles for fcc aluminum subjected to uniaxial extension. The observed evolution of second moments in the MD simulation with macroscopic deformation is not directly related to the transformation of atomic trajectories through the deformation gradient using generator functions. However, it is noteworthy that deformation leads to a change in the second moment of the trajectories. Correspondingly, the vibration part of the Piola stress becomes particularly significant at high temperature and high tensile strain as the crystal approaches the softening limit. In contrast to the eigenvectors of the deformation gradient, the eigenvectors of the vibration tensor show strong spatial heterogeneity in the vicinity of softening. More importantly, the elliptic distribution of local atomic density transitions to a dumbbell shape, before significant non-affinity in equilibrium positions has occurred. read less

Abstract: Department of Chemistry, CDMF, Federal University of S~ao Carlos, P.O. Box 676, S~ao Carlos 13565-905, Brazil Department of Physical Chemistry, University of Valencia, Burjassot 46100, Spain Faculdade de Ciências Exatas e Tecnol ogicas, Universidade Federal da Grande Dourados, Unidade II, CP 533, 79804-970, Dourados, Mato Grosso do Sul, Brazil Department of Condensed Matter Physics, Institute of Physics ‘Gleb Wataghin’, State University of Campinas, 13083-970, Campinas, S~ao Paulo, Brazil Department of Physical Chemistry, Institute of Chemistry, State University of Campinas, 13083-970, Campinas, S~ao Paulo, Brazil Department of Biotechnology and Exact Sciences, Federal University of Tocantins, 77410-530, Gurupi, Tocantins, Brazil read less

Abstract: We present a novel distributed-memory parallel implementation of the concurrent atomistic-continuum (CAC) method. Written mostly in Fortran 2008 and wrapped with a Python scripting interface, the CAC simulator in PyCAC runs in parallel using Message Passing Interface with a spatial decomposition algorithm. Built upon the underlying Fortran code, the Python interface provides a robust and versatile way for users to build system configurations, run CAC simulations, and analyze results. In this paper, following a brief introduction to the theoretical background of the CAC method, we discuss the serial algorithms of dynamic, quasistatic, and hybrid CAC, along with some programming techniques used in the code. We then illustrate the parallel algorithm, quantify the parallel scalability, and discuss some software specifications of PyCAC; more information can be found in the PyCAC user’s manual that is hosted on http://www.pycac.org . read less

Abstract: ABSTRACT We report the fabrication of nickel nanowires with parallel growth-twin structures (‘twin lamella’ along the wire axis) by electrochemical deposition, and demonstrate an interesting twin ‘unzipping’ phenomenon in such nanotwinned nanowires under bending. Through in situ TEM, we found that ‘unzipping’ of twin lamella was achieved by gradually increasing twin spacing along the wire axis via a layer-by-layer twin boundary migration process. Molecular dynamics simulations suggest that partial dislocation slip is responsible for activating the ‘unzipping’, with a multi-step-process involving dislocation loop initiation, expansion and partially annihilation. Our work could provide new insights into the deformation mechanisms of nanotwinned 1-D metallic nanostructures. GRAPHICAL ABSTRACT IMPACT STATEMENT Nickel nanowires with parallel-twin structures were fabricated and demonstrated an interesting twin lamella ‘unzipping’ behavior upon flexural bending, which provides new insights into the deformation mechanisms of nanotwinned metallic materials. read less

Abstract: Atomistic simulations are used to determine the resolved shear stress necessary for equilibrium and the resulting geometry of nanoscale dislocation shear loops in Al. Dislocation loops with different sizes and shapes are created via superposition of elemental triangular dislocation displacement fields in the presence of an externally imposed shear stress. First, a bisection algorithm is developed to determine systematically the resolved shear stress necessary for equilibrium at 0 K. This approach allows for the identification of dislocation core structure and a correlation between dislocation loop size, shape and the computed shear stress for equilibrium. It is found, in agreement with predictions made by Scattergood and Bacon, that the equilibrium shape of a dislocation loop becomes more circular with increasing loop size. Second, the bisection algorithm is extended to study the influence of temperature on the resolved shear stress necessary for stability. An approach is presented to compute the effective lattice friction stress, including temperature dependence, for dislocation loops in Al. The temperature dependence of the effective lattice friction stress can be reliably computed for dislocation loops larger than 16.2 nm. However, for dislocation loops smaller than this threshold, the effective lattice friction stress shows a dislocation loop size dependence caused by significant overlap of the stress fields on the interior of the dislocation loops. Combined, static and finite temperature atomistic simulations provide essential data to parameterize discrete dislocation dynamics simulations. read less

Abstract: Molecular dynamics (MD) simulations under different mechanical and thermal constraints are carried out with a nanovoid embedded inside a single-crystal, face-centred-cubic copper. The dislocation emission angles measured from MD plots under 0.1 K, uniaxial-strain simulation are in line with the theoretical model. The dislocation density calculated from simulation is qualitatively consistent with the experimental measurement in terms of a saturation feature. The ‘relatively farthest-travelled’ atoms are employed to reflect the correlation between the dislocation structure and the void growth. At a smaller scale, the incomplete shear dislocation loops on the slip plane contribute to the local material transport. At a larger scale, the dislocation structures formed by those incomplete shear loops further facilitate the growth of nanovoid. Compared to the uniaxial-strain case, the void growth under the uniaxial-stress is very limited. The uniaxial-strain loading results in an octahedron void shape. The uniaxial-stress loading turns the nanovoid into a prolate ellipsoid along the loading direction. In the simulation, the largest specimen contains 12 million atoms and the lowest strain rate applied is 2 × 106 s−1. Under all the different thermomechanical constraints concerned, the formation of incomplete shear dislocation loops are found capable of growing the void. read less

Abstract: Previous HotQC studies of Cu nanovoids undergoing volumetric expansion conducted by the authors have uncovered a quasistatic-to-dynamic transition at a critical strain rate of the order of . At low strain rates nanovoid expansion takes place under essentially isothermal conditions, whereas at high strain rates it happens under essential adiabatic conditions. In this paper, we present a comparative study concerned with two different scenarios, each representing a variation on the reference case presented in [1]: (i) aluminium (Al) nanovoids undergoing volumetric expansion; and (ii) copper (Cu) nanovoids undergoing uniaxial deformation. Scenario (i) addresses material specificity by replacing Cu by Al in the reference case, whereas scenario (ii) addresses the effect of triaxiality by replacing volumetric expansion by uniaxial expansion in the reference case. We find a distinct quasistatic-to-dynamic transition in both scenarios, which suggests that the transition is indeed universal, i.e. material and strain-triaxiality independent. By contrast, the fine structure of the dislocation mechanisms that mediate void growth are strongly material and loading specific. read less

Abstract: Using molecular dynamics simulations, we study computational self-assembly of one-component three-dimensional dodecagonal (12-fold) quasicrystals in systems with two-length-scale potentials. Existing criteria for three-dimensional quasicrystal formation are quite complicated and rather inconvenient for particle simulations. So to localize numerically the quasicrystal phase, one should usually simulate over a wide range of system parameters. We show how to universally localize the parameter values at which dodecagonal quasicrystal order may appear for a given particle system. For that purpose, we use a criterion recently proposed for predicting decagonal quasicrystal formation in one-component two-length-scale systems. The criterion is based on two dimensionless effective parameters describing the fluid structure which are extracted from the radial distribution function. The proposed method allows reduction of the time spent for searching the parameters favoring a certain solid structure for a given system. We show that the method works well for dodecagonal quasicrystals; this result is verified on four systems with different potentials: the Dzugutov potential, the oscillating potential which mimics metal interactions, the repulsive shoulder potential describing effective interactions for the core/shell model of colloids and the embedded-atom model potential for aluminum. Our results suggest that the mechanism of dodecagonal quasicrystal formation is universal for both metallic and soft-matter systems and it is based on competition between interparticle scales. read less

Abstract: Dislocation/stacking fault interactions play an important role in the plastic deformation of metallic nanocrystals and polycrystals. These interactions have been explored in atomistic models, which are limited in scale length by high computational cost. In contrast, multiscale material modeling approaches have the potential to simulate the same systems at a fraction of the computational cost. In this paper, we validate the concurrent atomistic-continuum (CAC) method on the interactions between a lattice screw dislocation and a stacking fault (SF) in three face-centered cubic metallic materials—Ni, Al, and Ag. Two types of SFs are considered: intrinsic SF (ISF) and extrinsic SF (ESF). For the three materials at different strain levels, two screw dislocation/ISF interaction modes (annihilation of the ISF and transmission of the dislocation across the ISF) and three screw dislocation/ESF interaction modes (transformation of the ESF into a three-layer twin, transformation of the ESF into an ISF, and transmission of the dislocation across the ESF) are identified. Our results show that CAC is capable of accurately predicting the dislocation/SF interaction modes with greatly reduced DOFs compared to fully-resolved atomistic simulations. read less

Abstract: The deformation behavior and grain boundary (GB) response of nanocrystalline Ni under cyclic shear loading are investigated by molecular dynamics simulations. The GB migration hysteresis phenomenon, in which the GB migration displacement lags behind the change in nominal shear strain, is observed in the symmetric tilt GBs for the first time. The elementary structure transformation occurring at the two end segments of the observed GB during GB migration produces a disordered and irreversible state, while the transformation in the middle segment is reversible. Both dislocation retraction and nucleation occur during unloading. Relatively large cyclic strain amplitudes lead to disordered GB segments of greater length, such that the residual GB migration displacement increases with increasing cyclic amplitude. GB migration hysteresis vanishes after the GB becomes immobile owing to a cyclic shear induced transition to a disordered state along its entire length. read less

Abstract: Cation doping is often used to stabilize the cubic or tetragonal phase of zirconia for enhanced thermomechanical and electrochemical properties. In the present paper we report a combined density functional theory (DFT) and molecular dynamics study of the effect of Sc, Y, and Ce dopants on properties of Ni/ZrO2 interfaces and nickel sintering. First, we develop an MD model that is based on DFT data for various nickel/zirconia interfaces. Then, we employ the model to simulate Ni nanoparticles coalescing on a zirconia surface. The results show the possibility of particle migration by means of fast sliding over the surface when the work of separation is small (<1.0J m−2). The sliding observed for the O-terminated Ni(1 1 1)/ZrO2(1 1 1) interface is not affected by dopants in zirconia because the work of separation of the doped interface stays small. The most pronounced effect of the dopants is observed for the Zr-terminated Ni(1 1 1)/ZrO2(1 1 1) interface, which possesses a large work of separation (4.4J m−2) and thus restricts the sliding mechanism of Ni nanoparticle migration. DFT calculations for the interface revealed that dopants with a smaller covalent radius result in a larger energy barriers for Ni diffusion. We analyze this effect and discuss how it can be used to suppress nickel sintering by using the dopant selection. read less

Abstract: The structures and behaviors of grain boundaries (GBs) have profound effects on the mechanical properties of polycrystalline materials. In this paper, twist GBs in aluminum were investigated with molecular dynamic simulations to reveal their atomic structures, energy and interactions with dislocations. One hundred twenty-six twist GBs were studied, and the energy of all these twist GBs were calculated. The result indicates that and twist GBs have lower energy than twist GBs because of their higher interplanar spacing. In addition, 12 types of twist GBs in aluminum were chosen to explore the deformation behaviors. Low angle twist GBs with high density of network structures can resist greater tension because mutually hindering behaviors between partial dislocations increase the twist GB strength. read less

Abstract: The effects of grain size and intragranular dislocation on yield mechanism and subsequent plastic deformation in ultra ne-grained (UFG) Al and Cu were investigated by large-scale atomic simulations. Polycrystalline atomic models with and without intragranular dislocation sources were used to elucidate the relationship between mechanical properties and defect texture. It is found that the intragranular dislocation plays a signi cant role in both incipient yield and grain boundary mediated dislocation nucleation. In addition UFG Cu yields earlier than UFG Al because partial dislocations in Cu are more likely to activate from grain boundaries, where the partial dislocation leaves deformation twin and secondary dislocation tends to move on twin boundary accompanied by the shift of twin boundary plane. [doi:10.2320/matertrans.MH201515] read less

Abstract: Tension-compression asymmetry could be a notable feature in many nanocrystalline (NC) materials. The scientific and practical research on the tension-compression asymmetry may play an important role of improving the mechanical behavior of NC materials. Using large-scale molecular dynamics (MD) simulations at the strain rate of 109 s−1, both tension and compression tests are complemented in twin-structural polycrystalline Ni nanowires (NWs). The MD simulation suggests that twin boundaries spacing (TBS) has an interesting effect on the tension-compression asymmetry. For NW (radius = 9 nm) with different TBSs, the flow stresses are totally higher under compression than under tension. The asymmetry gets a minimum value at a particular TBS. Such results can be explained by the interplay of the work of dislocations mechanism under various TBSs and the free surface in NWs. read less

Abstract: We investigate the mechanisms of incipient plasticity at low angle twist and asymmetric tilt boundaries in fcc metals. To observe plasticity of grain boundaries independently of the bulk plasticity, we simulate nanoindentation of bicrystals. On the low angle twist boundaries, the intrinsic grain boundary (GB) dislocation network deforms under load until a dislocation segment compatible with glide on a lattice slip plane is created. The half loops are then emitted into the bulk of the crystal. Asymmetric twist boundaries considered here did not produce bulk dislocations under load. Instead, the boundary with a low excess volume nucleated a mobile GB dislocation and additional GB defects. The GB sliding proceeded by motion of the mobile GB dislocation. The boundary with a high excess volume sheared elastically, while bulk-nucleated dislocations produced plastic relaxation. read less

Abstract: In situ bending tests and dynamic modeling simulations are for the first time revealing the mechanical behavior of copper nanowires (NW) with radially grown fivefold twin structures on the atomic scale. Combining the simulations with the experimental results it is shown that both the twin boundaries (TBs) and the twin center act as dislocation sources. TB migration and L-locks are readily observed in these types of radially grown fivefold-twin structures. read less

Abstract: The fracture behavior of a single-crystal Al-nanoplate with an edge crack under tensile loading was simulated using a molecular statics technique to evaluate crack growth resistance in Al. The crack length was determined using a stiffness method. A parabolic function fitted from simulation results was used to predict the crack length from the stiffness value extracted from unloading curves. Based on energy considerations, crack growth resistance was calculated. Crack growth resistance rose sharply in the initial stages of crack growth, and with an additional crack extension, it increased gradually to converge to a constant far exceeding the fracture toughness predicted by the Griffith criterion. This trend in the crack growth resistance curve was closely related to the amorphous zone formed at the crack tip after the onset of crack propagation. read less

Abstract: The mechanical properties of Ni/Ni3Al monocrystal of nanophases with varying temperatures, strain rates, and phase sizes have been studied using molecular dynamics simulation. The simulation results show that the primary deformation mechanisms in Ni/Ni3Al monocrystal of nanophases were slip bands and antiphase boundaries at room temperature. The studies on the effects of temperature showed that the yield strain, yield strength, and elastic module decreased as temperature increased. However, the influences of strain rate and phase size on the mechanical properties of Ni/Ni3Al monocrystal of nanophases showed that the high strain rate led to the increase of yield stress, and the phase sizes had no significant influence on the maximum yield stress. In addition, the behavior of crack propagation in the model of Ni/Ni3Al interface was investigated under cyclic loading, and it was found that the interface of Ni/Ni3Al was resistance to the fatigue crack propagation. read less

Abstract: Using results of equilibrium molecular dynamics simulation in conjunction with the Green–Kubo formalism, we present a general treatment of thermal impedance of a crystal lattice with a monatomic unit cell. The treatment is based on an analytical expression for the heat current autocorrelation function which reveals, in a monatomic lattice, an energy gap between the origin of the phonon states and the beginning of the energy spectrum of the so-called acoustic short-range phonon modes. Although, we consider here the f.c.c. Al model as a case example, the analytical expression is shown to be consistent for different models of elemental f.c.c. crystals over a wide temperature range. Furthermore, we predict a frequency ‘window’ where the thermal waves can be generated in a monatomic lattice by an external periodic temperature perturbation. read less

Abstract: The material strengths of polycrystalline metals have been widely predicted according to the grain size, where yield stress is governed by slip transfer through the grain boundary (GB). The transferability of a dislocation across a GB is enormously important in the deformation process as well as the interaction between a dislocation and GB. This paper proposes a new criterion for the transferability of dislocations through a GB that considers both the intergranular crystallographic orientation of slip systems and the applied stress condition. Atomistic simulations were carried out to investigate the slip transfer event of simple bicrystals composed of Σ3(112) GB than Σ3(111) GBs under uniaxial deformation and to illustrate the availability of this criterion. As a result, in contrast to the predictions of conventional criteria such as the M-value, dislocations propagated more easily across the Σ3(111) and Σ3(112) GB under given stress states, reflecting a larger L′-value of Σ3 bicrystal associated with... read less

Abstract: By using a molecular dynamics method with EAM potential, we study the evolution phenomena of metal twist grain boundaries (GBs) in the [100], [111] and [110] orientations, together with their bimetal interface, under anticlockwise and clockwise torsions. Our results show that there are different evolution behaviors of the GB screw dislocations for single metals (Al and Ni) and their bimetal interface (Al/Ni) under torsion. Specifically, for the single metals in the [100] and [111] orientations, their GBs evolve toward lower or higher angle twist GBs depending on the twist direction. For Ni in the [110] orientation, the dislocations spread not only in the GB region but also in the grain interior. However, for the bimetal interface, the propagation of dislocations is not only reduced dramatically but also limited to the interface region, showing that there is an inhibition effect. Therefore, such an inhibition effect can enhance the stability of nanomaterials which is very useful for the further design of nanodevices. read less

Abstract: Ultrafast laser experiments on metals usually induce a high electron temperature and a low ion temperature and, thus, an energy relaxation process. The electron heat capacity and electron-phonon coupling factor are crucial thermal quantities to describe this process. We perform ab initio theoretical studies to determine these thermal quantities and their dependence on density and electron temperature for the metals aluminum and beryllium. The heat capacity shows an approximately linear dependence on the temperature, similar to free electron gas, and the compression only slightly affects the capacity. The electron-phonon coupling factor increases with both temperature and density, and the change observed for beryllium is more obvious than that for aluminum. The connections between thermal quantities and electronic/atomic structures are discussed in detail, and the different behaviors of aluminum and beryllium are well explained. read less

Abstract: Persistent slip bands (PSB) is the important and typical microstructure generated during fatigue crack initiation. Intensive works have been done to investigate the mechanisms of the formation of persistent slip bands in the past decade. In this paper, a molecular dynamics (MD) simulation associated with embedded atom model (EAM) is applied on the PSBs formation in nickel-base superalloys with different microstructure and temperature under tensile-tensile loadings. Simulation results show that PSBs formed within the γ phase by massive dislocations pile-up and propagation which can penetrate the grain. Also, the temperature will affect the material fatigue performance and blur PSBs appearance. The simulation results are in strong agreement with the experimental test results published before. read less

Abstract: The line tension Γ ?> of a dislocation is an important and fundamental property ubiquitous to continuum scale models of metal plasticity. However, the precise value of Γ ?> in a given material has proven difficult to assess, with literature values encompassing a wide range. Here results from a multiscale simulation and robust analysis of the dislocation line tension, for dislocation bow-out between pinning points, are presented for two widely-used interatomic potentials for Al. A central part of the analysis involves an effective Peierls stress applicable to curved dislocation structures that markedly differs from that of perfectly straight dislocations but is required to describe the bow-out both in loading and unloading. The line tensions for the two interatomic potentials are similar and provide robust numerical values for Al. Most importantly, the atomic results show notable differences with singular anisotropic elastic dislocation theory in that (i) the coefficient of the ln(L) ?> scaling with dislocation length L differs and (ii) the ratio of screw to edge line tension is smaller than predicted by anisotropic elasticity. These differences are attributed to local dislocation core interactions that remain beyond the scope of elasticity theory. The many differing literature values for Γ ?> are attributed to various approximations and inaccuracies in previous approaches. The results here indicate that continuum line dislocation models, based on elasticity theory and various core-cut-off assumptions, may be fundamentally unable to reproduce full atomistic results, thus hampering the detailed predictive ability of such continuum models. read less

Abstract: An interatomic potential for the Ni–Al system is presented within the third-generation charge optimized many-body (COMB3) formalism. The potential has been optimized for Ni3Al, or the γ′ phase in Ni-based superalloys. The formation energies predicted for other Ni–Al phases are in reasonable agreement with first-principles results. The potential further predicts good mechanical properties for Ni3Al, which includes the values of the complex stacking fault (CSF) and the anti-phase boundary (APB) energies for the (1 1 1) and (1 0 0) planes. It is also used to investigate dislocation propagation across the Ni3Al (1 1 0)–Ni (1 1 0) interface, and the results are consistent with simulation results reported in the literature. The potential is further used in combination with a recent COMB3 potential for Al2O3 to investigate the Ni3Al (1 1 1)–Al2O3 (0 0 01) interface, which has not been modeled previously at the classical atomistic level due to the lack of a reactive potential to describe both Ni3Al and Al2O3 as well as interactions between them. The calculated work of adhesion for this interface is predicted to be 1.85 J m−2, which is in agreement with available experimental data. The predicted interlayer distance is further consistent with the available first-principles results for Ni (1 1 1)–Al2O3 (0 0 0 1). read less

Abstract: The crack growth and stress evolution of single crystal nickel under three different crystal orientations (X[100], Y[010], Z[001]; X[110], Y[10], Z[001]; X[111], Y[10], Z [2]) were studied by introducing a cohesive zone model (CZM) based on molecular dynamics (MD) simulation. The results indicated that different crystal orientations have significant effect on the fracture mechanisms and stress distribution characteristics. Under the X[100], Y[010], Z[001] orientation, void nucleation and growth was observed during the crack propagation; under the X[110], Y[10], Z[001] orientation, atomic configurations basically remained unchanged throughout the crack growth, which represented a brittle process; dissimilarly, blunting and slip bands occurred at the front of the crack tip for the X[111], Y[10], Z[2] orientation. These different mechanisms resulted in different stress distributions along the crack path and crack growth rates. Moreover, based on the calculation of the CZM, the relationship between stress and opening displacement was obtained, which provided useful information for understanding the crystal orientation dependent on the atomic-scale fracture mechanisms and associated mechanical properties. read less

Abstract: This work presents the development and applications of a new empirical, variable charge potential for Al2O3 systems within the charge optimized many-body (COMB) potential framework. The potential can describe the fundamental physical properties of Al2O3, including cohesive energy, elastic constants, defect formation energies, surface energies and phonon properties of α-Al2O3 comparable to that obtained from experiments and first-principles calculations. The potential is further employed in classical molecular dynamics (MD) simulations to validate and predict the properties of the Al (1 1 1)–Al2O3 (0 0 0 1) interface, tensile properties of Al nanowires, Al2O3 nanowires, Al2O3-covered Al nanowires, and defective Al2O3 nanowires. The results demonstrate that the potential is well-suited to model heterogeneous material systems involving Al and Al2O3. Most importantly, the parameters can be seamlessly coupled with COMB3 parameters for other materials to enable MD simulations of a wide range of heterogeneous material systems. read less

Abstract: Molecular dynamics (MD) simulations are performed to investigate the effects of stress on generalized stacking fault (GSF) energy of three fcc metals (Cu, Al, and Ni). The simulation model is deformed by uniaxial tension or compression in each of [111], [11-2], and [1-10] directions, respectively, before shifting the lattice to calculate the GSF curve. Simulation results show that the values of unstable stacking fault energy (γusf), stable stacking fault energy (γsf), and unstable twin fault energy (γutf) of the three elements can change with the preloaded tensile or compressive stress in different directions. The ratio of γsf/γusf, which is related to the energy barrier for full dislocation nucleation, and the ratio of γutf/γusf, which is related to the energy barrier for twinning formation are plotted each as a function of the preloading stress. The results of this study reveal that the stress state can change the energy barrier of defect nucleation in the crystal lattice, and thereby can play an important role in the deformation mechanism of nanocrystalline material. read less

Abstract: The fracture toughness values of nanosized Cu and Ni single crystals with an edge nanocrack were determined under quasi-static loading conditions. Molecular statics (MS) simulations that can essentially capture the discreteness and the nonlinearity of materials were used in the present study. Different crack lengths were used to evaluate the effects of crack size on the fracture toughness. Based on MS simulations, the energy release rate was calculated using the energies obtained from two models with neighboring crack lengths under the same loading conditions. Furthermore, continuum counterparts of the atomistic models were used to calculate the toughness by the finite element method for linear elastic fracture mechanics (LEFM). The reasons behind the discrepancies between the toughness values obtained using different methods were discussed, and the applicable ranges of the toughness and the LEFM were indicated in terms of the lattice constants. read less

Abstract: Deformation and fracture processes in engineering materials often require simultaneous descriptions over a range of length and time scales, with each scale using a different computational technique. Here we present a high-performance parallel 3D computing framework for executing large multiscale studies that couple an atomic domain, modeled using molecular dynamics and a continuum domain, modeled using explicit finite elements. We use the robust Coupled Atomistic/Discrete-Dislocation (CADD) displacement-coupling method, but without the transfer of dislocations between atoms and continuum. The main purpose of the work is to provide a multiscale implementation within an existing large-scale parallel molecular dynamics code (LAMMPS) that enables use of all the tools associated with this popular open-source code, while extending CADD-type coupling to 3D. Validation of the implementation includes the demonstration of (i) stability in finite-temperature dynamics using Langevin dynamics, (ii) elimination of wave reflections due to large dynamic events occurring in the MD region and (iii) the absence of spurious forces acting on dislocations due to the MD/FE coupling, for dislocations further than 10 Å from the coupling boundary. A first non-trivial example application of dislocation glide and bowing around obstacles is shown, for dislocation lengths of ∼50 nm using fewer than 1 000 000 atoms but reproducing results of extremely large atomistic simulations at much lower computational cost. read less

Abstract: Crystal defects can be identified by their fingerprint in coherent X-ray diffraction patterns. Realistic defects in face-centred cubic nanocrystals are studied numerically, revealing various signatures in diffraction patterns depending on the Miller indices and providing an identification method. read less

Abstract: The fracture behavior of precracked nanocrystals with grain size gradients is simulated using the molecular dynamics method. A large grain size gradient is found to elevate resistance to crack propagation and transform the fracture mode from intergranular to intragranular when the crack is obstructed by a coarse grain. But the intragranular crack is nipped in its bud due to the difficulty of intragranular fracture. However, intergranular fractures can be always kept in nanocrystals with a small grain size gradient. Both the Schmid factors for the slip systems of grains near the crack tip and the critical stress intensity factors are calculated, and energy partitioning is conducted to analyze the mechanisms behind this phenomenon. The research exhibits the key role of grain size gradient in improving the antifracture ability of nanocrystals. read less

Abstract: In this study, we propose a combination of molecular dynamics (MD) and the master sintering curve (MSC) approach to analyze the activation energy of sintering for porous materials. MD calculations were performed by using a porous structure with various initial densities, and the change of relative density with simulation time was analyzed. To relate the MD results with long-term sintering behaviors of porous materials, we established a method to obtain the MSC, which is able to determine the activation energy of sintering, on the basis of these MD simulation results. We have successfully obtained sintering behavior and activation energies of sintering depending on temperature range and particle diameter. These activation energies obtained by our approach are in agreement with experimental observations. In addition, the temperature dependence of activation energy of sintering is also in good agreement with that of surface diffusion, which indicates that surface diffusion is the dominant sintering mechanism. read less

Abstract: Molecular dynamics simulations were used to quantify mechanically induced structural evolution in nanocrystalline Al with an average grain size of 5 nm. A polycrystalline sample was cyclically strained at different temperatures, while a recently developed grain tracking algorithm was used to measure the relative contributions of novel deformation mechanisms such as grain rotation and grain sliding. Sample texture and grain size were also tracked during cycling, to show how nanocrystalline plasticity rearranges overall grain structure and alters the grain boundary network. While no obvious texture is developing during cycling, the processes responsible for plasticity act collectively to alter the interfacial network. Cyclic loading led to the formation of many twin boundaries throughout the sample as well as the occasional coalescence of neighboring grains, with higher temperatures causing more evolution. A temperature-dependent cyclic strengthening effect was observed, demonstrating that both the structure and properties of nanocrystalline metals can be dynamic during loading. read less

Abstract: The paper establishes a quantitative link between the bulk hydrogen content and the local concentration in the core of the symmetric tilt grain boundary in Al. A detailed map of approximate segregation energies is obtained by combined semi-empirical and ab initio calculations. Even if the density of trap sites and the binding to the core are large, it is shown that segregation alone is not expected to lead to a significant loss of cohesion below 1000 ppm bulk concentration. Other mechanisms should be involved, like H-induced structural changes, to explain the experimentally observed failure of the interfaces at low H concentration. An example of such mechanism is reported. read less

Abstract: Grain boundary energy is very important in determining properties of ultra fine grain and nano structure materials. Molecular dynamics were used to simulate grain boundary energy at different misorientations for Al, Cu and Ni elements. Obtained results indicated well compatibility with theoretic predictions. It was obtained that higher cohesive energy results in higher grain boundary energy and depth of CSLs. In this manner, Ni had the highest and Al had the lowest cohesive energy and grain boundary energy. Also, a linear correlation was obtained between GBE of elements, which was related to relative cohesive energy. read less

Abstract: Molecular dynamics (MD) and density functional theory (DFT) studies were performed to investigate the influence of vacancy defects on generalized stacking fault (GSF) energy of fcc metals. MEAM and EAM potentials were used for MD simulations, and DFT calculations were performed to test the accuracy of different common parameter sets for MEAM and EAM potentials in predicting GSF with different fractions of vacancy defects. Vacancy defects were placed at the stacking fault plane or at nearby atomic layers. The effect of vacancy defects at the stacking fault plane and the plane directly underneath of it was dominant compared to the effect of vacancies at other adjacent planes. The effects of vacancy fraction, the distance between vacancies, and lateral relaxation of atoms on the GSF curves with vacancy defects were investigated. A very similar variation of normalized SFEs with respect to vacancy fractions were observed for Ni and Cu. MEAM potentials qualitatively captured the effect of vacancies on GSF. read less

Abstract: In a number of fcc materials such as copper or aluminum, as well as more complex materials such as twinning induced plasticity (TWIP) steels, the interaction between dislocations and other defects such as stacking faults or twins plays an important role in the hardening behavior of such materials. Interactions of dislocation and twin or stacking fault layers have been studied in this work using molecular dynamics. Depending on the material and the loading conditions, possible interaction modes include (i) penetration of the dislocation into the faulted layer, (ii) reduction of the faulted layer after interaction, (iii) growth of the faulted layer after interaction. Such studies up to this point have been performed without temperature control near zero K (0 to 2 K). In this work, we extend the previous studies to higher temperature with the help of two methods, both based on molecular dynamics (MD) modeling. (© 2013 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim) read less

Abstract: The energy-density and stress-density schemes (Shiihara et al 2010 Phys. Rev. B 81 075441) within the projector augmented wave (PAW) method based on the generalized gradient approximation (GGA) have been applied to tilt and twist grain boundaries (GBs) and single vacancies in Cu and Al. Local energy and local stress at GBs and defects are obtained by integrating the energy and stress densities in each local region by the Bader integration using a recent algorithm (Yu et al 2011 J. Chem. Phys. 134 064111) as well as by the layer-by-layer integration so as to settle the gauge-dependent problem in the kinetic terms. Results are compared with those by the fuzzy-Voronoi integration and by the embedded atom method (EAM). The features of local energy and local stress at GBs and vacancies depend on the bonding nature of each material. Valence electrons in Al mainly located in the interatomic regions show remarkable response to structural disorder as significant valence charge redistribution or bond reconstruction, often leading to long-range variations of charges, energies and stresses, quite differently from d electrons in Cu mainly located near nuclei. All these features can be well represented by our local energy and local stress. The EAM potential for Al does not reproduce correct local energy or local stress, while the EAM potential for Cu provides satisfactory results. read less

Abstract: Molecular dynamics simulations are used to investigate strain localization in a model nanocrystalline metal. The atomic mechanisms of such catastrophic failure are first studied for two grain sizes of interest. Detailed analysis shows that the formation of a strain path across the sample width is crucial, and can be achieved entirely through grain boundary deformation or through a combination of grain boundary sliding and grain boundary dislocation emission. Pronounced mechanically-induced grain growth is also found within the strain localization region. The effects of testing conditions on strain localization are also highlighted, to understand the conditions that promote shear banding and compare these observations to metallic glass behavior. We observed that, while strain localization occurs at low temperatures and slow strain rates, a shift to more uniform plastic flow is observed when either strain rate or temperature is increased. We also explore how external sample dimensions influence strain localization, but find no size effect for the grain sizes and samples sizes studied here. read less

Abstract: Twin formation energy is an intrinsic quantity for bulk crystals. At the nanoscale, the twin formation energy of covalent SiC nanowires goes up with decreasing dimension. In contrast, this article reports that the twin formation energy of metallic nanowires goes down with decreasing dimension. This result is based on classical molecular statics simulations of four representative metals. Cu and Al represent face-centered cubic (FCC) metals of low and high twin formation energies. Ta represents a body-centered cubic (BCC) metal, and Mg represents a hexagonal close-packed (HCP) metal. For all the four metals, the dependence of twin formation energy on size correlates with lower twin formation energy near surfaces, according to atomic-level analysis. Based on this atomic-level insight, the authors propose a core–shell model that reveals the twin formation energy as inversely proportional to the diameter of nanowires. This dependence is in agreement with the results of molecular statics simulations. read less

Abstract: An effective multiscale simulation which concurrently couples the quantum-mechanical and molecular-mechanical calculations based on the position continuity of atoms is presented. By an iterative procedure, the structure of the dislocation core in face-centered cubic metal is obtained by first-principles calculation and the long-range stress is released by molecular dynamics relaxation. Compared to earlier multiscale methods, the present work couples the long-range strain to the local displacements of the dislocation core in a simpler way with the same accuracy. read less

Abstract: Annihilation of vacancy-type non-screw dipolar dislocations is studied with molecular dynamics in fcc metals Al, Cu and Ni, and intermetallic γ-TiAl. Contrary to common belief, dipoles do not simply disappear. Instead, they transform into a series of defects depending on their height, orientation and temperature. At low temperatures, hollow structures, reconstructed configurations and faulted dipoles are formed. At high temperatures, with the help of short-range diffusion, isolated or interconnected vacancy clusters and stacking-fault tetrahedra are formed within a simulation time of 1 ns. Employing saddle-point-search methods, the formation of the above by-products is explained by accelerated diffusion paths along the dipole cores. read less

Abstract: Σ3 grain boundaries form as a result of either growth twinning or deformation twinning in face centered cubic (fcc) metals and play a crucial role in determining the mechanical and electrical properties and microstructural stability. We studied the structure and stability of Σ3 grain boundaries (GBs) in fcc metals by using topological analysis and atomistic simulations. Atomistic simulations were performed for Cu and Al with empirical interatomic potentials to reveal the influence of stacking fault energy on the morphology of the twinned grains. Three sets of tilt Σ3 GBs were studied with respect to the tilt axis parallel to ⟨111⟩, ⟨112⟩, and ⟨110⟩, respectively. We showed that Σ3{111} and Σ3{112} GBs are thermodynamically stable and the others will dissociate into terraced interfaces regardless of the stacking fault energy. The morphology of the nano-twinned grains in Cu is predicted from the above analysis and found to match with experiments. read less

Abstract: In his last few years, Patrick Veyssière became involved in atomic-scale molecular dynamics simulations that give access to the full atomistic details of dislocation/dislocation and dislocation/defect interactions. Such simulations are, however, very limited in time and do not allow the study of diffusional processes, such as vacancy migration. For such processes, one needs to explore the configuration space in search of activated states, whose energy barriers control the slow thermal-activated kinetics of defects. Here, we present work, performed in the context of a France–China project and initiated by Patrick Veyssière, where we study the long-term evolution of vacancy supersaturations in fcc metals. We employed a combination of saddle-point search methods (activation–relaxation technique, automated basin climbing method and nudged elastic band method) and kinetic Monte Carlo simulations to reach experimental timescales while retaining atomistic fidelity. The simulations identified a specific and, so far, unidentified cluster, the pentavacancy, made of five vacancies forming a bcc unit cell in the fcc lattice, which plays a central role in vacancy clustering in aluminium. read less

Abstract: Two of the central recurring themes in nanomechanics are strength and plasticity [1–3]. They are naturally coupled because plastic deformation is a major strength-determining mechanism and understanding the resistance to deformation (strength) is a principal motivation for studying plasticity. Many phenomena of interest in mechanics can be discussed in the framework of microstructural evolution of a system where defects like cracks and dislocations are formed and evolve interactively. Microstructure evolution at the nanoscale is particularly relevant from the standpoint of probing unit processes of deformation, such as advancement of a crack front by a lattice unit or propagation of a dislocation core by a Burgers vector. These atomic-level details can reveal the mechanisms of deformation, which are the essential inputs to describing microstructure evolution at the mesoscale − the next length and time scales. This hierarchical relation is the essence of multiscale modeling and simulation paradigm [4–6]. The purpose of this chapter is to discuss the atomistic approach to describe the evolution of crystalline defects [1–3,6]. We focus on a method, which is becoming widely used, that allows one to track the microstructure evolution through the sampling of a read less

Abstract: Plasticity in small volumes is significantly different than in large, bulk-like specimens. The strength of materials in the micron and sub-micron regime depends not only on the microstructure but also on the size of the material. Given that a number of important mechanical processes occur at these length scales, it is important to build a comprehensive understanding of these materials and processes. This work examines several different processes in small volumes. The motion of dislocations in the drag regime are examined as a function of material volume which exhibits higher drag than in the bulk. The stability of single arm sources are also investigated using molecular dynamics. Finally, dislocation nucleation from the free surface is investigated using both atomistic as well as continuum models. These results provide a better understanding of plastic processes in confined volumes. read less

Abstract: Single arm sources have been utilized to explain source-dependent strength in a variety of small-scale structures and have been observed to control plasticity in experiments. In this work, we investigate the stability of single arm sources using molecular dynamics focusing on Lomer–Cottrell dislocations as pinning points. We show that these segments are not stable enough to create static pinning points. We also show that some artificially created pinning points can act as stable sources, which allows us to investigate the strength of single arm sources and their dynamics in nanopillars. Finally, we show using constant strain rate molecular dynamics that single arm sources can be created and destroyed by interacting dislocations nucleated from free surfaces and grain boundaries. read less

Abstract: The effects of stacking fault energy, unstable stacking fault energy, and unstable twinning fault energy on the fracture behavior of nanocrystalline Ni are studied via quasicontinuum simulations. Two semi-empirical potentials for Ni are used to vary the values of these generalized planar fault energies. When the above three energies are reduced, a brittle-to-ductile transition of the fracture behavior is observed. In the model with higher generalized planar fault energies, a nanocrack proceeds along a grain boundary, while in the model with lower energies, the tip of the nanocrack becomes blunt. A greater twinning tendency is also observed in the more ductile model. These results indicate that the fracture toughness of nanocrystalline face-centered-cubic metals and alloys might be efficiently improved by controlling the generalized planar fault energies. read less

Abstract: Molecular dynamics simulations are used to show that cyclic mechanical loading can relax the non-equilibrium grain boundary (GB) structures of nanocrystalline metals by dissipating energy and reducing the average atomic energy of the system, leading to higher strengths. The GB processes that dominate deformation in these materials allow low-energy boundary configurations to be found through kinematically irreversible structural changes during cycling, which increases the subsequent resistance to plastic deformation. read less

Abstract: The techniques of grain boundary engineering are rapidly gaining significance microstructural design. To understand individual grain boundary characteristics, the influence of grain boundaries on the elastic and plastic deformation behaviors of copper bicrystals with Σ3(1¯11) twin and Σ3(1¯12) grain boundaries were investigated by large scale molecular statics simulation. These grain boundaries were chosen as examples of coherent and incoherent grain boundaries. Nanoindentation tests perpendicular to the grain boundary plane were used to investigate local deformation properties. Our results showed that an incoherent boundary experiences a reduction in elastic resistance due to the increase in excess free volume and structure-dependent local indentation modulus, while a coherent boundary has little effect on the elastic deformation. The propagation of plastic deformation is strongly blocked by the dissociation into a displacement shift complete (DSC) lattice dislocation which explains the superficial absor... read less

Abstract: With the increasing use of molecular simulation to understand deformation mechanisms in transition metals, it is important to assess how well the simulations reproduce physical behavior both near equilibrium and under more extreme conditions. In particular, it is important to examine whether simulations predict unusual deformation paths that are competitive with those experimentally observed. In this work we compare generalized planar fault energy landscapes and surface energies for various interatomic potentials with those from density functional theory calculations to examine how well these more complicated planar faults and surface energies are captured and whether any deformations are energetically competitive with the {111}⟨112⟩ slip observed in FCC crystals. To do this we examine not just the (111) fault orientation, but also the (100), (110), (210), (211), (311), and (331) orientations to test behavior outside of the fitting range of the interatomic potentials. We find that the shape of the (111)[11 ] stacking fault energy curve varies significantly with potential, with the ratio of unstable to stable stacking fault energies ranging from 1.22 to 14.07, and some deformation paths for non-(111) orientations give activation barriers less than 50% higher than the unstable stacking fault energies. These are important considerations when choosing an interatomic potential for deformation simulations. read less

Abstract: The dynamics of complex systems often involve thermally activated barrier crossing events that allow these systems to move from one basin of attraction on the high dimensional energy surface to another. Such events are ubiquitous, but challenging to simulate using conventional simulation tools, such as molecular dynamics. Recently, E and Zhou [Nonlinearity 24(6), 1831 (2011)] proposed a set of dynamic equations, the gentlest ascent dynamics (GAD), to describe the escape of a system from a basin of attraction and proved that solutions of GAD converge to index-1 saddle points of the underlying energy. In this paper, we extend GAD to enable finite temperature simulations in which the system hops between different saddle points on the energy surface. An effective strategy to use GAD to sample an ensemble of low barrier saddle points located in the vicinity of a locally stable configuration on the high dimensional energy surface is proposed. The utility of the method is demonstrated by studying the low barrier saddle points associated with point defect activity on a surface. This is done for two representative systems, namely, (a) a surface vacancy and ad-atom pair and (b) a heptamer island on the (111) surface of copper. read less

Abstract: We present and discuss an algorithm to identify and characterize the long icosahedral structures (staggered pentagonal nanowires with 1-5-1-5 atomic structure) that appear in Molecular Dynamics simulations of metallic nanowires of different species subjected to stretching. The use of this algorithm allows the identification of pentagonal rings forming the icosahedral structure as well as the determination of its number np , and the maximum length of the pentagonal nanowire Lpm. The algorithm is tested with some ideal structures to show its ability to discriminate between pentagonal rings and other ring structures. We applied the algorithm to Ni nanowires with temperatures ranging between 4K and 865K, stretched along the [111], [100] and [110] directions. We studied statistically the formation of pentagonal nanowires obtaining the distributions of length Lpm and number of rings np as function of the temperature. The Lpm distribution presents a peaked shape, with peaks located at fixed distances whose separation corresponds to the distance between two consecutive pentagonal rings. read less

Abstract: The physical vapour deposition of Ni atoms on α-Fe(001) surface under different deposition temperatures were simulated by molecular dynamics to study the intermixing and microstructure of the interfacial region. The results indicate that Ni atoms hardly penetrate into Fe substrate while Fe atoms easily diffuse into Ni deposition layers. The thickness of the intermixing region is temperature-dependent, with high temperatures yielding larger thicknesses. The deposited layers are mainly composed of amorphous phase due to the abnormal deposition behaviour of Ni and Fe. In the deposited Ni-rich phase, the relatively stable metallic compound B2 structured FeNi is found under high deposition temperature conditions. read less

Abstract: The vibration and depinning of an edge dislocation in Al initially anchored at an array of voids is studied by molecular dynamics simulations. The dislocation behaviour is studied under cyclic loading in the frequency range 10–160 GHz and varying stress amplitudes. It is found that for stress amplitudes smaller than 50 MPa the vibration amplitude and phase shift are in good agreement with an atomistically informed Granato–Lücke model. At larger stresses relativistic effects cannot be neglected anymore. The depinning stress is determined under quasistatic and cyclic loading conditions. For frequencies close to the resonant frequency depinning of the dislocation is observed at significantly lower stresses than under quasistatic loading. Possible consequences of this dynamical depinning effect are discussed. read less

Abstract: Density-functional theory (DFT) calculations of interphase boundary energies relevant to hexagonal-close-packed (hcp) $\ensuremath{\gamma}$-precipitate formation were performed within approximate unit cells that mirror the experimental conditions in face-centered-cubic (fcc) Al-Ag solid solutions. In Al-rich, fcc Al-Ag, $\ensuremath{\gamma}$ precipitates are observed to form rapidly with large $(300+)$ aspect ratios even though the Al stacking-fault energy is high (approximately $130\text{ }\text{mJ}/{\text{m}}^{2}$), which should suppress hcp ribbon formation according to standard arguments. Our DFT results show why high-aspect ratio plates occur and why previous estimates based on Wulff construction were orders of magnitude less than observed values. Using DFT, we obtain a Gibbs free-energy diagram that gives the relevant hcp equilibrium precipitate structure occurring at $50\text{ }\text{at}\text{.}\text{ }\mathrm{%}$ Ag. We derive the critical nucleation parameters for $\ensuremath{\gamma}$-precipitate formation, which require our calculated bulk-driving force for nucleation and interphase boundary energies. From our DFT-based nonequilibrium estimate for precipitation that accounts for growth via coarsening by ledge and edge migrations, we obtain time-dependent aspect ratio that agrees well with experiment. The same energetics and growth model are relevant to other phenomena, such as lath morphology in martensites or island coarsening. read less

Abstract: We present the results of a large-scale atomistic study of tensile deformation in a virtual fcc polycrystalline sample with columnar grain structure and a [110] texture. The atomic interaction was described by a volume-dependent central interatomic potential based on first principle calculations and experimental data for fcc Ni. The sample contained nine grains of 40 nm average size, created using a Voronoi construction with a common [110] axis, so that the grain boundaries were all pure tilt with random misorientation angles and crystallographic orientation of the grain boundary plane. We report the stress–strain behavior of the sample and the particular details of dislocation emission and dislocation interaction. Different grain boundaries acted as emission sites at different stresses due to their different local structure and orientation with respect to the applied stress. It was found that boundaries close to a twin misorientation can emit dislocations easily and become closer to the twin misorientation as a result of the emission process. Low angle boundaries were observed to disappear as a result of the deformation process. The emission of leading and trailing Shockley partials was observed and as the deformation proceeds, dislocation debris accumulates in the sample. The results also show that, as the deformation proceeds, the strain can localize in certain grains and grain regions, driven solely by the particular local structure and orientation of the various grain boundaries. read less

Abstract: An accurate description of the thermoelastic response of solids is central to classical simulations of compression- and deformation-induced condensed matter phenomena. To achieve the correct thermoelastic description in classical simulations, a new approach is presented for determining interatomic potentials. In this two-step approach, values of atomic volume and the second- and third-order elastic constants measured at room temperature are extrapolated to T = 0 K using classical thermo-mechanical relations that are thermodynamically consistent. Next, the interatomic potentials are fitted to these T = 0 K pseudo-values. This two-step approach avoids the low-temperature quantum regime, providing consistency with the assumptions of classical simulations and enabling the correct thermoelastic response to be recovered in simulations at room temperature and higher. As an example of our approach, an EAM potential was developed for aluminum, providing significantly better agreement with thermoelastic data compared with previous EAM potentials. The approach presented here is quite general and can be used for other potential types as well, the key restriction being the inapplicability of classical atomistic simulations when quantum effects are important. read less

Abstract: A concurrent multiscale modeling methodology that embeds a molecular dynamics (MD) region within a finite element (FEM) domain is used to study plastic processes at a crack tip in a single crystal of aluminum. The case of mode I loading is studied. A transition from deformation twinning to full dislocation emission from the crack tip is found when the crack plane is rotated around the [ 111 ] crystallographic axis. When the crack plane normal coincides with the [112] twinning direction, the crack propagates through a twinning mechanism. When the crack plane normal coincides with the [011] slip direction, the crack propagates through the emission of full dislocations. In intermediate orientations, a transition from full dislocation emission to twinning is found to occur with an increase in the stress intensity at the crack tip. This finding confirms the suggestion that the very high strain rates, inherently present in MD simulations, which produce higher stress intensities at the crack tip, over-predict the tendency for deformation twinning compared to experiments. The present study, therefore, aims to develop a more realistic and accurate predictive modeling of fracture processes. read less

Abstract: Molecular dynamics simulations were performed to gain fundamental insight into crystal plasticity, and its size effects in nanowires deformed by spherical indentation. This work focused on <111>-oriented single-crystal, defect-free Ni nanowires of cylindrical shape with diameters of 12 and 30 nm. The indentation of thin films was also comparatively studied to characterize the influence of free surfaces in the emission and absorption of lattice dislocations in single-crystal Ni. All of the simulations were conducted at 300 K by using a virtual spherical indenter of 18 nm in diameter with a displacement rate of 1 m·s^−1. No significant effect of sample size was observed on the elastic response and mean contact pressure at yield point in both thin films and nanowires. In the plastic regime, a constant hardness of 21 GPa was found in thin films for penetration depths larger than 0.8 nm, irrespective of variations in film thickness. The major finding of this work is that the hardness of the nanowires decreases as the sample diameter decreases, causing important softening effects in the smaller nanowire during indentation. The interactions of prismatic loops and dislocations, which are emitted beneath the contact tip, with free boundaries are shown to be the main factor for the size dependence of hardness in single-crystal Ni nanowires during indentation. read less

Abstract: Through detailed comparisons between Embedded Atom Method (EAM) and first-principles calculations for Al, we find that EAM tends to fail when there are large electron density gradients present. We attribute the observed failures to the violation of the uniform density approximation (UDA) underlying EAM. To remedy the insufficiency of UDA, we propose a gradient-corrected EAM model which introduces gradient corrections to the embedding function in terms of exchange-correlation and kinetic energies. Based on the perturbation theory of "quasiatoms" and density functional theory, the new embedding function captures the essential physics missing in UDA, and paves the way for developing more transferable EAM potentials. With Voter-Chen EAM potential as an example, we show that the gradient corrections can significantly improve the transferability of the potential. read less

Abstract: Through multiscale simulations, we explore the influence of both smooth and atomically rough indenter tips on the nucleation of dislocations during nanoindentation of single-crystal aluminum. We model the long-range strain with finite element analysis using anisotropic linear elasticity. We then model a region near the indenter atomistically and perform molecular dynamics with an embedded atom method interatomic potential. We find that smooth indenters nucleate dislocations below the surface but rough indenters can nucleate dislocations both at the surface and below. Increasing temperature from 0 to 300 K creates prenucleation defects in the region of high stress and decreases the critical depth. read less

Abstract: Innovative thin film technology has enabled the development of finer electronic devices, but a greater understanding of the atomic level process of film growth and its relationship with film characterization is needed to successfully manufacture these devices. Kinetic Monte Carlo (KMC) simulations can treat phenomena with a µs order time scale in thin film growth. Most of the previous KMC simulations were conducted on a film surface that consisted of identical atoms with incident particles, that is homoepitaxial growth. In this study, KMC parameters that define heteroepitaxial growth are characterized by a simple Lennard-Jones type two-body interaction. KMC simulations were then performed until dislocations were created. KMC parameters, activation energy and attempt frequency, were shifted by the expansion and contraction of atomic bonds at the heteroepitaxial surface. The amount of the shift depended on the distance from the interface, but they saturated at more than three deposited layers. Thus, the KMC simulation of heteroepitaxial growth can be conducted using only a few sets of KMC parameter tables that are adopted according to the layer. The layer dependence of the KMC parameters for the Ni/Cu interface was also investigated using the embedded-atom (EAM) potential as the interatomic interaction for describing realistic materials. KMC simulations of Cu film growth on a Ni(111) surface was conducted and compared with homoepitaxial Cu film growth on Cu(111). The activation energies of diffusions around an island on the heteroepitaxial surface decreased more than those on the homoepitaxial surface. Thus, the shape of the island for the heteroepitaxial growth of Cu on Ni(111) was more simplified than the homoepitaxial growth of Cu on Cu(111). read less

Abstract: Many atomically ordered grain boundaries (GBs) couple to applied mechanical stresses and are moved by them, producing shear deformation of the lattice they traverse. This process does not require atomic diffusion and can be implemented at low temperatures by deformation and rotation of structural units. This so-called coupled GB motion occurs by increments and can exhibit dynamics similar to the stick-slip behavior known in atomic friction. We explore possible dynamic regimes of coupled GB motion by two methods. First, we analyze a simple one-dimensional model in which the GB is mimicked by a particle attached to an elastic rod and dragged through a periodic potential. Second, we apply molecular dynamics (MD) with an embedded-atom potential for Al to simulate coupled motion of a particular tilt GB at different temperatures and velocities. The stress-velocity-temperature relationships established by both methods are qualitatively similar and indicate highly nonlinear dynamics at low temperatures and/or large velocities. At high temperatures and/or slow velocities, the character of the GB motion changes from stick slip to driven random walk and the stress-velocity relation becomes approximately linear. The MD simulations also reveal multiple GB jumps due to dynamic correlations at high velocities, and a transition from coupling to sliding at high temperatures. read less

Abstract: The physical mechanisms and processes underlying the erosion of a surface induced by cluster bombardment or short-pulse laser irradiation are highlighted. When the average energy delivered per atom in the vicinity of the surface becomes comparable to the cohesive energy of the solid, sputtering from a so-called spike may result. Such a spike leads to abundant sputtering (surface erosion) and crater formation. Direct atomization in the region of highest energy deposition, as well as melt flow and gas flow contribute to the erosion. The materials phenomena occurring after ultra-fast laser irradiation of a metal in the ps- or fs-regime are reviewed. With increasing laser fluence, the film melts, voids are formed, the film tears (spallation), and finally fragments to form a multitude of clusters. These processes are universal in the sense that they occur in widely differing materials such as metals or van-der-Waals bonded materials. We investigate a Lennard-Jones solid as well as four different metals (Al, Cu, Ti, W), which vary widely in their cohesive energy, melting temperature, bulk modulus, and crystal structure. When the energy transfer starting the process is scaled to the cohesive energy of the material, the thresholds of these processes adopt similar values. A comparison of the similarities and differences of the mechanisms underlying surface erosion under cluster ion impact and ultrafast laser irradiation will be drawn. read less

Abstract: The dynamic deformation upon stretching of Ni nanowires as those formed with mechanically controllable break junctions or with a scanning tunneling microscope is studied both experimentally and theoretically. Molecular dynamics simulations of the breaking process are performed. In addition, and in order to compare with experiments, we also compute the transport properties in the last stages before failure using the first-principles implementation of Landauer's formalism included in our transport package ALACANT. read less

Abstract: Atomistic simulations were employed to investigate the structure and energy of asymmetric tilt grain boundaries in Cu and Al. In this work, we examine the Σ5 and Σ13 systems with a boundary plane rotated about the ⟨ 100 ⟩ misorientation axis, and the Σ9 and Σ11 systems rotated about the ⟨ 110 ⟩ misorientation axis. Asymmetric tilt grain boundary energies are calculated as a function of inclination angle and compared with an energy relationship based on faceting into the two symmetric tilt grain boundaries in each system. We find that asymmetric tilt boundaries with low index normals do not necessarily have lower energies than boundaries with similar inclination angles, contrary to previous studies. Further analysis of grain boundary structures provides insight into the asymmetric tilt grain boundary energy. The Σ5 and Σ13 systems in the ⟨ 100 ⟩ system agree with the aforementioned energy relationship; structures confirm that these asymmetric boundaries facet into the symmetric tilt boundaries. The Σ9 and Σ11 systems in the ⟨ 110 ⟩ system deviate from the idealized energy relationship. As the boundary inclination angle increases towards the Σ9 (221) and Σ11 (332) symmetric tilt boundaries, the minimum energy asymmetric boundary structures contain low index {111} and {110} planes bounding the interface region. read less

Abstract: Abrupt growth of displacement observed in the relationship between indent load and indent depth in nanoindentation of crystalline materials, so-called displacement burst, has been recognized as one of the representative examples of nanoscale plastic behavior (nanoplasticity). This phenomenon corresponds to the early stage of plastic deformation and is greatly influenced by the collective dislocation emission. In the present paper a simplified model is constructed for the first displacement burst with use of the elastic theory based on both the Hertzian contact theory and the classical dislocation theory to evaluate the displacement burst in nanoindentation. The result of the analytical model for the energy equilibrium revealed that there is a strong correlation between burst width and critical indent depth that corresponds to the dislocation emission. Finally, it is shown that more than one hundred high-density dislocations are generated simultaneously and surface step corresponding to the Burgers vector of dislocation dipole of each emitted dislocation causes significant displacement burst. read less

Abstract: Molecular dynamics (MD) simulations in combination with an atomistic path technique are used to examine energies required to impinge a screw dislocation on a coherent twin boundary (CTB) in Al and Cu. At large distances, we find that the dislocation-CTB interaction is characterized by repulsive forces which can be attributed to both the elasticity mismatch and distortion (shift and rotation) of deformation fields across the twin boundary. The repulsive forces are determined as a function of distance between the dislocation and the twin boundary based on our MD data and the classical dislocation theory. At short distances, the interaction is significantly influenced by the shear strength of the CTB: relatively low CTB shear strength can induce close-range attractive forces and cause slip to be absorbed into the twin plane. read less

Abstract: A majority of computational mechanical analyses of nanocrystalline materials or nanowires have been carried out using classical molecular dynamics (MD). Due to the fundamental reason that the MD simulations must resolve atomic level vibrations, they cannot be carried out at a time scale of the order of microseconds in a reasonable computing time. Additionally, MD simulations have to be carried out at very high loading rates (∼108 s−1) rarely observed during experiments. In this investigation, a modified hybrid Monte Carlo (HMC) method that can be used to analyze time-dependent (strain-rate-dependent) atomistic mechanical deformation of nanostructures at higher time scales than currently possible using MD is established for a Cu nanowire and for a nanocrystalline Ni sample. In this method, there is no restriction on the size of MD time step except that it must ensure a reasonable acceptance rate between consecutive Monte Carlo (MC) steps. In order to establish the method, HMC analyses of a Cu nanowire defo... read less

Abstract: Novel shape memory behaviour was discovered recently in single-crystalline fcc nanowires of Cu, Ni and Au with lateral dimensions below 5 nm. Under proper thermomechanical conditions, these wires can recover elongations up to 50%. This phenomenon only exists at the nanoscale and is associated with reversible lattice reorientations within the fcc lattice structure driven by surface stresses. Whereas the propagation of partial dislocations and twin planes specific to fcc metals are the required mechanism, only materials with higher propensities for twinning (e.g. Cu and Ni) show this behaviour and those with lower propensities for twinning (e.g. Al) do not. This paper provides an overview of this novel behaviour with a focus on the transformation mechanism, driving force, reversible strain, size and temperature effects and energy dissipation. A mechanism-based micromechanical continuum model for the tensile behaviour is developed. This model uses a decomposition of the lattice reorientation process into a reversible, smooth transition between a series of phase-equilibrium states and a superimposed irreversible, dissipative propagation of a twin boundary. The reversible part is associated with strain energy functions with multiple local minima and quantifies the energy conversion process between the twinning phases. The irreversible part is due to the ruggedness of the strain energy landscape, associated with dislocation nucleation, gliding and annihilation, and characterizes the dissipation during the transformation. This model captures all major characteristics of the behaviour, quantifies the size and temperature effects and yields results which are in excellent agreement with data from molecular dynamics simulations. read less

Abstract: Molecular dynamics (MD) methods are opening new opportunities for simulating the fundamental processes of material behavior at the atomistic level. However, increasing the size of the MD domain quickly presents intractable computational demands. A robust approach to surmount this computational limitation has been to unite continuum modeling procedures such as the finite element method (FEM) with MD analyses thereby reducing the region of atomic scale refinement. The challenging problem is to seamlessly connect the two inherently different simulation techniques at their interface. In the present work, a new approach to MD-FEM coupling is developed based on a restatement of the typical boundary value problem used to define a coupled domain. The method uses statistical averaging of the atomistic MD domain to provide displacement interface boundary conditions to the surrounding continuum FEM region, which, in return, generates interface reaction forces applied as piecewise constant traction boundary conditions to the MD domain. The two systems are computationally disconnected and communicate only through a continuous update of their boundary conditions. With the use of statistical averages of the atomistic quantities to couple the two computational schemes, the developed approach is referred to as an embedded statistical coupling method (ESCM) as opposed to a direct coupling method where interface atoms and FEM nodes are individually related. The methodology is inherently applicable to three-dimensional domains, avoids discretization of the continuum model down to atomic scales, and permits arbitrary temperatures to be applied. read less

Abstract: The effects of grain boundary structure on the mechanical properties of polycrystalline aluminum are investigated by using molecular dynamics and quasicontinuum simulations. Three problems are simulated: (a) tensile deformation of nanocrystalline aluminum with different grain boundary misorientation distributions, (b) interaction between edge dislocations and tilt grain boundaries, and (c) grain boundary motion under shear deformation. It is found that the arrangements of grain boundary dislocations, which are determined not only by the misorientation angle but also by the deviation angle, strongly control the grain boundary motion; therefore, it is concluded that individual grain boundary structures influence the macroscopic mechanical properties in nanostructured materials. read less

Abstract: The initial yield process and the subsequent formation of prismatic dislocation loops around a spherical inclusion embedded in a single-crystalline Al matrix are studied by atomistic simulations. In conjunction with linear elastic theory, it is confirmed that the maximum shear stress is created at the inclusion–matrix interface on the {1 1 1} plane intersecting the spherical inclusion at a height of (RP: radius of spherical inclusion). The critical pressure, shear stress and strain for dislocation nucleation are then quantitatively determined. Afterwards, prismatic dislocation loops, which are constructed by four pure edge dislocations with the same Burgers vector but not on the same slip planes, are formed by energetically unstable interactions around the inclusion. Consequently, analytical considerations and atomistic simulation provide a clear explanation of experimental observations and an instructive insight into the precipitation problem. read less

Abstract: The dynamic deformation upon stretching of Ni nanowires as those formed in mechanically controllable break junctions is studied. In order to compare with experiments, we also compute the transport properties in the last stages before failure using the first-principles implementation of Landauer's formalism included in our transport package ALACANT. read less

Abstract: The collective grain movement of nanocrystalline metals and its temperature dependence are studied by using molecular dynamics simulations. First, a unit structure that consists of eight aluminium grains in the regular hexagonal shape with 5 nm grain size is prepared, and then an analysis model is made by arranging the same 144 unit structures in the two-dimensional periodicity. Thus the total number of grains is 1152. Various collective grain deformations occur at different temperatures under tensile loading. In the case of 100 K, shear bands formed by the collective grain deformation can be observed remarkably. On the other hand, in the case of 300 or 500 K, no remarkable inhomogeneous deformation such as shear bands occurs. This might be due to the different accommodation mechanism for geometrical misfits by local shear deformation at each different temperature. In order to investigate the effect of the collective grain deformation on the macro-scale mechanical properties, the stress–strain curve for the model with 144 unit structures and an averaged strain–stress curve for the 60 cases of a model with one unit structure are compared. Consequently, it is found that the inhomogeneous plastic deformation mode such as a shear band can influence the strength of nanocrystalline metals. read less

Abstract: Preliminary simulations of simple shear deformation and indentation simulations using different radii of a spherical indenter are performed using molecular dynamics in order to uncover the internal stress state for elastic deformation and subsequent initial plasticity under nano-indentation. An atomic single-crystalline aluminium model containing up to 1,372,000 atoms and an ideal friction-free spherical indenter are presented in a set of simulations. Effects of the stress distribution using several kinds of spherical radii of indenters on the critical condition of dislocation emissions are discussed with much emphasis. The critical shear stress for the dislocation emission under indentation is well accorded with the shear strength under the simple shear deformation exposed to the equivalent external stresses to the indentation-induced stress states. It is confirmed that shear strength strongly depends on the external stress component and therefore, high compressive stress states generated beneath the indenter lead to the much higher critical shear stress than μ/2π. read less

Abstract: The close-range interaction of dislocations and solute clusters in the Al–Mg binary system is studied by means of atomistic simulations. We evaluate the binding energy per unit length of dislocations to the thermodynamically stable solute atmospheres that form around their cores, at various temperatures and average solid solution concentrations. A measure of the cluster size that renders linear the relationship between the binding energy per unit length and the cluster size is identified. The variation of the interaction energy between a dislocation and a cluster residing at a finite distance from its core is evaluated and it is shown that the interaction is negligible once the separation is larger than approximately 15 Burgers vectors. The data are relevant for the dynamics of dislocation pinning during dynamic strain ageing in solid solution alloys and for static ageing. read less

Abstract: Atomistic models were used to determine the properties of dislocation core fields and stacking fault fields in Al and Cu using embedded atom method (EAM) potentials. Long-range, linear elastic displacement fields due to nonlinear behaviour within dislocation cores, the core field, for relevant combinations of Shockley partial dislocations for edge, screw, and mixed (60° and 30°) geometries were obtained. Displacement fields of stacking faults were obtained separately and used to partition the core field of dissociated dislocations into core fields of partial dislocations and a stacking fault expansion field. Core field stresses were derived from which the total force, including the Volterra field plus core field, between dislocations for several dislocation configurations was determined. The Volterra field dominates when the distance between dislocations exceeds about 50b but forces due to core fields are important for smaller separation distances and were found to affect the equilibrium angle of edge dislocation dipoles and to contribute to the force between otherwise non-interacting edge and screw dislocations. Interactions among the components of a dissociated dislocation modify the equilibrium separation for Shockley partials suggesting that methods that determine stacking fault energies using measurements of separation distances should include core fields. read less

Abstract: The strength of nanocrystalline aluminum has been studied using molecular dynamics simulation. Nanocrystalline models consisting of hexagonal grains with grain size $d$ between 5 nm and 80 nm are deformed by the application of tension. A transition from grain-size hardening to grain-size softening can be observed in the region where $d\ensuremath{\approx}30\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, which is the optimum grain size for strength. In the grain-size hardening region, nanocrystalline models primarily deform by intragranular deformation. Consequently, a pile-up of dislocations can be observed. When the grain size becomes less than 30 nm, where the thickness of the grain boundaries cannot be neglected in comparison to the grain sizes, the dominant deformation mechanism of nanocrystalline metals is intergranular deformation by grain boundary sliding. Further, geometrical misfits by grain boundary sliding are accommodated by the grain rotation mechanism. Moreover, cooperative grain boundary sliding occurs in the 5 nm model. The optimum grain size is controlled by the relationship between resistance to intergranular deformation by grain boundary processes and intragranular deformation resisted by the grain boundary. Therefore, the primary role of the grain boundary changes in the region where the optimum grain size is observed. read less

Abstract: We investigate the mechanisms of fatigue behavior in nanocrystalline metals at the atomic scale using empirical force laws and molecular level simulations. A combination of molecular statics and molecular dynamics was used to deal with the time scale limitations of molecular dynamics. We show that the main atomistic mechanism of fatigue crack propagation in these materials is the formation of nanovoids ahead of the main crack. The results obtained for crack advance as a function of stress intensity amplitude are consistent with experimental studies and a Paris law exponent of about 2. read less

Abstract: An efficient approach is presented for performing efficient molecular dynamics simulations of solute aggregation in crystalline solids. The method dynamically divides the total simulation space into "active" regions centered about each minority species, in which regular molecular dynamics is performed. The number, size, and shape of these regions is updated periodically based on the distribution of solute atoms within the overall simulation cell. The remainder of the system is essentially static except for periodic rescaling of the entire simulation cell in order to balance the pressure between the isolated molecular dynamics regions. The method is shown to be accurate and robust for the Environment-Dependant Interatomic Potential (EDIP) for silicon and an Embedded Atom Method potential (EAM) for copper. Several tests are performed beginning with the diffusion of a single vacancy all the way to large-scale simulations of vacancy clustering. In both material systems, the predicted evolutions agree closely with the results of standard molecular dynamics simulations. Computationally, the method is demonstrated to scale almost linearly with the concentration of solute atoms, but is essentially independent of the total system size. This scaling behavior allows for the full dynamical simulation of aggregation under conditions that are more experimentally realizable than would be possible with standard molecular dynamics. read less

Abstract: This paper reports a molecular statics study of Cu surface diffusion barriers, particularly the facet–facet and step–facet barriers. The study focuses on two high-symmetry surfaces or facets, Cu{111} and Cu{100}. Our results show that these two barriers are distinct from conventional step barriers and are independent of facet size once it is beyond three atomic layers. Usually, the facet–facet barrier is substantially larger than diffusion barriers on flat surfaces or down monolayer steps, and the step–facet barrier is substantially larger than diffusion barriers along or across monolayer steps. Exceptions do exist. When two Cu{100} facets are involved, the two barriers decrease as the size of the ending facet increases from one layer to two layers, and then increase from two to three (or more) layers. As a result of the large facet–facet and step–facet barriers, surfaces of Cu thin films are of the order of 100 nm. The small facet–facet and step–facet barriers between two Cu{100} facets, when the ending facet is two to three layers, make it difficult to form another Cu{100} facet near one Cu{100} facet. For the same reason, nanowires along 100/{100} on the Cu{100} are unlikely, while nanowires along 110/{111} are feasible. read less

Abstract: The mechanical properties and deformation mechanisms of nano-polycrystalline (NPC) materials under tension are studied using molecular dynamics. The embedded atom method (EAM by Mishin et al. 1999) and the effective medium theory (EMT by Jacobsen et al. 1987) are adopted as the interatomic potentials of Al to investigate the influence of stacking fault energy (SFE) on the phenomena. The main difference between EAM and EMT potentials is that the latter underestimates the SFE of Al. Simulations using three different models are carried out to study the grain size dependence of the mechanical properties under different strain rate conditions. For all cases, the dependency of maximum stress on grain size can be expressed as an inverse Hall-Petch relation. This tendency is considered and may be directly explained by the volume effect of the grain boundary (GB). Both crystal slips and GB sliding are observed, but GB sliding is predominant in the small-grain model. Most of the crystal slips are caused by motion of perfect dislocation in EAM potential cases, but in EMT potential cases most of the crystal slips are caused by motion of Shockley's partial dislocation. Because the EMT potential underestimates the SFE, the core length of the extended dislocation is comparable to the grain size, and hence many stacking faults remain in grains. During the unique deformation process by partial dislocation movement, a strange rotation of grains is also observed. That is, the deformation mechanism of NPC materials is strongly influenced by the SFE. Another interesting deformation mechanism observed is the grain switching process. From the macroscopic viewpoint, this is quite similar to the switching mechanism proposed by Ashby and Verrall (1973). However, there is a significant difference: in the present study both the GB migration and sliding are predominant but in Ashby and Verrall's study, the mechanism of switching is based on diffusion. read less

Abstract: In this paper, a multiscale modelling strategy is used to study the effect of grain-boundary sliding on stress localization in a polycrystalline microstructure with an uneven distribution of grain size. The development of the molecular dynamics (MD) analysis used to interrogate idealized grain microstructures with various types of grain boundaries and the multiscale modelling strategies for modelling large systems of grains is discussed. Both molecular-dynamics and finite-element (FE) simulations for idealized polycrystalline models of identical geometry are presented with the purpose of demonstrating the effectiveness of the adapted finite-element method using cohesive zone models to reproduce grain-boundary sliding and its effect on the stress distribution in a polycrystalline metal. The yield properties of the grain-boundary interface, used in the FE simulations, are extracted from a MD simulation on a bicrystal. The models allow for the study of the load transfer between adjacent grains of very different size through grain-boundary sliding during deformation. A large-scale FE simulation of 100 grains of a typical microstructure is then presented to reveal that the stress distribution due to grain-boundary sliding during uniform tensile strain can lead to stress localization of two to three times the background stress, thus suggesting a significant effect on the failure properties of the metal. read less

Abstract: This letter presents a concept of surface kinetic barrier: The step–facet barrier. This concept is demonstrated for two face-centered-cubic metals, aluminum and copper, through molecular statics calculations. Our numerical results show that the step–facet barrier is substantially larger than step–step or diffusion barriers on flat surfaces; true for both metals. Based on the relative magnitudes of kinetic barriers, we discuss implications of the step–facet barrier on surface processing, particularly the step flow. This discussion shows that the kinetic barrier potentially may enable us to pattern nanowires on a metal surface. read less

Abstract: It is shown that coverage evolution during atomic deposition on a substrate may be described, on mesoscopic scales, by dynamic models of the reaction–diffusion type. Such models combine reaction terms representing adsorption–desorption and chemical processes and nonlinear diffusion terms which are of the Cahn-Hilliard type. This combination may lead, below a critical temperature, to the instability of uniform deposited layers. This instability leads to the formation of nanostructures which correspond to regular spatial variations in substrate coverage. Patterns’ wavelengths and symmetries are selected by the dynamics and not by variational arguments. For increasing coverage, one should observe, in layers with isotropic atomic diffusion, a succession of structures going from hexagonal arrays of high-coverage dots, to stripes and finally to hexagonal arrays of low-coverage dots. For anisotropic diffusion, stripes perpendicular to the high-mobility direction should be selected. The relevance of this approach to the study of deposited Al, Ti and TiN monolayers is discussed. read less

Abstract: The present work deals with the atomic mechanism responsible for the emission of partial dislocations from grain boundaries (GB) in nanocrystalline metals. It is shown that, in a 12 nm grain-size sample, GBs containing grain-boundary dislocations (GBDs) can emit a partial dislocation during deformation by local atomic shuffling and stress-assisted free-volume migration. As in previous work, the nucleation occurs at a GBD, which, upon nucleation and propagation, is removed. In the present case, free-volume migration occurs away from the nucleation region both before and after the nucleation event. read less

Abstract: This paper presents the results of a large-scale atomistic simulation study of the process of emission of multiple dislocations in Al. We use embedded-atom method potentials based on ab-initio data and molecular statics and dynamics techniques to study the configuration of the crack tip as the dislocation emission process evolves. In the configuration studied, the crack is oriented in a {111}-type plane with a [110]-type crack front and the dislocations are emitted in adjacent inclined {111}-type planes. The dislocations are Shockley partials and they form a twinned region. The number of dislocations emitted increases with increasing applied stress intensity and is limited if the dislocations are not allowed to reach their equilibrium positions The shielding effect of the emitted dislocations decreases the total stress intensity factor at the crack tip but also causes a net decrease in the mode-II stress intensity factor projected on the slip plane of the emitted dislocations. Most importantly, this lower stress intensity along the slip plane limits the emission of new dislocations and, after a number of dislocations are emitted, the crack advances by cleavage for several lattice periods. The process is then repeated, resulting in a combined dislocation emission–crack propagation process. These results suggest a mechanism for the brittle-to-ductile transition that depends strongly on dislocation mobility and pinning behaviour. read less

Abstract: Dislocation emission from a crack tip is a necessary mechanism for crack tip blunting and toughening. A material is intrinsically ductile under Mode I loading when the critical stress intensity KIe for dislocation emission is lower than the critical stress intensity KIc for cleavage. In intrinsically ductile fcc metals, a first partial dislocation is emitted, followed either by a trailing partial dislocation (“ductile” behavior) or a twinning partial dislocation (“quasi-brittle”). K Ie for the first partial dislocation emission is usually evaluated using the approximate Rice theory, which predicts a dependence on the elastic constants and the unstable stacking fault energy γusf . Here, atomistic simulations across a wide range of fcc metals show that K Ie is systematically larger (10–30%) than predicted. However, the critical crack-tip shear displacement is up to 40% smaller than predicted. The discrepancy arises because Mode I emission is accompanied by the formation of a surface step that is not considered in the Rice theory. A new theory for Mode I emission is presented based on the ideas that (i) the stress resisting step formation at the crack tip creates “lattice trapping” against dislocation emission such that (ii) emission is due to a mechanical instability at the crack tip. The new theory naturally includes the energy to form the step, and reduces to the Rice theory (no trapping) when the step energy is small. The new theory predicts a higher K Ie at a smaller critical shear displacement, rationalizing deviations of simulations from the Rice theory. The twinning tendency is estimated using the Tadmor and Hai extension of the Rice theory. Atomistic simulations reveal that the predictions of the critical stress intensity factor K Ie for crack tip twinning are also systematically lower (20–35%) than observed. Energy change during nucleation reveal that twining partial emission is not accompanied by creation of a surface step while emission of the trailing partial creates a step. The absence of the step during twinning motivates a model for twinning nucleation that accounts for the fact that nucleation does not occur directly at the crack tip. New predictions are in excellent agreement with all simulations that show twinning. A second mode of twinning is found wherein the crack first advances by cleavage and then emits the twinning partial at the new crack tip. The stacking fault stress dependence is analyzed through (i) the generalized stacking fault potential energy (GSFE) and (ii) the generalized stacking fault enthalpy (GSFH). At an imposed shear displacement, there is also an associated inelastic normal displacement ∆n around the fault. Atomistic simulations with interatomic potentials and/or first principle calculations reveal that read less

Abstract: Dislocations in fcc crystals are studied here in several length and time scale regimes starting from atomistic calculations up to continuum models. Temperature-dependence of the stacking fault free energy (SFFE) for Fe is calculated utilizing the thermodynamic integration and a reference free energy model for solids based on the quasi-harmonic approximation. The underlying molecular dynamics (MD) simulation is based on the bond order potential for Fe of Müller et al. (2007). The SFFE of Fe at 0 K is calculated to be −20 mJ/m, negative due to the fact that the fcc phase is unstable at this temperature. The SFFE increases with temperature and becomes positive at around 200 K. Depending on system size, an SFFE for Fe between 5.5 and 9.1 mJ/m is obtained at 298 K, increasing to between 70 and 80 mJ/m at 1000 K. Next, the interaction between dislocations and stacking faults at low temperatures is studied with the help of MD. Observed interaction types in Cu include annihilation, penetration, and growth. Of particular importance is the mixed screw-edge character of the partial dislocations involved and the fact that the screw part cross slips more easily than its edge counterpart. The interaction of curved dislocations with twinned crystal is also studied with MD. In two of the in-plane shear loading directions, jerky stress flow is observed. Upon closer investigation, the jerky behavior is related to the fast motion of twin boundary. Next, the Peierls-Nabarro (PN) and Volterra (V) dislocation models are employed for dislocation-mediated bulk twin nucleation and growth. The dynamic model is applied to the modeling of variable dislocation separation in the twin. In this context, dislocations are closest together at the twin tip and increase in separation away from the tip. The phase field model for dislocation is based on periodic microelasticity (Wang et al. 2001, Bulatov & Cai 2006, Wang & Li 2010) to model the strongly non-local elastic interaction of dislocation lines via their (residual) strain fields. The energy storage is modeled here with the help of the ”interface” energy concept and model of Cahn & Hilliard (1958) (see also Allen & Cahn 1979, Wang & Li 2010). The current approach is applied to determine the phase field free energy for Al and Cu. The identified models are then applied to simulate dislocation dissociation, stacking fault formation, glide and dislocation reactions in these materials. Transport and pile-up of infinite discrete dislocation walls driven by non-local interaction and external loading is also studied. The underlying model for dislocation wall interaction is based on the non-singular PN model. The influence of strongly non-local (SNL; long-range) interaction, and its approximation as weakly non-local (WNL; short-range), are studied. The pile-up behavior predicted by the current SNL-based continuous wall distribution modeling is consistent with that predicted by discrete wall distribution modeling (e.g., Roy et al. 2008, de Geus et al. 2013). Both deviate substantially from the pile-up behavior predicted by WNL-based continuous wall distribution modeling (e.g., Dogge 2014, Chapter 2). read less

Abstract: Molecular dynamic (MD) simulation has been conducted to study the effect of interface structure on the mechanical response of eight symmetric tilt grain boundaries in high stacking-fault Al. It is found that the grain boundaries with E structure unit (SU) have higher energy, but the grain boundary energy alone cannot be used as a parameter to determine the mechanical properties of grain boundary. The SUs, especially E units, do have an influence on the mechanical response of grain boundaries. Our results show that the dislocation imitates from E units preferably, but this depends on the dissociation at grain boundary. read less

Abstract: Oxides like silicates, alumina or periclase, are materials with significant properties and are therefore investigated extensively in experiment and in theory. The aim of this PhD thesis was to propose and further to develop methods, which make molecular dynamic simulations of oxides with large particle numbers and for long simulation times possible. The work consists of three parts. In the first one the already existing methods for simulating oxides will be discussed, while in the second one their methodological progress will be presented. The third chapter is solely reserved for the phenomenon of flexoelectricity, which has been discovered during the visualization of the crack propagation in alumina. Oxide, wie z.B. Silikate, Korund oder Periklas, sind bedeutende Funktionswerkstoffe und werden daher experimentell wie theoretisch intensiv untersucht. Ziel dieser Dissertation war es, Verfahren vorzustellen und derart zu optimieren, dass sie Molekulardynamiksimulationen von Oxiden mit grosen Teilchenzahlen und uber lange Zeiten ermoglichen. Die Arbeit gliedert sich dabei in drei Bereiche. Im ersten Teil wird auf die einzelnen bereits vorhandenen Methoden zur Simulation von Oxiden eingegangen, im zweiten Kapitel deren Verbesserung vorgestellt. Der dritte Bereich widmet sich ausschlieslich dem Phanomen der Flexoelektrizitat, welche durch die geschickte Visualisierung der Rissausbreitung in Korund entdeckt wurde. read less

Abstract: Dislocation multiplication from the Frank­Read source is investigated in aluminum by applying atomic models. To express the dislocation bow-out motion and dislocation loop formation, we introduce cylindrical holes as a strong pinning point to the dislocation-bowing segment. The critical configuration for dislocation bow-out in atomic models exhibits an oval shape, which agrees well with the results obtained by the line tension model. The critical shear stress for the dislocation bow-out in atomic models continuously increases with decreasing length L of the Frank­Read source (even at the nanometer scale). This is expressed by the function L11 lnL, which is obtained by a continuum model based on elasticity theory. The critical shear stresses for the Frank­Read source are compared with those for grain boundary dislocation sources, as well as the ideal shear strength. [doi:10.2320/matertrans.MA201319] read less

Abstract: 相転移温度は、着目相の相安定性を測る定量指標となる ため、工業的な材料やプロセス開発には最も重要な物性値 の一つである。例えば、凝固現象において重要な物性値と なる融点は、工業的な固体材料においても高温耐性を定性 的に表す重要な指標としてしばしば利用されている。 代表的な一次相転移となる融解現象(固液相転移)では、 “固体の融解の本質とはいかなるものか“という基本的な 問題が現在でも存在する。この問題に対して、融解前の固 体相に着目し、固体物性論の立場から融解現象の本質に迫 ろうという思想は古く、前世紀初頭の Lindemannや Born の仕事に遡る。彼らは、それぞれ、Lindemann criterionあ るいは Born criterionと今日呼ばれる、「相転移は連続反応 (successive reactions)である」とする Ostwaldの Step Rule的 基準概念を 、固液相転移に対し提案した。前者は、固体 における原子振動の振幅がある閾値に達することが融解現 象につながるとするもの 、後者は、融解現象には固体の 弾性物性が関与し剛性率 (shear modulus)あるいはその異方 性がゼロとなる点が融点であるとするものである 。相転 移前の相の状態の energetics(あるいは instability)と相転 移物性の相関を捉えようとするこのような思想は、ミクロ 理論からの直接的導出が難しい転移温度のような物理量を あからさまに求めることなく、材料のトレンドを把握する 指導原理のようなものにもなり得るため、材料設計という 観点からも有用となる。 本稿では、結晶固体が持つ弾性物性が、熱力学相安定性 に関する物性と、ミクロ的観点からはどのようにつながる のかを著者達の理論研究 もベースに概説し、鋼のマルテ ンサイト変態過程の弾性物性その場測定実験の研究報告に 橋渡ししたい。 2.遷移金属における Friedel モデル read less

Abstract: Microscopic yielding can be realized by the transfer of a dislocation across a grain boundary (GB), or by incorporation between the residual GB dislocation and the dislocations nucleated in the near-field of a GB due to the applied stress. These phenomena are determined by the crystallographic orientation and the multiaxial stress state around a GB. In the present paper, a new boundary interaction criterion of Lor L'-value is proposed, which considers both the contributions of the geometric relationship between two grains and a GB, and the stress state applied to the near-field of a GB. This value and the others so far proposed were calculated for <110>, <100>, and <111> symmetric tilt grain boundaries under uniaxial compression normal to the GB. The dynamic transfer and incorporation of the dislocations nucleated under uniaxial compression normal to the GB plane were then examined using 3-dimensional molecular dynamics simulations. Two kinds of <110> symmetric tilt boundaries of copper 3A and 3B were atomistically modeled. The individual reaction process between the dislocations nucleated from an artificial Frank-Read source introduced in one grain and a GB under uniaxial compression was resolved in detail. Combinations of the preferential slip systems across GBs were discussed in reference to the proposed boundary interaction criterion. The case of 3B with an easier slip transfer across a GB is identical to that predicted, while incorporation of the displacement shift complete (DSC) dislocation migrated on a 3A boundary plane complicates the defect reaction, necessitating a larger critical stress for slip transfer across a GB. read less

Abstract: The competition between free surfaces and internal grain boundaries as preferential sites for dislocation nucleation during plastic deformation in aluminum bicrystalline nanowires is investigated using molecular dynamics simulations at room temperature. A number of nanowires containing various minimum energy interfaces are studied under uniaxial compression at a constant applied strain rate to provide a broad, inclusive look at the competition between the two types of sources. In addition, we conduct a detailed study on the role of the grain boundaries to act as a source, sink, or obstacle for lattice dislocations, as a function of grain boundary structure. This work compares the behavior of bicrystalline nanowires containing both random high-angle boundaries and a series of symmetric tilt grain boundaries to further elucidate the effect of interface structure on its behavior. The results show that grain boundaries in nanowires can be preferred nucleation sites for dislocations and twin boundaries, in addition to efficient sinks and pinning points for migrating dislocations. Plastic deformation behavior at high imposed strains is linked to the underlying deformation processes, such as twinning, dislocation pinning, or dislocation exhaustion/starvation. We also detail some important reactions between lattice dislocations and grain boundaries observed in the simulations, along with the activation of a single-arm source. This work suggests that the cooperation of numerous mechanisms and the structure of internal grain boundaries are crucial in understanding the deformation of bicrystalline nanowires. read less

Abstract: We utilize the relationship between the shape matrix and strain tensor of calculation cell in Parrinello-Rahman's algorithm, which is used in molecular dynamics (MD) methods, and formulate a strain-controlled MD algorithm based on the finite deformation theory of continuum mechanics. We simulate the atomic behavior in a nano-sized polycrystalline aluminum specimen under two different loading conditions. The first loading condition is a simple shear, and the other is a simple shear after compression, which mimics the loading conditions of actual ECAP processing. Atomic strain measure (ASM) is introduced to investigate the distribution of strain at the atomic scale within the specimen. In both cases, it is observed that grain boundary-structured atoms bordering the dislocation emitted from existing grain boundaries (GB) expand onto the slip plane and develop into a new GB plane, accommodating the rotation of adjacent grains. We propose a possible mechanism for grain refinement under severe plastic deformation. Common neighbor analysis indicates that precompression restricts dislocation emission from the GB. ASM results indicate that this deterioration of dislocation emission is caused by configurational change in the GB region that acts as a dislocation source. That is, preferential accumulation of compressive strain is observed near the GB under compression. In addition, precompression promotes GB sliding. read less

Abstract: The minimum energy motions of pure edge and screw dislocations in aluminum were investigated by atomistic transition state analysis. While the Peierls-Nabarro model and its modifications duplicate the essential nature of a dislocation within a crystalline lattice, the atomic-level relaxation of the dislocation core should be considered to estimate the minimum energy barrier. The relaxed atomic structure within and around the dislocation core is derived from the material’s inherent intrinsic properties and is therefore difficult to solve solely by simple analytical models. In this study, the minimum energy barriers and core structures for the quasi-static motions of pure edge and screw dislocations were investigated by the parallelized nudged elastic band method with the embedded atom method potential. We found that the local potential energy is distributed asymmetrically around the dislocation line for the most stable state and that it is bilaterally symmetrical at the transition state of the dislocation motion. The short-ranged structural relaxation of the core rearrangement as well as the wide-ranging elastic stress field is of great importance in realistic dislocation motion. read less

Abstract: We investigate the compressive yielding of Ni single crystals by performing atomistic simulations with the sample diameters in the range of 5nm ∼ 40 nm. Remarkable effects of sample sizes on the yield strength are observed in the nanopillars with two different orientations. The deformation mechanisms are characterized by massive dislocation activities within a single slip system and a nanoscale deformation twining in an octal slip system. A dislocation dynamics-based model is proposed to interpret the size and temperature effects in single slip-oriented nanopillars by considering the nucleation of incipient dislocations. read less

Abstract: Kinetic Monte Carlo (KMC) method realizes the millisecond or second order atomistic thin film growth. Twenty five kinds of events which may occur on Al(111) surface were classified. An attempt frequency and an activation energy of each event were defined using vibration analyses and nudged elastic band (NEB) method by which the minimum energy path (MEP) can be reasonably predicted. Temperature and deposition rate dependences of Al(111) film growth were intensively investigated in the present paper. The higher temperature and the lower rate drive the layer-by-layer film structural change. Two types of islands (fcc and hcp) were seen by modeling without considering the events of diffusion of dimer and trimer, while only fcc islands remain with considering such events. Thus, we find that the primitive events of diffusion of dimer and trimer take important roles in determination of surface morphology. read less

Abstract: Brittle-ductile transition (BDT) behaviour was investigated in low carbon steel deformed by an accumulative roll-bonding (ARB) process. The temperature dependence of its fracture toughness was measured by conducting four-point bending tests at various temperatures and strain rates. The fracture toughness increased while the BDT temperature decreased in the specimens deformed by the ARB process. Arrhenius plots between the BDT temperatures and the strain rates indicated that the activation energy for the BDT did not change due to the deformation with the ARB process. It was deduced that the decrease in the BDT temperature by grain refining was not due to the increase in the dislocation mobility controlled by short-range obstacles. Molecular dynamics simulations revealed that moving dislocations were impinged against grain boundaries, creating a shielding field, and were reemitted from there with increasing strain. Grain refining led to an increase in the fracture toughness at low temperatures and a decrease in the BDT temperature. In the present paper, the roles of grain boundaries have been discussed in order to explain the enhancement in the fracture toughness of fine-grained materials at low temperatures, and the decrease in the BDT temperature. read less

Abstract: The effect of extrinsic grain boundary dislocations (EGBDs) in nonequilibrium grain boundaries on the mechanical properties of ultrafine-grained metals is investigated by molecular dynamics simulations. Aluminum bicrystal models containing cracks and EGBDs impinged from the crack tips are prepared. First, the dependence of the local grain boundary structure on the accommodation mechanism of EGBDs, and on its stress field is studied. Then, the shielding effect of EGBDs on the emissions of dislocations from crack tips is investigated, and the effect of nonequilibrium grain boundaries on the intragranular deformation is discussed. Finally, to investigate the relationship between EGBDs and intergranular deformations, shear loading is applied to the bicrystal models. It is found that extrinsic grain boundaries function as the intergranular deformation source, and the Burgers vector components of the EGBDs lead to anisotropic grain boundary sliding. read less

Abstract: Recent advances in miniaturization and highly-accurate measurement techniques have allowed mechanical properties to be measured at the nanometer scale. Nanoindentation has been widely used because of its applicability in ambient conditions. Unstable displacement burst or the abrupt growth of indent displacement after homogeneous elastic deformation observed in crystalline materials is a unique plastic deformation characteristic (nanoplasticity). In the present paper, a series of atomistic simulations of nanoindentation in single crystalline aluminum and copper are performed in analyzing the critical state for dislocation nucleation and interaction between dislocations beneath the indenter. With reference to the Hertzian solution based on isotropic linear elastic theory, both the anisotropic effect and nonlinear behavior of nanoindentation are discussed in detail. The discovery was made that the incipient yield process is strongly related to the triaxial stress state created beneath the indenter, and that energetically unfavorable interactions accompanied with cross slip induce the formation of prismatic dislocations. read less

Abstract: *** Anisotropic growth process of two kinds of steps on Al (111) substrates is performed using kinetic Monte Carlo (kMC) method. Employed kMC parameters of activation energy and attempt frequency are estimated by nudged elastic band (NEB) method and transition state theory. Obtained set of results suggest that degree of the anisotropic growth clearly depends on substrate temperature and deposition rate. We find microscopic origin of the anisotropic growth is difference of diffusion rates along {111} and {100} steps, and there is a particular growth condition in which strong anisotropy is observed. At high deposition rate and low temperature, new islands which are easily generated on terraces, hinder the growth anisotropy weaker. read less

Abstract: A multiscale modeling strategy is developed to study grain boundary fracture in polycrystalline aluminum. Atomistic simulation is used to model fundamental nanoscale deformation and fracture mechanisms and to develop a constitutive relationship for separation along a grain boundary interface. The nanoscale constitutive relationship is then parameterized within a cohesive zone model to represent variations in grain boundary properties. These variations arise from the presence of vacancies, interstitials, and other defects in addition to deviations in grain boundary angle from the baseline configuration considered in the molecular-dynamics simulation. The parameterized cohesive zone models are then used to model grain boundaries within finite element analyses of aluminum polycrystals. read less

Abstract: Multiscale modeling methods for the analysis of metallic microstructures are discussed. Both molecular dynamics and the finite element method are used to analyze crack propagation and stress distribution in a nanoscale aluminum bicrystal model subjected to hydrostatic loading. Quantitative similarity is observed between the results from the two very different analysis methods. A bilinear traction-displacement relationship that may be embedded into cohesive zone finite elements is extracted from the nanoscale molecular dynamics results. read less

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Abstract: In this study, we propose an atomistic spherical bicrystal model that employs the synthetic driving force method to investigate the dynamics of curved interfaces. The model allows tracking the migration and faceting of spherical grain boundaries. As a demonstration case, a simulation for a grain boundary with Σ3 misorientation is performed. A subsequent analysis of interface energy and stiffness is performed, whereby it is found that the dynamics of [111] and [1¯1¯2] segments in the Σ3 boundary plane fundamental zone follow energy and stiffness trends, while others, e.g., the [01¯1] segment, do not. read less

Abstract: Large language models (LLMs) are playing an increasingly important role in science and engineering. For example, their ability to parse and understand human and computer languages makes them powerful interpreters and their use in applications like code generation are well-documented. We explore the ability of the GPT-4 LLM to ameliorate two major challenges in computational materials science: i) the high barriers for adoption of scientific software associated with the use of custom input languages, and ii) the poor reproducibility of published results due to insufficient details in the description of simulation methods. We focus on a widely used software for molecular dynamics simulations, the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS), and quantify the usefulness of input files generated by GPT-4 from task descriptions in English and its ability to generate detailed descriptions of computational tasks from input files. We find that GPT-4 can generate correct and ready-to-use input files for relatively simple tasks and useful starting points for more complex, multi-step simulations. In addition, GPT-4's description of computational tasks from input files can be tuned from a detailed set of step-by-step instructions to a summary description appropriate for publications. Our results show that GPT-4 can reduce the number of routine tasks performed by researchers, accelerate the training of new users, and enhance reproducibility. read less

Abstract: New insights are provided into the role of vacancy-hydrogen (VaH) complexes, compared to the hydrogen atoms alone, on hydrogen embrittlement of nickel. The effect of the concentration of hydrogen atoms and VaH complexes is investigated in different crystal orientations on dislocation emission and propagation in single crystal of nickel using atomistic simulations. At first, embrittlement is studied on the basis of unstable and stable stacking fault energies as well as fracture energy to quantify the embrittlement ratio (unstable stacking fault energy/fracture energy). It is found that VaH complexes lead to high embrittlement compared to H atoms alone. Next, dislocation emission and propagation at pre-cracked single crystal crack-tip are investigated under Mode-I loading. Depending upon the elastic interaction energy and misfit volume, high local concentrations at the crack front lead to the formation of nickel-hydride and nickel-hydride with vacancies phases. These phases are shown to cause softening due to earlier and increased dislocation emission from the interface region. On the other hand, dislocation propagation under the random distribution of hydrogen atoms and VaH complexes at the crack front or along the slip plane shows that VaH complexes lead to hardening that corroborates well with the increased shear stresses observed along the slip plane. Further, VaH complexes lead to the disintegration of partial dislocation and a decrease in dislocation travel distance with respect to time. The softening during emission and hardening during propagation and disintegration of partial dislocation loops due to VaH complexes fit the experimental observations of various dislocation structures on fractured surfaces in the presence of hydrogen, as reported in literature. read less

Abstract: Modeling metal matrix composites in finite element software requires incorporating a cohesive zone model (CZM) to represent the interface between the constituent materials. The CZM determines the behavior of traction–separation (T–S) in this region. Specifically, when a diffusion zone is formed due to heat treatment, it becomes challenging to determine experimentally the equivalent mechanical properties of the interface. Additionally, understanding the influence of heat treatment and the creation of a diffusion zone on the T–S law is crucial. In this study, the molecular dynamics approach was employed to investigate the effect of the diffusion region formation, resulting from heat treatment, on the T–S law at the interface of a SiC/Al composite in tensile, shear, and mixed-mode loadings. It was found that the formation of a diffusion layer led to an increase in tensile and shear strengths and work of separation compared with the interfaces without heat treatment. However, the elastic and shear moduli were not significantly affected by the creation of the diffusion layer. Moreover, the numerical findings indicated that the shear strength in the diffusion region was higher when compared with the shear strength of the {111} slip plane within the fcc aluminum component of the composite material. Therefore, in the diffusion region, crack propagation did not occur in the pure shear loading case; however, shear sliding was observed at the aluminum atomic layers. read less

Abstract: ABSTRACT Enormous progress has been made in high-pressure research over the last decades in both, experiments and computer simulations, many challenges still remain. This is evidenced by controversial experimental and numerical data even for the simplest atomic systems exhibiting different types of bonding. Here we discuss the determination of the solid–liquid co-existence (melting) lines reviewing the computational techniques for studying the high-pressure melting of atomic systems based on molecular dynamic or Monte Carlo algorithms. Some emphasis is put on presenting the parallel-tempering Monte Carlo method that gives direct access to heat capacity curves and entropic information as a function of temperature allowing for an easy detection and interpretation of the melting transition. For molecular dynamics simulations there exist a variety of methods to extract melting information – here we include a more thorough discussion of thermodynamic integration as it is frequently used for high-pressure melting. Applications of these techniques and discussion for different atomic systems are presented including an overview of experimental and numerical results of the weakly, van-der-Waals bond noble gases, of diamond as a representative for covalent bonding and of alkali metals and iron. We conclude by summarizing some outstanding problems and challenges for numerical simulations. 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: Based on a molecular dynamics (MD) simulation, we investigated the nanohole propagation behaviors of single-crystal nickel (Ni) under different styles of Ni–Ni interatomic potentials. The results show that the MEAM (the modified embedded atom method potential) potential is best suited to describe the brittle propagation behavior of nanoholes in single-crystal Ni. The EAM/FS (embedded atom method potential developed by Finnis and Sinclair) potential, meanwhile, is effective at characterizing the plastic growth behavior of nanoholes in single-crystal Ni. Furthermore, the results show the difference between the different styles of interatomic potentials in characterizing nanohole propagation in single-crystal Ni and provide a theoretical basis for the selection of interatomic potentials in the MD simulation of Ni crystals. read less

Abstract: Overcoming the time scale limitations of atomistics can be achieved by switching from the state-space representation of Molecular Dynamics (MD) to a statistical-mechanics-based representation in phase space, where approximations such as maximum-entropy or Gaussian phase packets (GPP) evolve the atomistic ensemble in a time-coarsened fashion. In practice, this requires the computation of expensive high-dimensional integrals over all of phase space of an atomistic ensemble. This, in turn, is commonly accomplished efficiently by low-order numerical quadrature. We show that numerical quadrature in this context, unfortunately, comes with a set of inherent problems, which corrupt the accuracy of simulations -- especially when dealing with crystal lattices with imperfections. As a remedy, we demonstrate that Graph Neural Networks, trained on Monte-Carlo data, can serve as a replacement for commonly used numerical quadrature rules, overcoming their deficiencies and significantly improving the accuracy. This is showcased by three benchmarks: the thermal expansion of copper, the martensitic phase transition of iron, and the energy of grain boundaries. We illustrate the benefits of the proposed technique over classically used third- and fifth-order Gaussian quadrature, we highlight the impact on time-coarsened atomistic predictions, and we discuss the computational efficiency. The latter is of general importance when performing frequent evaluation of phase space or other high-dimensional integrals, which is why the proposed framework promises applications beyond the scope of atomistics. read less

Abstract: Nanogrinding is one of the main technologies for machining complex surface shapes with nanometer-level precision. The subsurface deformation depth, as an important index of machining quality, directly affects the service life and mechanical properties of machined parts. In order to explore the factors that influence subsurface deformation depth, this work investigated the effects of three factors, namely, grinding speed, grinding depth and crystal orientation, along different crystal planes at the depth of the subsurface deformation layer in a monocrystalline nickel nanofabrication process. By combining molecular dynamics simulation and orthogonal tests, the results showed that, among the three aforementioned factors, the influence of crystal orientation at the depth of the subsurface deformation layer was the greatest, followed by that of grinding depth, while the influence of grinding speed was the weakest. Through the orthogonal tests, the factors affecting the significance of subsurface deformation depth were analyzed, and the results were found to be more meaningful compared with those of current single-factor studies. Meanwhile, in-depth exploration of the nanogrinding mechanism can provide the necessary theoretical basis for the development of nanomachining technology, which is of great significance for the improvement of ultra-precision cutting technology. read less

Abstract: The plastic deformation mechanisms of Ni/Al2O3 interface systems under tensile loading at high strain rates were investigated by the classical molecular dynamics (MD) method. A Rahman–Stillinger–Lemberg potential was used for modeling the interaction between Ni and Al atoms and between Ni and O atoms at the interface. To explore the dislocation nucleation and propagation mechanisms during interface tensile failure, two kinds of interface structures corresponding to the terminating Ni layer as buckling layer (Type I) and transition layer (Type II) were established. The fracture behaviors show a strong dependence on interface structure. For Type I interface samples, the formation of Lomer–Cottrell locks in metal causes strain hardening; for Type II interface samples, the yield strength is 40% higher than that of Type I due to more stable Ni-O bonds at the interface. At strain rates higher than 1×109 s−1, the formation of L-C locks in metal is suppressed (Type I), and the formation of Shockley dislocations at the interface is delayed (Type II). The present work provides the direct observation of nucleation, motion, and reaction of dislocations associated with the complex interface dislocation structures of Ni/Al2O3 interfaces and can help researchers better understand the deformation mechanisms of this interface at extreme conditions. read less

Abstract: The structural complexities of grain boundaries (GBs) result in their complicated property contributions to polycrystalline metals and alloys. In this study, we propose a GB structure descriptor by linearly combining the average two-point correlation function (PCF) and standard deviation of PCF via a weight parameter, to reveal the standard deviation effect of PCF on energy predictions of Cu, Al and Ni asymmetric tilt GBs (i.e., Σ3, Σ5, Σ9, Σ11, Σ13 and Σ17), using two machine learning (ML) methods; i.e., principal component analysis (PCA)-based linear regression and recurrent neural networks (RNN). It is found that the proposed structure descriptor is capable of improving GB energy prediction for both ML methods. This suggests the discriminatory power of average PCF for different GBs is lifted since the proposed descriptor contains the data dispersion information. Meanwhile, we also show that GB atom selection methods by which PCF is evaluated also affect predictions. read less

Abstract: This paper investigated the micromechanical behavior of different 6H-SiC/Al systems during the uniaxial tensile loading by using molecular dynamics simulations. The results showed that the interface models responded diversely to the tensile stress when the four low-index surfaces of the Al were used as the variables of the joint surfaces. In terms of their stress–strain properties, the SiC(0001)/Al(001) models exhibited the highest tensile strength and the smallest elongation, while the other models produced certain deformations to relieve the excessive strain, thus increasing the elongation. The SiC(0001)/Al(110) models exhibited the largest elongations among all the models. From the aspect of their deformation characteristics, the SiC(0001)/Al(001) model performed almost no plastic deformation and dislocations during the tensile process. The deformation of the SiC(0001)/Al(110) model was dominated by the slip of the 1/6 <112> Shockley partial dislocations, which contributed to the intersecting stacking faults in the model. The SiC(0001)/Al(111) model produced a large number of dislocations under the tensile loading. Dislocation entanglement was also found in the model. Meanwhile, a unique defect structure consisting of three 1/6 <110> stair-rod dislocations and three stacking faults were found in the model. The plastic deformation in the SiC(0001)/Al(112) interface model was restricted by the L-C lock and was carried out along the 1/6 <110> stair-rod dislocations’ direction. These results reveal the interfacial micromechanical behaviors of the 6H-SiC/Al composites and demonstrate the complexity of the deformation systems of the interfaces under stress. read less

Abstract: Development of high-performance lithium-ion batteries (LIBs) is boosted by the needs of the modern automotive industry and the wide expansion of all kinds of electronic devices. First of all, improvements should be associated with an increase in the specific capacity and charging rate as well as the cyclic stability of electrode materials. The complexity of experimental anode material selection is now the main limiting factor in improving LIB performance. Computer selection of anode materials based on first-principles and classical molecular dynamics modeling can be considered as the main paths to success. However, even combined anodes cannot always provide high LIB characteristics and it is necessary to resort to their alloying. Transmutation neutron doping (NTD) is the most appropriate way to improve the properties of thin film silicon anodes. In this review, the effectiveness of the NTD procedure for silicene/graphite (nickel) anodes is shown. With moderate P doping (up to 6%), the increase in the capacity of a silicene channel on a Ni substrate can be 15–20%, while maintaining the safety margin of silicene during cycling. This review can serve as a starting point for meaningful selection and optimization of the performance of anode materials. read less

Abstract: The mechanical properties of ceramic–metal nanocomposites are greatly affected by the equivalent properties of the interface of materials. In this study, the effect of vacancy in SiC on the interdiffusion of SiC/Al interfaces is investigated using the molecular dynamics method. The SiC reinforcements exist in the whisker and particulate forms. To this end, cubic and hexagonal SiC lattice polytypes with the Si- and C-terminated interfaces with Al are considered as two samples of metal matrix nanocomposites. The average main and cross-interdiffusion coefficients are determined using a single diffusion couple for each system. The interdiffusion coefficients of the defective SiC/Al are compared with the defect-free SiC/Al system. The effects of temperature, annealing time, and vacancy on the self- and interdiffusion coefficients are investigated. It is found that the interdiffusion of Al in SiC increases with the increase in temperature, annealing time, and vacancy. read less

Abstract: The crack propagation behavior of Al containing Mg–Si clusters is investigated using molecular dynamics (MD) simulations to demonstrate the relationship between the natural aging time in Al–Si–Mg alloys and ductility. Experimental results show that the elongation at failure decreases with natural aging. There are few studies on the relationship between natural aging and ductility because of the difficult observation of Mg–Si clusters. To solve the difficulty, cracked Al containing Mg–Si clusters of varying sizes are assumed for the MD simulations. A larger Mg–Si cluster in Al results in earlier crack opening and dislocation emission. Moreover, as the Mg–Si cluster size increases, the stress near the crack tip becomes more concentrated. This causes rapid crack propagation, a similar effect to that of crack tip sharpening. As a result of long-term natural aging, the cracks expand rapidly. The influence of geometry is also investigated. Crack lengthening and thickness reduction negatively impact the fracture toughness, with the former having a larger impact than the latter. Although there are several discrepancies in the practical deformation conditions, the simulation results can help to more thoroughly understand natural aging in Al–Si–Mg alloys. read less

Abstract: The mechanical properties of the SiC/Al interface are crucial in estimating the overall strength of this ceramic-metal composite. The present work investigates the interdiffusion at the SiC/Al interface using molecular dynamics simulations. One cubic and one hexagonal SiC with a higher probability of orientations in contact with Al are examined as two samples of metal-matrix nanocomposites with whisker and particulate reinforcements. These reinforcements with the Si- and C-terminated surfaces of the SiC/Al interfaces are also studied. The average main and cross-interdiffusion coefficients are evaluated using a single diffusion couple for each system. The effect of temperature and annealing time are analysed on the self- and interdiffusion coefficients. It is found that the diffusion of Al in SiC is similar in cubic and hexagonal SiC and as expected, the interdiffusion coefficient increases as the temperature and annealing time increase. The model after diffusion can be used to evaluate the overall mechanical properties of the interface region in future studies. read less

Abstract: Incipience of plastic flow in nanoporous metals under tension is an important point for the development of mechanical models of dynamic (spall) fracture. Here we study axisymmetric deformation with tension of nanoporous aluminum with different shapes and sizes of nanopores by means of molecular dynamics (MD) simulations. Random deformation paths explore a sector of tensile loading in the deformation space. The obtained MD data are used to train an artificial neural network (ANN), which approximates both an elastic stress–strain relationship in the form of tensor equation of state and a nucleation strain distance function. This ANN allows us to describe the elastic stage of deformation and the transition to the plastic flow, while the following plastic deformation and growth of pores are described by means of a kinetic model of plasticity and fracture. The parameters of this plasticity and fracture model are identified by the statistical Bayesian approach, using MD curves as the training data set. The present research uses a machine-learning-based approximation of MD data to propose a possible framework for construction of mechanical models of spall fracture in metals. read less

Abstract: ABSTRACT A high-dimensional neural network interatomic potential was developed and used in molecular dynamics simulations of condensed phase Ni and Ni systems with liquid–solid phase coexistence. The reference data set was generated by sampling the potential energy surface over a broad temperature-pressure domain using ab initio MD simulations to train a unified potential. Excellent agreement was achieved between bulk face-centred cubic nickel thermal expansion simulations and relevant experimental data. The same potential also yields accurate structures and diffusivities in the liquid state. The phase transition between liquid and solid phases was simulated using the two-phase interface method. The predicted melting point temperature is within a few kelvins of the literature value. The general methodology could be applied to describe crystals with much more complex phase behaviours. read less

Abstract: Although it has been demonstrated that BNNS/Al composites have excellent tensile and compressive properties, the effect of BNNS's distribution on the hardness of BNNS/Al composites has been rarely studied. In this work, molecular dynamics (MD) simulations are performed to obtain the nanoindentation process of the BNNS/Al models with different distribution angles of BNNS. The results show that the distribution angle of BNNS in the Al matrix has little effect on the hardness and dislocation density. However, different angles have a great influence on the deformation of the Al matrix. When the plane of BNNS is in the same direction as the slippage, the atomic strain of the Al matrix can extend to the bottom of the material. While other distributions have a hindering effect on the atomic strain of the Al matrix. In addition, the distribution of BNNS also changes the extension direction of the phase transition of the Al matrix. 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: This work presents first insights into the dynamics of free-surface release clouds from dynamically compressed polystyrene and pyrolytic graphite at pressures up to 200 GPa, where they transform into diamond or lonsdaleite, respectively. These ejecta clouds are released into either vacuum or various types of catcher systems, and are monitored with high-speed recordings (frame rates up to 10 MHz). Molecular dynamics simulations are used to give insights to the rate of diamond preservation throughout the free expansion and the catcher impact process, highlighting the challenges of diamond retrieval. Raman spectroscopy data show graphitic signatures on a catcher plate confirming that the shock-compressed PS is transformed. First electron microscopy analyses of solid catcher plates yield an outstanding number of different spherical-like objects in the size range between ten(s) up to hundreds of nanometres, which are one type of two potential diamond candidates identified. The origin of some objects can unambiguously be assigned, while the history of others remains speculative. read less

Abstract: Dynamic mechanical properties play an essential role in governing the intrinsic fatigue behavior of superalloys. In this work, [001](010), [110](−110), and [101](010) pre-existing center cracks model of nickel single crystals under increasing cyclic shear deformations were studied by molecular dynamics simulations. More importantly, we introduced three hyper-gravity forces, i.e. 3 × 1012 g, 4 × 1012 g, and 5 × 1012 g, during the fatigue deformation to simulate the high-speed rotation of the blade. The stress intensity factor for the first dislocation nucleation indicates that the critical stress is strongly dependent on the hyper-gravity intensities and temperatures. The fatigue life decreased rapidly with the elevated hyper-gravity strength. Moreover, the [001](010) crack propagation shows a brittle-to-ductile transition at temperatures below 300 K and is suppressed at high temperatures. The crack length in the relation to hyper-gravity intensities is discussed and shows anisotropy along the direction of hyper-gravity. No crack propagation is observed in [110](−110) and [101](010) central crack models. read less

Abstract: Grain boundaries (GBs) have been studied for decades, but it remains a challenging task to describe characteristic GB properties by simple structural descriptors, especially at the local atomic level. In this paper, we use the atomic descriptor based on the smooth overlap of atomic positions (SOAP) to study the atomic energy at GBs by using machine learning and propose a route to simplify it. It is found that, compared with conventional local atomic descriptors such as the Voronoi index, excess volume, centrosymmetry, or local entropy, the SOAP vector shows excellent predictive performance for the atomic energy among the 172 Al and 388 Ni coincidence site lattice (CSL) GBs as well as general GBs in the nanocrystalline model. Additionally, we successfully used the datasets of GBs and amorphous models to predict the atomic energy of one another, which proves the similarity between the local atomic environments (LAEs) in GBs and the amorphous state. Furthermore, the distribution of local distortion factors based on the SOAP vector shows the transition in atomic pack ordering from special CSL GBs to general GBs and the amorphous structures. The simplified descriptor we propose can reduce the original SOAP vector from > 1000 features to only a few yet still shows the superior predictive performance of the atomic energy at GBs in all cases than the conventional descriptors combined. It is expected that the simplification process can be adapted to study more complex GB behaviors. A simple and efficient descriptor of the LAEs should allow us to have a clearer picture of the structure-property correlation in GBs, which is essential for GB engineering. read less

Abstract: ABSTRACT In this article, the mechanical properties of the boron-nitride nano-sheet (BNNS)-reinforced aluminium (Al) nanocomposite are estimated by using molecular dynamics (MD) models. Different nanoscale representative volume elements (NRVEs) have been considered to predict the stiffness and strength properties of BNNS-Al NRVE under tensile and shear loading conditions. A comparison is also made between the stress–strain behaviour of graphene sheet (GS)-Al and BNNS-Al nanocomposites. BNNS-Al nanocomposites are found to be more ductile and less stiff than GS-Al nanocomposite. Effect of layering of BNNS on the stress–strain response is observed and found that, in contrast to the layering of graphene sheets, the stiffness properties of BNNS-reinforced Al nanocomposite are not sensitive to the layering of BNNS. It is established that the layering of BNNS marginally enhance the mechanical properties of the nanocomposite. It is also found that the chirality of BNNS marginally affects the stress–strain response of the nanocomposite irrespective of the loading condition i.e. tensile/shear loading. read less

Abstract: Detailed comparative molecular dynamics simulations of the diffusion process in a model quinary equiatomic FeNiCrCoCu FCC alloy are presented. Vacancy-assisted diffusion is studied by a statistical technique obtaining distributions of vacancy formation and migration energy values. In addition, vacancy migration is simulated using molecular dynamics at high temperatures and monitoring mean square displacements over time. To assess the role of compositional complexity, the results are compared to corresponding simulations in each of the pure individual components of the alloy as well as the corresponding “average atom” potential, with similar properties to the alloy but no compositional randomness. The comparison shows that the diffusion kinetics in the random alloy is not slower than in the average atom material or the average of the components, indicating that compositional fluctuations do not always result in “sluggish” diffusion. The results are compared with experimental data for self-diffusion in similar high-entropy alloys. read less

Abstract: : Nanoindentation on crystalline materials is generally believed to generate a 22 three-dimensional dislocation-dominated plastic zone, which has a semi-spherical shape with 23 a diameter no larger than a few times the indentation depth. Here, by observing 24 nanoindentation on aluminum in situ inside a transmission electron microscope, we 25 demonstrate that the conventional three-dimensional plasticity dominated by regular 26 dislocations triumph as the contact size upon yielding increases above ~100 nm. However, 27 when the contact diameter is less than ~50 nm, a narrow and long (hereafter referred to as 28 “one dimensional”) plastic zone can be created in front of the tip, as the indenter successively 29 injects prismatic dislocation loops/helices into the crystal. Interestingly, this one-dimensional 30 plastic zone can penetrate up to hundred times the indentation depth, far beyond the 31 prediction given by the Nix-Gao model. Our findings shed new light on understanding the 32 dislocation behavior during nanoscale contact. The experimental method also provides a potentially novel way to interrogate loop-defects interactions, and to create periodic loop arrays at precise positions for the modification of properties (e.g., strengthening). read less

Abstract: Coating an alloys film onto a metallic surface could dramatically improve the surface quality. This report studies the microstructure and intermixing phenomena of Ni50Co50 film deposited on Ni(001) substrate with flat, asperity and trench Ni surfaces by molecular dynamics (MD) simulation. The effects of the film thickness and loading velocity on the mechanical properties and deformation behaviours of the sample are also surveyed by indentation. The results represent that the intermixing and lattice structure of the film is enhanced after annealing. Moreover, the sample hardness is improved as the deposited Ni50Co50 film when the film thickness rising from 18 to 38 Å. In contrast, the structure transformation rate and dislocations density of the sample decrease when the Ni50Co50 film becomes thicker. Interestingly, the plastic deformation rate and dislocation density of the sample at the trench surface are higher than the flat one. Besides, the increase of the loading velocity gives rise to the plastic deformation and the local stress rates. The dislocation density of the Ni50Co50/Ni sample is reduced if the loading speed is high enough. read less

Abstract: The void coalescence under strong dynamic loading is a significant spallation process for ductile metals. Since the spallation is basically dominated by tension waves, most void coalescence studies have focused on the tension effect. However, it is known that in spallation, the material initially undergoes a strong compression wave, and then an irreversible deformation is produced by the compression wave inside the material. Therefore, in this paper, the effect of compression on the void coalescence is investigated using the molecular dynamics (MD) simulation. It was found that as the compressive strain increases, the yield strength decreases first and then increases. The results showed that due to the Bauschinger effect (BE), the yield strength decreases by 19.43% from 5.66 GPa without compressive loading to 4.56 GPa when the compressive strain is −7.5%, after which the yield strength increases. The voids do not coalesce when the compressive strain is −8%. In addition, it was found that during the compressive phase, the void surfaces would generate dislocations, which could obstruct the void coalescence in the tensile phase. Furthermore, under compressive loading, the temperature effect on the void coalescence was studied, and it was found that lower temperatures could suppress the void coalescence. read less

Abstract: When working out 3D building-up modes, it is necessary to predict the material properties of the resulting products. For this purpose, the crystallography of aluminum bronze grains after electron beam melting has been studied by EBSD analysis methods. To estimate the possibility of sample form changes by pressure treatment, we simulated structural changes by the method of molecular dynamics during deformation by compression of individual grains of established growth orientations. The analysis was carried out for free lateral faces and grain deformation in confined conditions. Simulation and experiments on single crystals with free lateral faces revealed the occurrence of stepwise deformation in different parts of the crystal and its division into deformation domains. Each domain is characterized by a shear along a certain slip system with the maximum Schmidt factor. Blocking the shear towards the lateral faces leads to selectivity of the shear along the slip systems that provide the required shape change. Based on the simulation results, the relationship between stress–strain curves and structural characteristics is traced. A higher degree of strain hardening and a higher density of defects were found upon deformation in confined conditions. The deformation of the columnar grains of the built material occurs agreed with the systems with the maximum Schmidt factor. read less

Abstract: The mechanical deformation of cellular structures in the selective laser melting (SLM) of aluminum was investigated by performing a series of molecular dynamics (MD) simulations of uniaxial tension tests. The effects of crystalline form, temperature, and grain orientation of columnar grains on the mechanical properties of SLM aluminum were examined. The MD results showed that the tensile strength of SLM aluminum with columnar grains at different temperatures was lower than that of single-crystal aluminum, but greater than that of aluminum with equiaxed grains. The tensile strength and Young’s modulus both decreased approximately linearly upon increasing the temperature. The deformation mechanisms of equiaxed and columnar grains included dislocation slip, grain boundary migration, and torsion, while the deformation mechanisms of single crystals included stacking fault formation and amorphization. Finally, the influence of the columnar grain orientation on the mechanical properties was studied, and it was found that the Young’s modulus was almost independent of the grain orientation. The tensile strength was greatly affected by the columnar grain orientation. Reasonable control of the grain orientation can improve the tensile strength of SLM aluminum. read less

Abstract: The penetration process has attracted increasing attention due to its engineering and scientific value. In this work, we investigate the deformation and damage mechanism about the nanoscale penetration of single-crystal aluminum nanorod with atomistic simulations, where distinct draw ratio (∅) and different incident velocities (up) are considered. The micro deformation processes of no penetration state (within 2 km/s) and complete penetration (above 3 km/s) are both revealed. The high-speed bullet can cause high pressure and temperature at the impacted region, promoting the localized plastic deformation and even solid-liquid phase transformation. It is found that the normalized velocity of nanorod reduces approximately exponentially during penetration (up < 3 km/s), but its residual velocity linearly increased with initial incident velocity. Moreover, the impact crater is also calculated and the corresponding radius is manifested in the linear increase trend with up while inversely proportional to the ∅. Interestingly, the uniform fragmentation is observed instead of the intact spallation, attributed to the relatively thin thickness of the target. It is additionally demonstrated that the number of fragments increases with increasing up and its size distribution shows power law damping nearly. Our findings are expected to provide the atomic insight into the micro penetration phenomena and be helpful to further understand hypervelocity impact related domains. read less

Abstract: Dislocations in high entropy alloys (HEAs) are wavy and have natural pinning points due to the variable chemical and energetic landscape surrounding the dislocation core. This can influence the critical shear stress necessary to initiate dislocation motion and the details associated with sustained dislocation glide. The objective of this work is to determine the relationship between Schmid shear stress and dislocation velocity in single phase FCC FeNiCrCoCu HEAs using molecular dynamics simulations, with comparisons made to dislocation motion in homogeneous Ni and Cu. Simulations are performed for four different dislocation character angles: 0◦ (screw), 30◦, 60◦ and 90◦ (edge). Several key differences are reported, compared to what is previously known about dislocation motion in homogeneous FCC metals. For example, the drag coefficient B in the phonon damping regime for HEAs has a nonlinear dependence on temperature, whereas this dependence is linear in Ni. Mobility relationships between different types of dislocations common in homogeneous FCC metals, such as the velocity of screw and 60◦ dislocations being lower than edge and 30◦ dislocations at the same shear stress, do not necessarily hold in HEAs. Dislocation waviness is measured and is found to correlate with the ability of dislocations to glide under an applied shear stress, including the temperature dependence of the drag coefficient B. These results confirm that the influence of HEA chemical complexity on dislocation motion is important and this data can be used to guide development of analytical or empirical models for dislocation mobility in HEAs. ∗Author to whom any correspondence should be addressed. 0965-0393/21/085017+24$33.00 © 2021 IOP Publishing Ltd Printed in the UK 1 Modelling Simul. Mater. Sci. Eng. 29 (2021) 085017 Y Shen and D E Spearot read less

Abstract: The development of novel materials has challenges besides their synthesis. Materials such as novel MXenes are difficult to probe experimentally due to their reduced size and low stability under ambient conditions. Quantum mechanics and molecular dynamics simulations have been valuable options for material properties determination. However, computational materials scientists may still have difficulty finding specific force field models for their simulations. Force fields are usually hard to parametrize, and their parameters’ determination is computationally expensive. We show the Lennard-Jones (2-body interactions) combined with the Axilrod-Teller (3-body interactions) parametrization process’ applicability for metals and new classes of materials (MXenes). Because this parametrization process is simple and computationally inexpensive, it allows users to predict materials’ behaviors under close-to-ambient conditions in molecular dynamics, independent of pre-existing potential files. Using the process described in this work, we have made the Ti2C parameters set available for the first time in a peer-reviewed work. read less

Abstract: Layering two-dimensional van der Waals materials provides a high degree of control over atomic placement, which could enable tailoring of vibrational spectra and heat flow at the sub-nanometer scale. Here, using spatially resolved ultrafast thermoreflectance and spectroscopy, we uncover the design rules governing cross-plane heat transport in superlattices assembled from monolayers of graphene (G) and MoS2 (M). Using a combinatorial experimental approach, we probe nine different stacking sequences, G, GG, MG, GGG, GMG, GGMG, GMGG, GMMG, and GMGMG, and identify the effects of vibrational mismatch, interlayer adhesion, and junction asymmetry on thermal transport. Pure G sequences display evidence of quasi-ballistic transport, whereas adding even a single M layer strongly disrupts heat conduction. The experimental data are described well by molecular dynamics simulations, which include thermal expansion, accounting for the effect of finite temperature on the interlayer spacing. The simulations show that an increase of ∼2.4% in the layer separation of GMGMG, relative to its value at 300 K, can lead to a doubling of the thermal resistance. Using these design rules, we experimentally demonstrate a five-layer GMGMG superlattice "thermal metamaterial" with an ultralow effective cross-plane thermal conductivity comparable to that of air. read less

Abstract: Fusion of metallic nanowires (NWs) is of increasing interest for fabricating printed devices. Atomistic simulations of inter‐NW neck growth during thermal fusion of vertically stacked silver nanowires (NWs) with nonorthogonal axes are performed, a geometric configuration that is commonly seen in applications. High NW rotation during fusion is uncovered surprisingly and found that it accelerates inter‐NW neck growth beyond that explainable by conventional geometric arguments. Rotation‐regulated surface diffusion and dislocation generation are found to be the culpable mechanisms and are shown to be dominant in distinct regimes of initial NW orientation. Motivated by these atomistic observations, an original analytical model of inter‐NW neck growth is formulated and validated. The model accurately predicts the unusual trends in neck growth with six orders of magnitude lesser computational effort than atomistic simulations. Further, it can handle nonisothermal temperature histories over millisecond time scales for NWs up to 100 nm in diameter, a capability that is beyond the reach of typical atomistic simulations. The impact of the revealed spatial disparity of nanoscale neck growth on the properties of random‐packed NW assemblies, and the foundational role of the model in rational design and processing of printed multi‐NW assemblies for a range of applications are discussed. read less

Abstract: Despite the fact that aluminum is one of the most commonly-used elements, experimental results on the value of its surface tension are largely scattered due to the high sensitivity of aluminum to the atmospheric conditions, leading to huge experimental challenges. In this study, the surface tension of pure Al and Al-O systems was studied in detail using Molecular Dynamics (MD) simulations. A force field that includes embedded atoms method and charge transfer ionic potential was applied to account for interatomic interactions. Simulations were performed at different temperatures (1000-2200 K) with different initial oxygen contents. The simulations allowed us to elucidate the effects of well-controlled atmospheric conditions on surface tension. Our results show that the surface tension of aluminum is sensitive to the amount of oxygen content at the surface, which depends on the total oxygen content and the temperature. At different temperatures, different amounts of oxygen atoms are needed to saturate the aluminum surface ( XSAT0 ). A relationship between XSAT0 and temperature was derived. Due to the scattered data in the literature, a new experiment was performed to measure the surface tension of pure aluminum at two different temperatures. Our MD simulations show a good agreement with these experimental results. We believe that this study can shed light on the underlying mechanisms controlling surface tension of aluminum and could offer routes to better engineer the surface properties of this liquid metal. read less

Abstract: Breathing modes are closely related to many physical properties of nanoclusters. Decades of research, however, failed to formulate a general and unambiguous definition. Here we present a straightforward and widely applicable definition of breathing modes based on power spectra of geometric quantities, namely, surface area, volume, etc. Applying group theory, normal-mode analysis, and molecular dynamics simulations, we have explored breathing modes of several ${\mathrm{Al}}_{n}$ clusters with high and low symmetries. The results suggest that our definition is able to cover not only common breathing modes but also some hidden modes. Our consistent definition also allows us to make a comprehensive and in-depth comparison with Lamb's continuous medium model, which reveals some high-frequency breathing modes are explicable only at the atomic level. read less

Abstract: Oriented attachment processes between nanocrystals provide a promising route for the synthesis of mesocrystals that, although made of the same elements as their usual crystal counterparts, neverthe... read less

Abstract: Shear-coupled grain boundary (GB) migration has been evidenced as an effective plastic mechanism in absence of the usual intragranular dislocation activity. GB migration occurs through the nucleation and further motion of disconnections. For perfect GBs, the activation barrier for migration is dominated by the disconnection nucleation. In this study, we examine the effects of a dipole of sessile disconnections on the shear-coupled GB migration using molecular dynamics simulations on a $\mathrm{\ensuremath{\Sigma}}41[001](540)$ symmetric tilt GB in aluminum. The first effect is observed on disconnection nucleation: we show that the disconnection dipole can operate as a source of mobile disconnections. The corresponding yield stress is weakly affected, but the GB migration energy barrier can be reduced by $35%$. The second effect is seen on the disconnection mobility: the sessile disconnection dipole impedes the disconnection motion by repulsing or attracting it. We conclude that the influence of such dipole on the GB migration favors the nucleation of mobile disconnections on one hand but slows down their motion on the other hand. read less

Abstract: In this work, the study of the regularities of shear deformation in the bulk of fcc single crystals was carried out on the basis of molecular dynamics modeling. The deformation (010) of single crystals in the form of a tetragonal prism with lateral faces {101} and {100} with different ratios of the sample width to its height has been studied. Modeling has shown that the sample vertices are the preferred sites for shear initiation. It was found that the formation of deformation domains and the interaction of shear planes are due to the geometry of shear planes in the bulk of the single crystal: their location relative to the base stress concentrators and their orientation relative to the lateral faces. read less

Abstract: The small atomic region containing a defect in a crystalline solid is nonlinear elastic. In this paper, we consider a solid containing defects as a composite wherein the defected regions have a bulk modulus, K, different from that of the perfect crystal. If the volume fraction of the defected regions is sufficiently small, Vegards Law should apply so that K of the composite depends linearly on the volume fraction. We identify the slope of that linear dependence as the signature, denoted S. Atomistic models of EAM Ni were used to compute K versus reciprocal volume for several kinds of defects, namely single, di-, and tri-vacancies and interstitials, dislocations, and grain boundaries. The results show 1) when the volume fraction of the defected region is small, Vegards Law does apply, and 2) S has a distinct value for each defect and is negative for vacancies but positive for the other defects. Thus, S depends on the effective modulus of the nonlinear region and its size. A spherical ball-in-hole composite model predicts that the variation of K with volume fraction is very nearly linear. The signs and magnitudes of S, although quite small, are discussed as a potential means of monitoring defect changes in the concentration via sensitive experimental measurements of the bulk modulus. read less

Abstract: This paper introduces Voxelized Atomic Structure (VASt) potentials as a machine learning (ML) framework for developing interatomic potentials. The VASt framework utilizes a voxelized representation of the atomic structure directly as the input to a convolutional neural network (CNN). This allows for high fidelity representations of highly complex and diverse spatial arrangements of the atomic environments of interest. The CNN implicitly establishes the low-dimensional features needed to correlate each atomic neighborhood to its net atomic force. The selection of the salient features of the atomic structure (i.e., feature engineering) in the VASt framework is implicit, comprehensive, automated, scalable, and highly efficient. The calibrated convolutional layers learn the complex spatial relationships and multibody interactions that govern the physics of atomic systems with remarkable fidelity. We show that VASt potentials predict highly accurate forces on two phases of silicon carbide and the thermal conductivity of silicon over a range of isotropic strain. read less

Abstract: The mechanical behavior of $\ensuremath{\langle}111\ensuremath{\rangle}$ ultrathin gold nanowires (NWs) and their dependence on the correlated parameters of size (i.e., diameter $\ensuremath{\sim}1--1.6$ nm), morphology (rectangular and hexagonal prism), and ultrahigh density twin boundaries was investigated using density functional theory calculations. The parameters of size, morphology, and the presence of twins all significantly influence the mechanical behavior of Au NWs, and their effects are interdependent. Importantly, an ultrahigh density of twins in NWs enhances their strength in both morphologies, and across all sizes studied. Our calculations reveal a remarkable range of Young's modulus (60--158 GPa) and ultimate tensile strengths (3.7--14.0 GPa), suggesting that the scatter observed in experimental research involving ultrathin gold nanowires is a feature of intrinsic complexity. In particular, a ``wrinkling'' of the atomic planes along the wire axis due to a combination of anisotropic surface stresses and wire core response, is demonstrated to be largely suppressed by the presence of high-density twins. The high tensile strengths of the studied nanowires with twins is attributed to a combination of the suppression of this anisotropic phenomena and slip plane disruption. This work demonstrates that precise control of these parameters is required during synthesis to achieve target mechanical behaviour in industrial device applications. Moreover, the work demonstrates that this system has the potential to be tuned for access to a vast range of materials design space. read less

Abstract: In this work, the interaction of the moving edge dislocation with obstacles in form of copper atoms is studied using the molecular dynamics simulations. The samples are aluminum monocrystals of 52 × 60 × 15 nm3 with axes oriented along directions [110], [111], [112]. The structure of copper solid solution is reproduced with following procedure: aluminum atoms are randomly selected and replaced by copper atoms. The concentration of copper atoms varies from 0.25% to 1%. The dislocation movement occurs under action of shear deformation. It is found that zones with a low concentration of copper atoms only slow down dislocation in an aluminum matrix, and the zones with a high local concentration of copper atoms not only produces stronger resistance to dislocation movement, but also they cause the change in the slip plane of the dislocation segment. When a significant part of a dislocation line moves to a neighboring slip plane, the complete transition of the dislocation to this slip plane can occur. It is also noted that such transitions of dislocation segments from one slip plane to another are accompanied by the formation of vacancies. Also the maximum value of the shear stress σxy is estimated-its value is approximately 250 MPa. read less

Abstract: Molecular dynamics simulations are performed to investigate the effect of temperature on the plastic deformation mechanism in aluminum single crystal. It is found that as temperature increases the Yield strength and Young’s modulus of the aluminum under compressive and tensile strain will reduce. Moreover, it is found that the higher temperature is, the easier the dislocation emission is. Under compressive strain, the proportion of 1/6<112> Shockley type of dislocations to total dislocations is found to increase with the temperature increasing. It is also found that only a large amount of dislocation occurring incipiently the strength of the material can be yield. read less

Abstract: Dislocation motion through a random alloy is impeded by its interactions with the compositional fluctuations intrinsic to the alloy, leading to strengthening. A recent theory predicts the strengthening as a function of the solute-dislocation interaction energies and composition. First-principles calculations of solute/dislocation interaction energies are computationally expensive, motivating simplified models. An elasticity model for the interaction reduces to the pressure field of the dislocation multiplied by the solute misfit volume. Here, the elasticity model is formulated and evaluated for cubic anisotropy in fcc metals, and compared to a previous isotropic model. The prediction using the isotropic model with Voigt-averaged elastic constants is shown to represent the full anisotropic results within a few percent, and so is the recommended approach for studying anisotropic alloys. Application of the elasticity model using accessible experimentally-measured properties and/or first-principles-computed properties is then discussed so as to guide use of the model for estimating strengths of existing and newly proposed alloys. read less

Abstract: Metal matrix nanocomposites have been actively studied to discover the characteristics of a new class of materials. In the present study, metal matrix nanocomposites are investigated using molecular dynamics simulations of the compressive behavior of nanoporous carbon nanotube (CNT)-aluminum (Al) composites that have a density of approximately 77% to that of pure Al. The weight-reduced nanocomposites exhibited an enhanced Young’s modulus of 138%, and a compressive strength degraded by 13% compared with pure Al. Through stress decomposition into CNT and Al constituents, it was observed that the Young’s modulus was enhanced due to the high stiffness of the CNTs; further, the reduced strength was primarily due to the early failure strain. The effects of CNT volume fractions and sizes are further analyzed using the rule of mixture, which is modified by the interphase area definition. In addition, the atomistic details of the structure and stress revealed a buckling behavior in the CNT as well as a massive slip behavior in the Al matrix during plastic deformation. The results presented in this study will have implications in the design and development of metal matrix nanocomposites for applications in high-performance lightweight materials. read less

Abstract: Coherent twin boundaries (CTBs) routinely form during the annealing of polycrystalline metals, in the absence of an applied stress. Molecular dynamics (MD) simulations of normal grain growth in nanocrystalline metals show such annealing twins as well the formation of twin junctions. MD simulations and theoretical analyses demonstrate how these junctions form and that their formation necessarily retards grain boundary (GB) migration. Both CTB and GB migration occurs via disconnection motion. We identify the types of disconnections important for CTB migration and show the disconnection pile-ups at TJs during GB migration are responsible for CTB formation in the vicinity of TJs. Analysis further shows that at least two types twinning partials are to be expected during TJ migration and that these give rise to the multiple twinning near migrating TJs observed in the MD simulations. read less

Abstract: In this atomistic study on the copper–nickel system, a new embedded-atom alloy potential between copper and nickel is fitted to experimental data on the mixing enthalpy, taking available potentials for the pure components from literature. The resulting phase boundaries of the new potential are in very good agreement with a recent CALPHAD prediction. Using this new potential, a high angle symmetrical tiltΣ5 and a coherent Σ3 twin grain boundary (GB) are chosen for a systematic investigation of equilibriumGB segregation in the semi-grandcanonical ensemble at temperatures from 400 K to 800 K. Applying thermodynamically accurate integration techniques, the GB formation energies are calculated exactly and as an absolute value for every temperature and composition, which also enables the evaluation of GB excess entropies. The thorough thermodynamic model of GBs developed by Frolov and Mishin is excellently confirmed by the simulations quantitatively, if the impact of both segregation and GB tension on the change in GB formation energy is accounted for. In the case of the Σ3 coherent GB, it turns out that the change in GB formation energy at low temperatures is for the most part attributed to the GB tension, while segregation only has a small influence. This demonstrated effect of GB tensions should also be taken into account in the interpretation of experiments. read less

Abstract: PUBLICATION LIST of VICTOR MANUEL VILLANUEVA SANDOVAL available in this PDF. read less

Abstract: From the Ising model and the Lennard-Jones fluid to water and the iron-carbon system, phase diagrams are an indispensable tool to understand phase equilibria. Despite the effort of the simulation community, the calculation of a large portion of a phase diagram using computer simulation is still today a significant challenge. Here, we propose a method to calculate phase diagrams involving liquid and solid phases by the reversible transformation of the liquid and the solid. To this end, we introduce an order parameter that breaks the rotational symmetry and we leverage our recently introduced method to sample the multithermal-multibaric ensemble. In this way, in a single molecular dynamics simulation, we are able to compute the liquid-solid coexistence line for entire regions of the temperature and pressure phase diagram. We apply our approach to the bcc-liquid phase diagram of sodium and the fcc-bcc-liquid phase diagram of aluminum. read less

Abstract: A complementary combination of long-time atomistic molecular dynamics simulations and real-time transmission electron microscopy (TEM) images has been utilized for unraveling, at an atomic resolution, the nature of the sintering process of Ag nanoparticles (NPs) induced on the surface of an α-Ag2WO4 crystal for the first time, under the exposure of a TEM electron beam (EB). Temporal evolution of calculated and experimental results highlights the role of the lattice plane matching and the stacking faults along the disorder-to-order transitions of the oriented attachment process. This phenomenon is considered as an example of surface plasmon resonances (SPRs), in which the EB has two effects: first, it provokes the formation of the Ag NPs that, due to the electron irradiation, become SPR electric dipoles, and, second, these Ag NPs undergo sintering processes that are controlled by dipole–dipole interactions forming larger clusters. The predictive power of the simulation model was verified experimentally, pa... 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: Thermal conductivity of a nickel-coated tri-wall carbon nanotube was studied using molecular dynamics where both the phonon and electron contributions were considered. Simulations predicted a significant effect of the metal coating on the thermal conductivity, i.e. 50% decrease for 1.2 nm of Ni coating. However, the decreasing rate of the thermal conductivity is minuscule for the metal thicker than 1.6 nm. The smaller thermal conductivity of the metal coating, phonon scattering at the interface, and less impacted heat transfer on the inner tubes of the carbon nanotube rationalized the observed trends. read less

Abstract: The temperature-dependent intrinsic stacking fault Gibbs energy is computed based on highly converged density-functional-theory (DFT) calculations for the three prototype face-centered cubic metals ... read less

Abstract: Nanosprings have drawn continuous attention due to their superior elongation and potential applications in stretchable devices. Based on molecular dynamics (MD) simulations, the deformation mechanism of Cu nanosprings with/without twin boundary (TB) structures is investigated in this work. It is found that dislocation-driven deformation mechanism of nanosprings mainly depends on their geometry parameters. During the plastic process, severe distortion caused by local dislocation emission is frequently observed, especially for nanosprings with large wire diameters. Small twin boundary spacings (TBSs) can effectively improve the mechanical properties of nanosprings through restricting dislocation emission and TB migration. In addition, the calculated spring constant reveals that the stiffness of nanosprings with larger wire diameters, smaller helix pitches or smaller TBSs will become larger. It is also worth mentioning that the classical theory is still valid in nanosprings with TB structures. These findings open a new avenue to design novel nanosprings for nanodevices. 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: This research was funded by the National Natural Science Foundation of China, grant number [51320105003] and [51674153], and the Chinese Scholarship Council (CSC). This research used the ALICE High Performance Computing Facility at the University of Leicester. The authors acknowledge the support from the Diamond Light Source for the provision of beam time and the National Laboratory for Information Science and Technology at Tsinghua University for access to supercomputing facilities. read less

Abstract: ABSTRACT Dislocations and heat conduction are essential components that influence properties and performance of crystalline materials, yet the modelling of which remains challenging partly due to their multiscale nature that necessitates simultaneously resolving the short-range dislocation core, the long-range dislocation elastic field, and the transport of heat carriers such as phonons with a wide range of characteristic length scale. In this context, multiscale materials modelling based on atomistic/continuum coupling has attracted increased attention within the materials science community. In this paper, we review key characteristics of five representative atomistic/continuum coupling approaches, including the atomistic-to-continuum method, the bridging domain method, the concurrent atomistic–continuum method, the coupled atomistic/discrete-dislocation method, and the quasicontinuum method, as well as their applications to dislocations, heat conduction, and dislocation/phonon interactions in crystalline materials. Through problem-centric comparisons, we shed light on the advantages and limitations of each method, as well as the path towards enabling them to effectively model various material problems in engineering from nano- to mesoscale. Abbreviations: AtC: atomistic-to-continuum; BCC: body-centred cubic; BDM: bridging domain method; CAC: concurrent atomistic–continuum; CADD: coupled atomistic/discrete-dislocation; DDD: discrete dislocation dynamics; DDf-MD: discrete diffusion-molecular dynamics; DOF: degree of freedom; ESCM: embedded statistical coupling method; FCC: face-centred cubic; GB: grain boundary; XFEM: extended finite element method; MD: molecular dynamics; MS: molecular statics; PK: Peach-Koehler; QC: quasicontinuum read less

Abstract: In the present work, molecular dynamics simulations were carried out to investigate the temperature distribution as well as the fundamental friction characteristics such as the coefficient of friction and wear in a disc-pad braking system. A wide range of constant velocity loadings was applied on metallic brake pads made of aluminium, copper and iron with different rotating speeds of a diamond-like carbon brake disc. The average temperature of Newtonian atoms and the coefficient of friction of the brake pad were investigated. The resulting relationship of the average temperature with the speed of the disc as well as the applied loading velocity can be described by power laws. The quantitative description of the volume lost from the brake pads was investigated, and it was found that the volume lost increases linearly with the sliding distance. Our results show that Archard's linear wear law is not applicable to a wide range of normal loads, e.g., in cases of low normal load where the wear rate was increased considerably and in cases of high load where there was a possibility of severe wear. In this work, a new formula for the brake pad wear in a disc brake assembly is proposed, which displays a power law relationship between the lost volume of the metallic brake pads per unit sliding distance and the applied normal load with an exponent of 0.62 ± 0.02. This work provides new insights into the fundamental understanding of the wear mechanism at the nano-scale leading to a new bottom-up wear law for metallic brake pads. read less

Abstract: The purpose of this study is to investigate polycrystalline lattices of aluminum (Al) under the stress–strain conditions in all-atom molecular dynamics simulations and Al alloys using X-ray diffrac... read less

Abstract: Stochastic Langevin dynamics has been traditionally used as a tool to describe nonequilibrium processes. When utilized in systems with collective modes, traditional Langevin dynamics relaxes all modes indiscriminately, regardless of their wavelength. We propose a generalization of Langevin dynamics that can capture a differential coupling between collective modes and the bath, by introducing spatial correlations in the random forces. This allows modeling the electronic subsystem in a metal as a generalized Langevin bath endowed with a concept of locality, greatly improving the capabilities of the two-temperature model. The specific form proposed here for the spatial correlations produces a physical wave-vector and polarization dependency of the relaxation produced by the electron-phonon coupling in a solid. We show that the resulting model can be used for describing the path to equilibration of ions and electrons and also as a thermostat to sample the equilibrium canonical ensemble. By extension, the family of models presented here can be applied in general to any dense system, solids, alloys, and dense plasmas. As an example, we apply the model to study the nonequilibrium dynamics of an electron-ion two-temperature Ni crystal. read less

Abstract: in the investigation of the fundamental mechanisms of lasermaterial interactions. The advancements in the computational methodology and fast growth of available computing resources are rapidly expanding the range of problems amenable to atomistic modeling. This chapter provides an overview of the results obtained in recent simulations of laser ablation of metal targets in vacuum, a background gas, and a liquid environment. A comparison of the Chapter 12 read less

Abstract: Metallic nanomaterials are widely used in micro/nanodevices. However, the mechanically driven microstructure evolution in these nanomaterials is not clearly understood, particularly when large stress and strain gradients are present. Here, we report the in situ bending experiment of Ni nanowires containing nanoscale twin lamellae using high-resolution transmission electron microscopy. We found that the large, localized bending deformation of Ni nanowires initially resulted in the formation of a low-angle tilt grain boundary (GB), consisting of randomly distributed dislocations in a diffuse GB layer. Further bending intensified the local plastic deformation and thus led to the severe distortion and collapse of local lattice domains in the GB region, thereby transforming a low-angle GB to a high-angle GB. Atomistic simulations, coupled with in situ atomic-scale imaging, unravelled the roles of bending-induced strain gradients and associated geometrically necessary dislocations in GB formation. These results offer a valuable understanding of the mechanically driven microstructure changes in metallic nanomaterials through GB formation. The work also has implications for refining the grains in bulk nanocrystalline materials. read less

Abstract: A molecular-dynamic study of the atomic mechanisms of the localized nonequilibrium structural state formation in nickel crystallite under a complex deformation scheme is carried out. Compressed and tensile forces were applied to the spaced captures at the boundary of the crystallite, which simulated the effect of interfaces of various types. It is shown that under such a loading scheme, a nanoband can be formed in the simulated crystallite with elastic curvature of the lattice. The nanoband arises between the captures and extends into the bulk of the crystallite. It is formed as a result of the vortex motion of atoms in the region of local loading. The nanoband is not uniformly curved in width and length. A comparison is made with the experimental results in which similar bands with a curved crystal lattice in metals are found. read less

Abstract: Molecular dynamics (MD) simulations were carried out to study the mechanical properties of co-continuous Al/SiC nanocomposites under tensile loading. Three cases of different models were implemented to investigate the influence of volume fraction (Vf) of SiC, thickness of SiC skeletons and shape of Al nanowire on the mechanical properties of the nanocomposites. It is found that the ultimate strength and Young's modulus of nanocomposites increase nonlinearly with the Vf of SiC, whereas the limit strains decrease with the increasing Vf of SiC. The Young's modulus obtained by MD simulations are in good agreement with the prediction by micromechanics methods and experimental results. In addition, the thickness of SiC skeletons and the shape of Al nanowire have a significant impact on the mechanical behaviors of co-continuous Al-SiC nanocomposites. This study on the mechanical properties of co-continuous Al-SiC nanocomposites will be helpful to further understanding the mechanical behaviors of the metal/ceramics co-continuous composites. read less

Abstract: This study focuses on the shock precursor decay phenomena in nanocrystalline aluminum (nc-al) atom systems using large scale molecular dynamics (MD) simulations. For this purpose, two different atom systems are considered: 1) a small 908 A thick (∼ 20 million atoms) with four different average grain sizes and 2) a large 0.5 micron thick (∼ 2 billion atoms) with three different grain sizes. To model shock wave propagation, a plate-on-plate configuration at different impact velocities between 0.7 km/s to 1.5 km/s was used. The results indicate that the effect of impact velocity on the shock precursor amplitudes decreases as the shock wave travels away from the impact plane towards the stress-free back surface. However, while smaller ( 180 A) show significant influence for all impact velocities. The combined results for both thinner and thicker atom systems showed that the precursor decay seamlessly continues across the nano-to macro-length scales, especially for the grain size of 180 A. An effort was also made to correlate defects generation in terms of dislocations to the precursor decay phenomenon.This study focuses on the shock precursor decay phenomena in nanocrystalline aluminum (nc-al) atom systems using large scale molecular dynamics (MD) simulations. For this purpose, two different atom systems are considered: 1) a small 908 A thick (∼ 20 million atoms) with four different average grain sizes and 2) a large 0.5 micron thick (∼ 2 billion atoms) with three different grain sizes. To model shock wave propagation, a plate-on-plate configuration at different impact velocities between 0.7 km/s to 1.5 km/s was used. The results indicate that the effect of impact velocity on the shock precursor amplitudes decreases as the shock wave travels away from the impact plane towards the stress-free back surface. However, while smaller ( 180 A) show significant influence for all impact velocities. The combined results for both thinner and thicker atom systems showed that the precursor decay seamlessly continues acro... read less

Abstract: Using a homemade, novel, in situ transmission electron microscopy (TEM) double tilt tensile device, plastic behavior of single crystalline Cu nanowires of around 150 nm are studied. Deformation twins occur during the tests as predesigned before the experiments. In situ observation of twin boundary sliding (TBS) caused by full dislocation (extended dislocation) is first revealed at the atomic scale which is confirmed by molecular dynamics (MD) simulation results. Combined with twin boundary migration and multiple dislocations nucleated from surface, TBS causes a superlarge fracture strain which is over 166% and a severe necking which is over 93%, far beyond the typical values for most nanomaterials without twins. read less

Abstract: The dynamic evolution and interaction of defects under the conditions of shock loading in nanocrystalline Al with an average grain size of 20 nm is investigated using molecular dynamics simulations for an impact velocity of 1 km/s. Four stages of defect evolution are identified during shock deformation and failure that correspond to the initial shock compression (I), the propagation of the compression wave (II), the propagation and interaction of the reflected tensile waves (III), and the nucleation, growth, and coalescence of voids (IV). The results suggest that the spall strength of the nanocrystalline Al system is attributed to a high density of Shockley partials and a slightly lower density of twinning partials (twins) in the material experiencing the peak tensile pressures. read less

Abstract: The rate of a homogeneous nucleation of nanovoids in expanded aluminum is researched in this work. Typical lifetime of the system in a metastable state at a negative pressure as well as coefficients of surface tension and nucleation frequency at the temperatures 300 and 500 K are obtained with the help of molecular dynamic simulation. The large value of the pre-exponential factor should be noted, which requires further detailed investigations. read less

Abstract: By means of molecular dynamics simulation, we investigate the interaction of picosecond-duration compression pulses excited by a flat impactor with flat and nano-structured rear surfaces of copper and aluminum samples. It is shown that protrusions on the rear surface can increase the threshold value of the impact velocity, leading to spallation. As the shock wave reaches the perturbed rear surface, an unloading on the lateral surfaces of the protrusions begins; it leads to an intensive plastic deformation in the surface layer of metal. A part of the compression pulse energy is spent on the plastic deformation that restricts the rarefaction wave amplitude and suppresses the spall fracture. An increase in threshold velocity can be observed for all investigated thicknesses of the targets. The increase is substantial with respect to comparability between the protrusion height and the compression pulse width (the impactor thickness). Another condition is the ratio of the protrusion cross-section to the total s... read less

Abstract: Low-mobility twin grain boundaries dominate the microstructure of grain boundary-engineered materials and are critical to understanding their plastic deformation behaviour. The presence of solutes, such as hydrogen, has a profound effect on the thermodynamic stability of the grain boundaries. This work examines the case of a grain boundary at inclinations from . The angle corresponds to the rotation of the (coherent) into the (lateral) twin boundary. To this end, atomistic models of inclined grain boundaries, utilising empirical potentials, are used to elucidate the finite-temperature boundary structure while grand canonical Monte Carlo models are applied to determine the degree of hydrogen segregation. In order to understand the boundary structure and segregation behaviour of hydrogen, the structural unit description of inclined twin grain boundaries is found to provide insight into explaining the observed variation of excess enthalpy and excess hydrogen concentration on inclination angle, but the explanatory power is limited by how the enthalpy of segregation is affected by hydrogen concentration. At higher concentrations, the grain boundaries undergo a defaceting transition. In order to develop a more complete mesoscale model of the interfacial behaviour, an analytical model of boundary energy and hydrogen segregation that relies on modelling the boundary as arrays of discrete disconnections is constructed. Furthermore, the complex interaction of boundary reconstruction and concentration-dependent segregation behaviour exhibited by inclined twin grain boundaries limits the range of applicability of such an analytical model and illustrates the fundamental limitations for a structural unit model description of segregation in lower stacking fault energy materials. read less

Abstract: This study contributes to the development of a ‘fundamental, atomistic basis’ to inform macro-scale models that can provide significant insights about the effect of dislocation microstructure evolution during plastic deformation. Within a mesoscale model, multi-dislocation interactions can be studied which are capable of driving high-stress effects such as dislocation nucleation under low applied stresses, due to stress-concentration in dislocation pile-ups at interfaces. This study establishes a methodology to evaluate a phenomenological model for atomic-scale crystal defect interactions from molecular dynamics simulations, which is a critical step for mesoscale studies of plastic deformation in metals. Dislocations are affected by thermally activated processes that become energetically favorable as the stress approaches a threshold value. The nudged elastic band technique is ideal for evaluating the energetic activation parameters from atomic simulations. With this method, the activation energy and volume were obtained for the process of homogeneous nucleation of a full dislocation loop in pure FCC aluminum. Using the (atomistic) activation parameters, a constitutive mathematical model is developed for simulations at the mesoscale, to evaluate the critical (local) shear stress threshold. The constitutive model is effective for extrapolating from an atomistic timeframe of femtoseconds to experimentally accessible timespans of seconds. 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 molecular dynamics study of edge cracks in Ni and Al single crystals under mode-I loading conditions is presented. Simulations are performed using embedded-atom method potentials for Ni and Al at a temperature of 0.5K. The results reveal that Ni and Al show different fracture mechanisms. Overall failure behavior of Ni is brittle, while fracture in Al proceeds through void nucleation and coalescence with a zig-zag pattern of crack growth. The qualitative nature of results is discussed in the context of vacancy-formation energies and surface energies of the two FCC metals. read less

Abstract: To investigate the barrier effect of grain boundaries on the propagation of avalanche-like plasticity at the atomic-scale, we perform three-dimensional molecular dynamics simulations by using simplified polycrystal models including symmetric-tilt grain boundaries. The cut-offs of stress-drop distributions following power-law distributions decrease as the size of the crystal grains decreases. We show that some deformation avalanches are confined by grain boundaries; on the other hand, unignorable avalanches penetrate all the grain boundaries included in the models. The blocking probability that one grain boundary hinders this system-spanning avalanche is evaluated by using an elemental probabilistic model. read less

Abstract: The computer simulation results on the atomic structure of the copper crystallite and its behavior in nanoindentation demonstrate the key role of local structural transformations in nucleation of plasticity. The generation of local structural transformations can be considered as an elementary event during the formation of higher scale defects, including partial dislocations and stacking faults. The cause for local structural transformations, both direct fcc-hcp and reverse hcp-fcc, is an abrupt local increase in atomic volume. A characteristic feature is that the values of local volume jumps in direct and reverse structural transformations are comparable with that in melting and lie in the range 5–7%. read less

Abstract: When a liquid is cooled well below its melting temperature at a rate that exceeds the critical cooling rate Rc, the crystalline state is bypassed and a metastable, amorphous glassy state forms instead. Rc (or the corresponding critical casting thickness dc) characterizes the glass-forming ability (GFA) of each material. While silica is an excellent glass-former with small Rc < 10(-2) K/s, pure metals and most alloys are typically poor glass-formers with large Rc > 10(10) K/s. Only in the past thirty years have bulk metallic glasses (BMGs) been identified with Rc approaching that for silica. Recent simulations have shown that simple, hard-sphere models are able to identify the atomic size ratio and number fraction regime where BMGs exist with critical cooling rates more than 13 orders of magnitude smaller than those for pure metals. However, there are a number of other features of interatomic potentials beyond hard-core interactions. How do these other features affect the glass-forming ability of BMGs? In this manuscript, we perform molecular dynamics simulations to determine how variations in the softness and non-additivity of the repulsive core and form of the interatomic pair potential at intermediate distances affect the GFA of binary alloys. These variations in the interatomic pair potential allow us to introduce geometric frustration and change the crystal phases that compete with glass formation. We also investigate the effect of tuning the strength of the many-body interactions from zero to the full embedded atom model on the GFA for pure metals. We then employ the full embedded atom model for binary BMGs and show that hard-core interactions play the dominant role in setting the GFA of alloys, while other features of the interatomic potential only change the GFA by one to two orders of magnitude. Despite their perturbative effect, understanding the detailed form of the intermetallic potential is important for designing BMGs with cm or greater casting thickness. read less

Abstract: Significance Richard Feynman famously described the hypothesis “All things are made of atoms” as among the most significant of all scientific knowledge. How atoms are arranged in “things” is an interesting and natural question. However, aside from perfect crystals and ideal gases, understanding these arrangements in an insightful yet tractable manner is challenging. We introduce a unified mathematical framework for classifying and identifying local structure in imperfect condensed matter systems using Voronoi topology. This versatile approach enables visualization and analysis of a wide range of complex atomic systems, including highly defected solids and glass-forming liquids. The proposed framework presents a new perspective into the structure of discrete systems of particles, ordered and disordered alike. Physical systems are frequently modeled as sets of points in space, each representing the position of an atom, molecule, or mesoscale particle. As many properties of such systems depend on the underlying ordering of their constituent particles, understanding that structure is a primary objective of condensed matter research. Although perfect crystals are fully described by a set of translation and basis vectors, real-world materials are never perfect, as thermal vibrations and defects introduce significant deviation from ideal order. Meanwhile, liquids and glasses present yet more complexity. A complete understanding of structure thus remains a central, open problem. Here we propose a unified mathematical framework, based on the topology of the Voronoi cell of a particle, for classifying local structure in ordered and disordered systems that is powerful and practical. We explain the underlying reason why this topological description of local structure is better suited for structural analysis than continuous descriptions. We demonstrate the connection of this approach to the behavior of physical systems and explore how crystalline structure is compromised at elevated temperatures. We also illustrate potential applications to identifying defects in plastically deformed polycrystals at high temperatures, automating analysis of complex structures, and characterizing general disordered systems. read less

Abstract: Resolving the mechanisms associated with grain boundary migration is a difficult task. This work is focused on detailing techniques to utilize atomistic simulations to better understand the energy, structure and mobility of grain boundaries (GBs). Various techniques are detailed on how to construct different simulation cells, select boundary plane normals, and measure simulated mobility. A limited number of GBs, inspired by experimental observations, are selected for the present work. The GBs studied indicate that the structure of the theoretical set of GBs has a structure consistent with an overall minimization of energy. The mobility simulations indicate that shear coupling is the preferred migration mechanism. However, when the same GBs are constrained, as might occur in polycrystalline networks, the relative mobilities of the set of GBs are significantly altered. Finally, the influence of deformation structures, as they pertain to GB migration during recrystallization, are discussed. read less

Abstract: We present a series of results from atomistic simulations in three different materials (3 crystal structures) that demonstrate that the multiplicity of grain boundary (GB) structures at fixed macroscopic GB degrees of freedom is both extremely large and ubiquitous. The GB energy vs. misorientation curve that is commonly discussed is in fact a wide band, with many GB states very close in energy. The existence of so many GB states suggests that GB configurational entropy Sc is important for GB properties. We demonstrate that the GB Sc consists of two major contributions, one of which is geometric in nature and one that depends on bonding. We then show how this concept can be employed to predict GB relaxation dynamics by analogy with Adam-Gibbs theory, originally derived to predict the properties of glass forming liquids. Finally, we apply these predictions to understand GB denuded zone size during irradiation. read less

Abstract: Structure quantification is key to successful mining and extraction of core materials knowledge from both multiscale simulations as well as multiscale experiments. The main challenge stems from the need to transform the inherently high dimensional representations demanded by the rich hierarchical material structure into useful, high value, low dimensional representations. In this paper, we develop and demonstrate the merits of a data-driven approach for addressing this challenge at the atomic scale. The approach presented here is built on prior successes demonstrated for mesoscale representations of material internal structure, and involves three main steps: (i) digital representation of the material structure, (ii) extraction of a comprehensive set of structure measures using the framework of n-point spatial correlations, and (iii) identification of data-driven low dimensional measures using principal component analyses. These novel protocols, applied on an ensemble of structure datasets output from molecular dynamics (MD) simulations, have successfully classified the datasets based on several model input parameters such as the interatomic potential and the temperature used in the MD simulations. read less

Abstract: We examine the effect of elastic stresses induced by growing voids on the diffusion vacancy fluxes using newly derived equations. One of the main goals of our work is to obtain a kinetic equation for the growth rate of voids in cubic metals. The diffusion equation for vacancies, in which the influence of elastic stress near the void on the flux is taken into account, is linearized and solved. Then after mathematical transformations that are similar to Lifshitz - Slyozov theory, kinetic equations for the growth rate of the voids in fcc and bcc metals are obtained. The kinetic equations contain additional terms due to developed strain. This feature distinguishes the present equation from known ones and changes the kinetic of void growth. The functional dependence on strain is determined by coefficients, which characterize the strain influence on diffusion (SID coefficients). These coefficients are very sensitive to the atomic structure in the nearest vicinity of the saddle-point configuration. We have built an advanced model to evaluate them. SID coefficient simulation is the next step of this work. Using the kinetic equations and the SID coefficients, we calculate the void growth rate in cubic metals under different conditions. read less

Abstract: Grain boundaries (GBs) are important microstructure features and can significantly affect the properties of nanocrystalline materials. Molecular dynamics simulation was carried out in this study to investigate the shear response and deformation mechanisms of symmetric and asymmetric Σ11<1 1 0> tilt GBs in copper bicrystals. Different deformation mechanisms were reported, depending on GB inclination angles and equilibrium GB structures, including GB migration coupled to shear deformation, GB sliding caused by local atomic shuffling, and dislocation nucleation from GB. The simulation showed that migrating Σ11(1 1 3) GB under shear can be regarded as sliding of GB dislocations and their combination along the boundary plane. A non-planar structure with dissociated intrinsic stacking faults was prevalent in Σ11 asymmetric GBs of Cu. This type of structure can significantly increase the ductility of bicrystal models under shear deformation. A grain boundary can be a source of dislocation and migrate itself at different stress levels. The intrinsic free volume involved in the grain boundary area was correlated with dislocation nucleation and GB sliding, while the dislocation nucleation mechanism can be different for a grain boundary due to its different equilibrium structures. read less

Abstract: We characterize the solute mobility behavior driven by interstitial mechanism in FCC diluted alloys using a classical molecular static technique (CMS). In the same line of ideas as the multi-frequency model, we calculate the tracer self- and solute diffusion coefficients. Specifically, we perform our calculations for the Al–U diluted alloy. We verify that in the Al–U system, mixed dumb-bells are observed to be unstable and U mobility is driven by crowdions. From previous results of diffusion in same alloys containing only vacancies, qualitatively we conclude that, experimental data are in perfect agreement with previous calculations of solute U diffusion driven by a vacancy mechanism. Also we give a possible migration path for solute U atoms through interstitial migration, where we have found that U enhances the Al mobility in the alloy. read less

Abstract: The low thermal conductivity of nano-crystalline materials is commonly explained via diffusive scattering of phonons by internal boundaries. In this study, we have quantitatively studied phonon-crystalline boundaries scattering and its effect on the overall lattice thermal conductivity of crystalline bodies. Various types of crystalline boundaries such as stacking faults, twins, and grain boundaries have been considered in FCC crystalline structures. Accordingly, the specularity coefficient has been determined for different boundaries as the probability of the specular scattering across boundaries. Our results show that in the presence of internal boundaries, the lattice thermal conductivity can be characterized by two parameters: (1) boundary spacing and (2) boundary excess free volume. We show that the inverse of the lattice thermal conductivity depends linearly on a non-dimensional quantity which is the ratio of boundary excess free volume over boundary spacing. This shows that phonon scattering across crystalline boundaries is mainly a geometrically favorable process rather than an energetic one. Using the kinetic theory of phonon transport, we present a simple analytical model which can be used to evaluate the lattice thermal conductivity of nano-crystalline materials where the ratio can be considered as an average density of excess free volume. While this study is focused on FCC crystalline materials, where inter-atomic potentials and corresponding defect structures have been well studied in the past, the results would be quantitatively applicable for semiconductors in which heat transport is mainly due to phonon transport. read less

Abstract: Materials used in soldier protective structures, such as armor, vehicles and civil infrastructures, are being improved for performance in extreme dynamic environments. Accordingly, atomistic molecular dynamics simulations were performed to study the spall response in a single crystal aluminum atom system. A planar 9.6 picoseconds (ps) shock pulse was generated through impacts with a shock piston at velocities ranging from 0.6 km/s to 1.5 km/s in three <1,0,0>, <1,1,0>, and <1,1,1> crystal orientations. In addition to characterizing the transient spall region width and duration, the spall response was characterized in terms of the traditional axial stress vs. axial strain response for gaining an understanding of the material failure in spall. Using an atom section averaging process, the snapshots, or the time history plots of the stress and strain axial distributions in the shock direction, were obtained from the MD simulations’ outputs of the atom level stresses and displacements. These snapshots guided the analyses to an estimation of the spall widths and spall transients. The results were interpreted to highlight the effects of crystal orientation and impact velocity on the spall width, spall duration, spall stress, strain rate, critical strain values at the void nucleation, and the void volume fraction at the void coalescence. For all the combinations of the crystal orientations and the impact velocities, the void nucleation was observed when the stress reached a peak hydrostatic state and the stress triaxiality factor reached a minimum, i.e. by the simultaneous occurring of these three conditions for the stress state: 1. pressure reaching a negative minimum, 2. axial stress reaching the magnitude value of the peak pressure, and 3. the effective stress reaching a zero value. At these conditions, void nucleation was mainly caused by atom de-bonding. In fact, the void nucleation strains were shown to have been preceded by the strains of the stress triaxiality condition in this study, thus confirming the stress triaxiality condition for the void nucleation in spall. Based on the observation that the axial 1 Army Research Laboratory, Comp. & Info. Sciences Directorate, APG, MD 21005. 2 Department of Materials Science & Engineering, Univer. Connecticut, Storrs, CT 06269. 3 Department of Mech. Engineering, University of Mississippi, University, MS 38677. E-mail: ramakrishna.r.valisetty.civ@mail.mil, dongare@uconn.edu 24 Copyright © 2014 Tech Science Press CMC, vol.44, no.1, pp.23-57, 2014 stress reached a maximum value of ∼6 GPa during the void nucleation phase in spall and stayed approximately at that value for different crystal orientations and impact velocities, the value was proposed as a material spall strength. read less

Abstract: Plastic deformation in face-centred cubic (or ‘FCC’) metals involves multi-scale phenomena which are initiated at atomic length and time scales (on order of 1.0e-15 seconds). Understanding the fundamental thresholds for plasticity at atomic and nano/meso scales requires rigorous testing, which cannot be feasibly achieved with current experimental methods. Hence, computer simulation-based investigations are extremely valuable. However, meso-scale simulations cannot yet accommodate atomically-informed grain boundary (or ‘GB’) effects and dislocation interactions. This study will provide a stress - strain analysis based on molecular dynamics simulations of a series of metastable grain boundaries with identical crystal orientations but unique grain boundary characteristics. Relationships between dislocation slip systems, resolved shear stresses and additional thermo-mechanical conditions of the system will be considered in the analysis of dislocation-grain boundary interactions, including GB penetration. This study will form the basis of new phenomenological relationships in an effort to enable accommodation of grain boundaries into meso scale dislocation dynamic simulations. read less

Abstract: Using a density functional theory-phase field dislocation dynamics model, we reveal a strong inverse relationship between the dislocation equilibrium core width and the normalized intrinsic stacking fault energy for nine face centered cubic (fcc) metals, in quantitative agreement with experiments but not with conventional continuum models. In addition, we show that due to an anomalous feature in its γ-surface, platinum has a fundamentally different core structure and a much wider equilibrium core width than expected. Based on ab initio electronic structure calculations, we attribute this anomaly to distinct differences in the directionality of charge transfer in platinum. read less

Abstract: The characteristic plasticity associated with the deformation of helium bubbles and voids in aluminium under shock compression is investigated by molecular dynamics (MD) simulations. The scenarios indicate that the emission of shear dislocation loops rather than prismatic loops is the mechanism by which helium bubbles and voids collapse. The tendency to favour dislocation nucleation and emission at the trailing side of a void but at both sides of a helium bubble is attributed to the distribution of the resolved shear stress along (111) planes. Under the same loading strength, the resolved shear stress of the leading side of a helium bubble is larger than that of a void due to the internal pressure of the bubble; therefore, the dislocation nucleation at the leading side of a helium bubble is easier than that for a void. Based on the Virial theorem, we find that the locations of the calculated maximum resolved shear stress are in good agreement with the locations of dislocation nucleation. The elastic model clearly shows that the resolved shear stress increases with the internal pressure of the helium bubble but that the location of the maximum resolved shear stress is not affected. The results from the model nicely explain the scenarios that emerged in our MD simulations. The detailed studies of the microscopic mechanism of plastic deformation are important to deeply understand the mechanical properties of irradiated materials. read less

Abstract: In this work, the role of atomistic-scale energetics on liquid-metal embrittlement of aluminum (Al) due to gallium (Ga) was explored. Ab-initio and molecular mechanics were employed to probe the formation energies of vacancies and segregation energies of Ga for , and symmetric tilt grain boundaries (STGBs) in Al. Results show that site-to-site variation of formation and segregation energies within the boundary are substantial, with the majority of sites having lower energies than the bulk values. Moreover, a few GBs such as Σ3(111)and Σ11(113) of different tilt axes with relatively high segregation energies (between 0 and −0.1 eV) at the boundary were also found, providing a new atomistic perspective in the GB engineering of material with smart GB networks to mitigate or control LME and more general embrittlement phenomena in alloys. read less

Abstract: This paper proposes a criterion for crack opening in FCC single crystals based on analyses of lattice orientation and interface energy of two adjacent crystals in a crystal plasticity finite element model (CPFEM). It also demonstrates the implementation of the criterion in Abaqus/Standard to simulate crack initiation and propagation in single-edged notch single crystal aluminium samples. Elements in the FEM mesh that have crystalline structures satisfying the crack opening criterion are removed from the mesh at the end of every loading step and FEM analyses are restarted on the new mesh in the next loading step. Removed elements effectively act as voids in the material due to crack nucleation. Similarly, the coalescence of newly removed elements at the end of a loading step with the existent ones simulates crack growth in the material. Two advantages of this approach are noted. Firstly, crack nucleation and its subsequent growth in the material is simulated solely based on lattice evolution history in the material without any presumptions of crack paths or regions where cracks are likely to occur. Secondly, as the criterion for crack nucleation is evaluated based on, and thus changes with, the lattice evolution during loading, a predefined energy criterion for crack opening, which could be erroneous, is avoided. Preliminary results of void nucleation and void growth around the notch tip in Cube and Brass oriented samples using CPFEM modelling appear to agree with molecular dynamics simulations of void growth in FCC single crystals. read less

Abstract: Presented here is a method by which Q-dependent dispersive dynamics may be measured (and used to generate a lattice dynamical model) for polycrystalline samples. This method is analogous to the measurement of dispersion curves for single-crystal samples. read less

Abstract: The role that grain boundary (GB) structure plays on the directional asymmetry of an intergranular crack (i.e. cleavage behaviour is favoured along one direction, while ductile behaviour along the other direction of the interface) was investigated using atomistic simulations for aluminium 〈1 1 0〉 symmetric tilt GBs. Middle-tension (M(T)) and Mode-I crack propagation specimens were used to evaluate the predictive capability of the Rice criterion. The stress–strain response of the GBs for the M(T) specimens highlighted the importance of the GB structure. The observed crack tip behaviour for certain GBs (Σ9 (2 2 1), Σ11 (3 3 2) and Σ33 (4 4 1)) with the M(T) specimen displayed an absence of directional asymmetry which is in disagreement with the Rice criterion. Moreover, in these GBs with the M(T) specimen, the dislocation emission from a GB source at a finite distance ahead of the crack tip was observed rather than from the crack tip, as suggested by the Rice criterion. In an attempt to understand discrepancy between the theoretical predictions and atomistic observations, the effect of boundary conditions (M(T), Mode-I and the edge crack) on the crack tip events was examined and it was concluded that the incipient plastic events observed were strongly influenced by the boundary conditions (i.e. activation of dislocation sources along the GB, in contrast to dislocation nucleation directly from the crack tip). In summary, these findings provide new insights into crack growth behaviour along GB interfaces and provide a physical basis for examining the role of the GB character on incipient event ahead of a crack tip and interface properties, as an input to higher scale models. read less

Abstract: Huge-scale atomistic simulations of shear deformation tests to the aluminum polycrystalline thin film containing the Frank­Read source are performed to elucidate the relationship between the interand intragranular plastic deformation processes and the mechanical properties of ultrafine-grained metals. Two-types of polycrystalline models, which consist of several grain boundaries reproducing easy and hard slip transfer, respectively, are prepared to investigate the effect of grain boundary on flow stress. While the first plastic deformation occurs by the dislocation bow-out motion within the grain region for both models, the subsequent plastic deformation is strongly influenced by the resistance of the slip transfer by dislocation transmission through grain boundaries. The influence of the competition between the intragranular dislocation nucleation and intergranular slip transfer on the material strength is considered. The nanostructured material’s strength depending on local defect structures associated with grain size and dislocation source length is assessed quantitatively. [doi:10.2320/matertrans.MH201313] read less

Abstract: The mechanism and dynamics of surface reconstructions of Pt(557) and Au(557) exposed to various coverages of carbon monoxide (CO) were investigated using molecular dynamics simulations. Metal–CO interactions were parametrized from experimental data and plane-wave density functional theory (DFT) calculations. The large difference in binding strengths of the Pt–CO and Au–CO interactions was found to play a significant role in step-edge stability and adatom diffusion constants. Various mechanisms for CO-mediated step wandering and step doubling were investigated on the Pt(557) surface. We find that the energetics of CO adsorbed to the surface can explain the step-doubling reconstruction observed on Pt(557) and the lack of such a reconstruction on the Au(557) surface. However, more complicated reconstructions into triangular clusters that have been seen in recent experiments were not observed in these simulations. read less

Abstract: The mechanical behavior of ceramics in extreme environments can be qualitatively different from that observed at ambient conditions and at typical loading rates. For instance, during shock loading the fracture of ceramics is not controlled by the largest flaw. Computer simulations play an increasingly important role in understanding and predicting material behavior, in particular under conditions in which experiments might be challenging or expensive. Here, we review the strengths and limitations of simulation techniques that are most commonly used to model the mechanical behavior of ceramics. We discuss specific application areas of simulations, focusing on the effects of high strain rate, confined deformation volume, altered material chemistry, and high temperature. We conclude by providing examples of future opportunities for modeling studies in this field. read less

Abstract: The internal microstructure evolution and atomic stress distribution around the crack tip of a pre-cracked single crystal nickel with unequal sample sizes are studied by molecular dynamics (MD) simulation. The simulated results indicate that the crack propagation dynamics and stress distributions around the crack tip are strongly dependent on the microstructure evolution caused by the change of sample size. Unequal sample sizes induce various atomic configurations around the crack tip during the crack propagation. When atomic configuration is invariable around the crack tip, the crack grows rapidly along the crack path, the stress concentration occurs at the crack tip of growing crack and the stress is monotonic along the crack path. Once the occurrence of microstructure evolution (void nucleation, deformation twinning) around the crack tip, the crack grows slowly and the stress value is variable along the crack path due to the peak stress is accompanied by the appearance of the void and deformation twinning ahead of the crack tip. The pre-cracked single crystal nickel under mode I loading condition shows the different crack propagation dynamics and stress distribution, which are closely related to the sample size induces void nucleation and deformation twinning mechanisms around the crack tip. read less

Abstract: It is widely considered that crystalline solids melt heterogeneously, i.e., near the equilibrium melting point ($T_{\text{m}}$) a liquid phase appears at the surface and gradually penetrates into the interior. However, via molecular dynamics (MD) simulations we find a quite new scenario of melting of mesoscale monatomic Lennard-Jones crystals with free surfaces. In the early stage of pre-melting, a quasi-liquid surface layer occurs which contains both liquidlike and solidlike atoms. Further heating leads to the homogeneous occurrence of liquidlike atoms in the interior. Liquidlike configuration grows fast with temperature and a pure liquid surface layer appears just in the next stage of pre-melting together with a homogeneous occurrence/growth of liquidlike atoms throughout the interior. Melting proceeds further by two different mechanisms: the heterogeneous one in the surface shell and the homogeneous one in the interior leading to the fast collapse of crystal lattice. Our findings are supported by the... read less

Abstract: In this article, mechanical properties of nanocrystalline Al+α-Al2O3 composites are investigated using molecular dynamics simulations. The configurations of matrix and volume fraction of α-Al2O3 may affect the mechanical properties of the particle reinforced metal-matrix composites and are taken into account. The potentials for the Al+α-Al2O3 system developed by Xin Lai et al. are adopted to depict the interactions between Al and α-Al2O3. Monocrystal Al and Bicrystal Al based α-Al2O3 particle reinforced nanocomposites are modelled respectively. Results show that: (1) volume fraction of the particles has no explicit effects on the elastic modulus and ultimate strength in both monocrystal Al and bicrystal Al based matrix nanocomposites, (2) disappearance of valley in the stress-strain curve of bicrystal Al results from existence of dislocation in matrix of various orientations. read less

Abstract: The structure and evolution of ultrashort shock waves generated by femtosecond laser pulses in single-crystal nickel films are investigated by molecular dynamics simulations. Ultrafast laser heating is isochoric, leading to pressurization of a 100-nm-thick layer below the irradiated surface. For low-intensity laser pulses, the highly pressurized subsurface layer breaks into a single elastic shock wave having a combined loading and unloading time ≈10–20 ps. Owing to the time-dependent nature of elastic-plastic transformations, an elastic response is maintained for shock amplitudes exceeding the Hugoniot elastic limit determined from simulations of steady shock waves. However, for high-intensity laser pulses (absorbed laser fluence >0.6 J/cm2), both elastic and plastic shock waves are formed independently from the initial high-pressure state. Acoustic pulses emitted by the plastic front support the motion of the elastic precursor resulting in a fluence-independent elastic amplitude; whereas the unsupported plastic front undergoes significant attenuation during propagation and may fully decay within the metal film. read less

Abstract: A phase field dislocation dynamics model that can model widely extended dislocations is presented. Through application of this model, we investigate the dependence of equilibrium stacking fault width (SFW) on the material γ-surface in fcc metals. This phase field model includes a direct energetic dependence on a parametrization of the entire γ-surface, which is directly informed by density functional theory. A wide range of materials are investigated and include both very low stacking fault energy (SFE) materials, such as silver, and high SFE materials, such as palladium. Additionally, analysis shows that by accounting for the unstable stacking fault energy, γU, we can better describe the material dependence of the equilibrium SFW rather than using only the intrinsic SFE, γI. Specifically, we see a direct dependence of the stable SFW between partial dislocations on the energy difference (γU − γI), which describes the energy barrier that partial dislocations must overcome in order to widen the stacking fault. read less

Abstract: Using molecular dynamics simulations, the influence of transverse tensile stresses on the plastic deformation behaviour of nanocrystalline (NC) Ni under tension has been investigated. The sample with an average grain size of 20 nm was created using a Voronoi construction, and two different tensile tests of the sample were performed at a constant strain rate. The simulation results revealed that more partials were emitted from the grain boundaries and propagate into the grain interiors after adding the transverse tensile stress, enhancing the dislocation density in the grain interiors. This increased dislocation density can cause additional strain hardening observed in the stress strain curve. Meanwhile, it was observed from microstructures that nanovoids are easier to form and coalesce into cracks under the biaxial stress state, causing strain softening. The two competing effects of the transverse tensile stress on the plastic deformation behaviour of NC Ni resulted in the flow stresses from 4% to 10% strain in the biaxial stress state slightly larger than those in the uniaxial stress state. read less

Abstract: Molecular dynamics simulations of the melting curves of six metals including Ag, Cu, Al, Mg, Ta, and Mo for the pressure range (0 to 15) GPa are reported. The melting curves of Ag, Cu, Al, and Mg fully confirm measurements and previous calculations. Meanwhile, the melting curves of Ta and Mo are consistent with previous calculations but diverge from laser-heated diamond-anvil cells values at high pressure. Our results suggest that the melting slope at 100 kPa is related to the electronic configuration of the element. In addition, the pressure dependence of fusion entropy and fusion volume are calculated up to 15 GPa. The overall fusion entropy is separated into topological entropy of fusion (ΔSD) due to the configuration change in melting and the volume entropy of fusion (ΔSV) due to the latent volume change in melting. Furthermore, we checked the R ln 2 rule under high pressure, according to which the value of ΔSD is a constant at ambient pressure. Result shows that the value of ΔSD is close to R ln 2 at... read less

Abstract: We investigated the melting properties of bulk nickel and the crystallization of nickel nanocrystals via molecular dynamics using a potential in the framework of the second moment approximation of tight-binding theory. The melting behavior was simulated with the hysteresis approach by subsequently heating and cooling gradually the system over a wide range of temperatures. The crystallization of nickel nanoclusters consisting of 55, 147 and 309 atoms was achieved after repeatedly annealing and quenching the corresponding quasicrystals several times to avoid being trapped in a local energy minimum. The time over which the global minimum was reached was found to increase with the cluster size. read less

Abstract: The mechanical process of single-crystal aluminium thin films under uniaxial tensile strain was simulated with molecular dynamics method at different temperature. The stress–strain curve and potential energy–strain curve of thin aluminium film under uniaxial tensile deformation were obtained by molecular dynamics simulations. With the changes of sample temperatures in uniaxial extension, the variation characteristics of stress–strain curves are alike at the elastic stage and different at the plastic one below and above 370 K, respectively. From the stress–strain curves, we gained the first local maximum stress-temperature curve and the strain at the first local maximum stress-temperature curve, and found that the strange temperature dependence of first local maximum stress: when the temperature is above 370 K, the stress goes down quickly with temperature, and when below 370 K, it descends slowly. With increasing temperature, the difference between two strain values corresponding to two maximal potential energies changes slowly below and above 370K but it goes up quickly about 370K. By these dependences, we have identified the critical temperature （370K） for the transition of plastic flow mechanism. read less

Abstract: Materials for armor applications are increasingly being required to be strong and light-weight as a consequence of increasing threat levels. We focus on materials response subjected to impact loads, understanding deformation and failure mechanisms, and developing validated mechanism-based models capable of predicting materials response under high rate loading conditions. As a specific example, we will examine the dynamic behavior of nanocrystalline aluminum using atomistic simulations. The dynamic behavior of this material is discussed in terms of competing deformation mechanisms--slip and twinning. Insights from high strain rate atomistic simulations were used in developing a fundamental mechanism-based analytical model to assist in the microstructural design of advanced materials to tailor their macroscopic properties. read less

Abstract: The lattice-inversion embedded-atom-method interatomic potential developed previously by us is extended to alkaline metals including Li, Na, and K. It is found that considering interatomic interactions between neighboring atoms of an appropriate distance is a matter of great significance in constructing accurate embedded-atom-method interatomic potentials, especially for the prediction of surface energy. The lattice-inversion embedded-atom-method interatomic potentials for Li, Na, and K are successfully constructed by taking the fourth-neighbor atoms into consideration. These angular-independent potentials markedly promote the accuracy of predicted surface energies, which agree well with experimental results. In addition, the predicted structural stability, elastic constants, formation and migration energies of vacancy, and activation energy of vacancy diffusion are in good agreement with available experimental data and first-principles calculations, and the equilibrium condition is satisfied. read less

Abstract: In order to investigate the mechanism behind the improvement of fracture toughness in ultrafinegrained metals at low temperatures, the strain rate dependence of the transition of dislocationsources from crack tips to grain boundaries is studied by the combination of molecular dynamicssimulations and the linear elastic theory. As the strain rate decreases, grain boundaries becomeanother stress consented site due to the pile-up of dislocations against the grain boundaries. Theamount of stress concentration became larger than that of crack tip as the number of dislocationsemitted from the crack tip increases. It was clearly indicated that dislocations were impinged intothe grain boundary when a new dislocation was emitted from there. It indicates the transition ofdislocation sources from the crack tip to grain boundaries at lower applied stresses. The distributionof dislocations between the crack tip and grain boundary is strongly related to the strain rate ; namely, a larger number of dislocations are distributed very close to the crack tip as the strain rate increases.It induces the smaller stress concentration at the grain boundary since the number of piling-updislocations is decreased around the grain boundary. Consequently, as the strain rate increases, thematerial becomes brittle, indicating that it will fail in a brittle mode and no longer deform plastically. read less

Abstract: Molecular-dynamics simulations of the ablation of nanocrystalline Al films by ultrashort laser pulses in the low-fluence (no-ionization) regime (0-2.5 times the ablation threshold, F{sub th}) are reported. The simulations employ an embedded-atom method potential for the dynamics of the ions and a realistic two-temperature model for the electron gas (and its interactions with the ion gas), which confers different electronic properties to the monocrystalline solid, nanocrystalline solid, and liquid regions of the targets. The ablation dynamics in three nanocrystalline structures is studied: two dense targets with different crystallite sizes (d=3.1 and 6.2 nm on average) and a d=6.2 nm porous sample. The results are compared to the ablation of monocrystalline Al. Significant differences are observed, the nanocrystalline targets showing, in particular, a lower ablation threshold and a larger melting depth, and yielding pressure waves of higher amplitude than the monocrystalline targets. Furthermore, it is shown that nanocrystalline targets experience no residual stress associated with thermal expansion and lateral constraints, and that little crystal growth occurs in the solid during and after ablation. Laser-induced spallation of the back surface of the films is also investigated; we find, in particular, that the high-strain fracture resistance of nanocrystalline samples is significantly reduced inmore » comparison to the crystalline material.« less read less

Abstract: In order to improve the fracture toughness in ultrafine-grained metals, we investigate the interactions among crack tips, dislocations, and grain boundaries in aluminum bicrystal models containing a crack and $\ensuremath{\langle}112\ensuremath{\rangle}$ tilt grain boundaries using molecular dynamics simulations. The results of previous computer simulations showed that grain refinement makes materials brittle if grain boundaries behave as obstacles to dislocation movement. However, it is actually well known that grain refinement increases fracture toughness of materials. Thus, the role of grain boundaries as dislocation sources should be essential to elucidate fracture phenomena in ultrafine-grained metals. A proposed mechanism to express the improved fracture toughness in ultrafine-grained metals is the disclination shielding effect on the crack tip mechanical field. Disclination shielding can be activated when two conditions are present. First, a transition of dislocation sources from crack tips to grain boundaries must occur. Second, the transformation of grain-boundary structure into a neighboring energetically stable boundary must occur as dislocations are emitted from the grain boundary. The disclination shielding effect becomes more pronounced as antishielding dislocations are continuously emitted from the grain boundary without dislocation emissions from crack tips, and then ultrafine-grained metals can sustain large plastic deformation without fracture with the drastic increase of the mobile dislocation density. Consequently, it can be expected that the disclination shielding effect can improve the fracture toughness in ultrafine-grained metals. read less

Abstract: (Received 18 February 2011; revised manuscript received 25 March 2011; published 9 June 2011)By combining atomic force microscopy (AFM) and large-scale molecular dynamics (MD) simulations, weexamineatcomparablescalestheatomisticprocessesgoverningnanohardnessinelectrodepositednanocrystallineNi with a mean grain diameter of 18.6 nm under confined contact deformation. Notably, this mean graindiameter represents the “strongest” size for Ni and other nanocrystalline materials where both crystal slipand grain-boundary deformation processes are intertwined to accommodate plastic flow. Accurate hardnessmeasurements were obtained from shallow nanoindentations, less than 10 nm in depth, using an AFM diamondtip.Weshowevidencethatthecontrollingyieldingmechanisminthepeakofhardnessasafunctionofpenetrationdepth corresponds to the emission of partial dislocations from grain boundaries. However, MD simulations alsoreveal for this grain size that the crystalline interfaces must undergo significant sliding at small penetrationdepths in order to initiate crystal slip. The strong interplay between intergranular and intragranular deformationprocesses found in this model nanocrystalline metal is discussed and shown to considerably reduce the localdependenceofnanohardnessontheinitialmicrostructureatthisscale,unlikepastobservationsofnanoindentationinNielectrodepositswithlargergrainsizes.Thesenewfindingsthereforeconstituteanimportantstepforwardtounderstanding the contribution of nanoscale grain-boundary networks on permanent deformation and hardnessrelevant for nanoscale materials and structures.DOI: 10.1103/PhysRevB.83.224101 PACS number(s): 62 read less

Abstract: Classical fracture mechanics as well as modern strain gradient plasticity theories assert the existence of stress concentration (or strain gradient) ahead of a notch tip, albeit somewhat relaxed in ductile materials. In this study, we present experimental evidence of extreme stress homogenization in nanocrystalline metals that result in immeasurable amount of stress concentration at a notch tip. We performed in situ uniaxial tension tests of 80 nm thick (50 nm average grain size) freestanding, single edge notched aluminum specimens inside a transmission electron microscope. The theoretical stress concentration for the given notch geometry was as high as 8, yet electron diffraction patterns unambiguously showed absence of any measurable stress concentration at the notch tip. To identify possible mechanisms behind such an anomaly, we performed molecular dynamics simulations on scaled down samples. Extensive grain rotation driven by grain boundary diffusion, exemplified by an Ashby-Verrall type of grain switching process, was observed at the notch tip to relieve stress concentration. We conclude that in the absence of dislocations, grain realignment or rotation may have played a critical role in accommodating externally applied strain and neutralizes any stress concentration during the process. read less

Abstract: In ductile materials, fracture involves void nucleation, growth and coalescence. The objective of this research is to understand how crystallographic orientation influences void growth in uniaxial tensile deformation of aluminum. We used molecular dynamics to simulate void growth in a spherical void embedded cubic specimen with periodic boundary conditions under remote uniaxial tension. The simulation results reveal how crystallographic orientation affects the yield stress and void growth corresponding to dislocation nucleation from the void surface and resulting in shear loops in perfect FCC lattice. Varying dislocation patterns/shear loops occur according to the specimens different orientations, thereby affirming the effect of crystallographic orientation. Consequently, atomistic simulations of this type can indeed inform continuum void growth models for application in multiscale models. read less

Abstract: The mechanisms of deformation and failure in face-centered cubic (FCC) nickel nanowires subjected to uniaxial tensile loading are investigated using molecular dynamics (MD) simulation, and the size effect on mechanical properties of FCC metal nanowires is studied. Simulation reveals that the surface free energy has great influence on the deformation and failure mechanism of metal nanowires. As a result of free surfaces and their reconstruction, the surface atoms depart from the perfect crystal lattice positions, leading to the appearance of nanocavities on the surfaces that are exposed to external load. The deformation process of nanowires undergoes expansion and connection of nanocavities from surface into inner lattices. Slip occurs during the deformation process, which is consistent with experimental phenomena. Elastic stiffness, yield, and fracture strength of nickel nanowires with various cross-sectional sizes are obtained, and the size effect on these mechanical properties is further analyzed. Based... read less

Abstract: The structures of Ni/MgO nanoparticles are studied by means of global optimization searches. The results from four different model potentials, sharing the same functional forms but different parametrizations, are reported and compared. Two parametrizations over four give qualitatively correct results, and one of them is also quantitatively satisfactory. The other models fail to explain some qualitative features observed in the experiments, such as the formation of hcp nanodots at small sizes or the transition to fcc structures at large sizes. The important features that an atomistic potential must present for the correct prediction of Ni cluster structures are discussed and generalized. read less

Abstract: We show that the generation of stacking faults in perfect face-centered-cubic (fcc) crystals, uniaxially compressed along [001], is due to transverse-acoustic phonon instabilities. The position in reciprocal space where the instability first manifests itself is not a point of high symmetry in the Brillouin zone. This model provides a useful explanation for the magnitude of the elastic limit, in addition to the affects of box size, temperature, and compression on the time scale for the generation of stacking faults. We observe this phenomenon in both simulations that use the Lennard-Jones potential and embedded atom potentials. Not only does this work provide fundamental insight into the microscopic response of the material but it also describes certain behavior seen in previous molecular dynamics simulations of single-crystal fcc metals shock compressed along the principal axis. read less

Abstract: Kinetic Monte Carlo simulations of the chemical and electrolytic etching processes of nano-scale particles in two-phase materials were performed. Etching produces a surface relief, which can subsequently be studied by optical, scanning electron and atomic force microscopy to obtain quantitative information on the size, shape and spatial arrangement of the particles. The present simulations yield insight into the dependence of the etched relief on the strengths of the atomic bonds in the two phases and on the shape of the particles. Lower limits for the difference in bond energies necessary (i) to reveal the particles and (ii) to avoid over-etching are established. The results of the simulations are discussed with reference to own actual etching experiments performed for nano-scale precipitates. read less

Abstract: We have previously reported that the fracture behavior of nanocrystalline (NC) Ni is via the nucleation and coalescence of nano-voids at grain boundaries and triple junctions, resulting in intergranular failure mode. Here we show in large-scale molecular dynamics simulations that partial-dislocation-mediated plasticity is dominant in NC Cu with grain size as small as ~ 10 nanometers. The simulated results show that NC Cu can accommodate large plastic strains without cracking or creating damage in the grain interior or grain boundaries, revealing their intrinsic ductile properties compared with NC Ni. These results point out different failure mechanisms of the two face-centered-cubic (FCC) metals subject to uniaxial tensile loading. The insight gained in the computational experiments could explain the good plasticity found in NC Cu not seen in Ni so far. read less

Abstract: Brittle-ductile transition (BDT) behaviour was investigated in low carbon steel deformed by an accumulative roll-bonding (ARB) process. The temperature dependence of its fracture toughness was measured by conducting four-point bending tests at various temperatures and strain rates. The fracture toughness increased while the BDT temperature decreased in the specimens deformed by the ARB process. Arrhenius plots between the BDT temperatures and the strain rates indicated that the activation energy for the controlling process of the BDT was not changed by the deformation with the ARB process. It was deduced that the decrease in the BDT temperature by grain refining was not due to the increase in the dislocation mobility controlled by short-range barriers. Quasi-three-dimensional simulations of dislocation dynamics, taking into account of crack tip shielding due to dislocations, were performed to investigate the effect of a dislocation source spacing along a crack front on the BDT. The simulation indicated that the BDT temperature is decreased with decreasing in the dislocation source spacing. Molecular dynamics simulations revealed that moving dislocations were impinged against grain boundaries and were reemitted from there with increasing strain. It indicates that grain boundaries can be new sources in ultra-fine grained materials, which increases toughness at low temperatures. read less

Abstract: General theory of semi-empirical potential methods including embedded-atom method and modified-embedded-atom method (MEAM) is reviewed. The procedures to construct these potentials are also reviewed. A multi-objective optimization (MOO) procedure has been developed to construct MEAM potentials with minimal manual fitting. This procedure has been applied successfully to develop a new MEAM potential for magnesium. The MOO procedure is designed to optimally reproduce multiple target values that consist of important material properties obtained from experiments and first-principle calculations based on density-functional theory. The optimized target quantities include elastic constants, cohesive energies, surface energies, vacancy-formation energies, and the forces on atoms in a variety of structures. The accuracy of the present potential is assessed by computing several material properties of Mg including their thermal properties. We found that the new MEAM potential shows a significant improvement over previously published potentials, especially for the atomic forces and melting temperature calculations. read less

Abstract: Recent experiments have demonstrated that plastic strains in nanocrystalline aluminum and gold films with grain sizes on the order of 50 nm are partially recoverable. To reveal the mechanisms behind such strain recovery, we perform large scale molecular dynamics simulations of plastic deformation in nanocrystalline aluminum with mean grain sizes of 10, 20, and 30 nm. Our results indicate that the inhomogeneous deformation in a polycrystalline environment results in significant residual stresses in the nanocrystals. Upon unloading, these internal residual stresses cause strain recovery via competitive deformation mechanisms including dislocation reverse motion/annihilation and grain-boundary sliding/diffusion. By tracking the evolution of each individual deformation mechanism during strain recovery, we quantify the fractional contributions by grain-boundary and dislocation deformation mechanisms to the overall recovered strain. Our analysis shows that, even under strain rates as high as those in molecular dynamics simulations, grain-boundary-mediated processes play important roles in the deformation of nanocrystalline aluminum. read less

Abstract: In this work we introduce a hybrid ab initio-classical simulation methodology designed to incorporate the chemistry into the description of phenomena that, intrinsically, require very large systems to be properly described. This hybrid approach allows us to conduct large-scale atomistic simulations with a simple classical potential (embedded atom method (EAM), for instance) while simultaneously using a more accurate ab initio approach for critical embedded regions. The coupling is made through shared atomic shells where the two atomistic modeling approaches are relaxed in an iterative, self-consistent manner. The magnitude of the incompatibility forces arising in the shared shell is analyzed, and possible terminations for the embedded region are discussed, as a way to reduce such forces. As a test case, the formation energy of a single vacancy in aluminum at different distances from an edge dislocation is studied. Results obtained using the hybrid approach are compared to those obtained using classical methods alone, and the range of validity for the classical approach is evaluated. read less

Abstract: High resolution transmission electron microscopy of nanotwinned Cu films revealed Σ3 {112} incoherent twin boundaries (ITBs), with a repeatable pattern involving units of three {111} atomic planes. Topological analysis shows that Σ3 {112} ITBs adopt two types of atomic structure with differing arrangements of Shockley partial dislocations. Atomistic simulations were performed for Cu and Al. These studies revealed the structure of the two types of ITBs, the formation mechanism and stability of the associated 9R phase, and the influence of stacking fault energies on them. The results suggest that Σ3 {112} ITBs may migrate through the collective glide of partial dislocations. read less

Abstract: Using molecular dynamics simulation ( approximately 1 mus) in combination with the embedded atom method we have investigated interdiffusion and structural transformations at 1000 K in an initial core-shell nanoparticle (diameter approximately 4.5 nm). This starting particle has the f.c.c. structure in which a core of Ni atoms ( approximately 34%) is surrounded by a shell of Pd atoms ( approximately 66%). It is found that in such nanoparticles reactive diffusion accompanying nucleation and growth of a Pd(2)Ni ordering surface-sandwich structure takes place. In this structure, the Ni atoms mostly accumulate in a layer just below the surface and, at the same time, are located in the centres of interpenetrating icosahedra to generate a subsurface shell as a Kagomé net. Meanwhile, the Pd atoms occupy the vertices of the icosahedra and cover this Ni layer from the inside and outside as well as being located in the core of the nanoparticle forming (according to the alloy composition) a Pd-rich solid solution with the remaining Ni atoms. The total atomic fraction involved in building up the surface-sandwich shell of the nanoparticle in the form of the Ni Kagomé net layer covered on both side by Pd atoms is estimated at approximately 70%. These findings open up a range of opportunities for the experimental synthesis and study of new kinds of Pd-Ni nanostructures exhibiting Pd(2)Ni surface-sandwich ordering along with properties that may differ significantly from the corresponding bulk Pd-Ni alloys. Some of these opportunities are discussed. read less

Abstract: We report a molecular-dynamics study of the mechanical response to dynamic biaxial tensile straining of nanometer-scale-thick Al, Cu, and Ni films. We find that the mechanical behavior of such films of face-centered cubic metals with moderate-to-high propensity for stacking-fault formation (Cu and Ni) is significantly different from those where such propensity is low (Al). The plastic strain rate in Cu and Ni films is greater than that in Al ones, leading to an extended easy-glide stage in Cu and Ni but not in Al films. These differences arise due to the different dislocation annihilation mechanisms in the two film categories. read less

Abstract: The interaction between dislocations and grain boundaries is the principal factor for determining the mechanical properties and the plastic deformation behavior of metals. It is possible to control the grain-boundary microstructure and the macroscopic behavior has been widely exploited for scientific and industrial applications. In atomic scale, however, specific interaction characteristics such as the reaction energy and pathway have yet to be revealed. We have investigated the interaction process between a dislocation and an energetically stable grain boundary, and the quantitative characteristics were determined via atomistic transition state analysis. As a result, the interaction energy is found to be $1.16\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}1}\text{ }\text{eV}/\text{\AA{}}$, which is ${10}^{4}$ times higher than the Peierls potential. The lattice dislocations subsequently experience anomalous dissociations on the grain boundary, which becomes a key factor for the previously unexplained dislocation disappearance and grain-boundary migration. read less

Abstract: Understanding the plasticity and strength of crystalline materials in terms of the dynamics of microscopic defects has been a goal of materials research in the last 70 years. The size-dependent yield stress observed in recent experiments of submicrometer metallic pillars provides a unique opportunity to test our theoretical models, allowing the predictions from defect dynamics simulations to be directly compared with mechanical strength measurements. Although depletion of dislocations from submicrometer face-centered-cubic (FCC) pillars provides a plausible explanation of the observed size-effect, we predict multiplication of dislocations in body-centered-cubic (BCC) pillars through a series of molecular dynamics and dislocation dynamics simulations. Under the combined effects from the image stress and dislocation core structure, a dislocation nucleated from the surface of a BCC pillar generates one or more dislocations moving in the opposite direction before it exits from the surface. The process is repeatable so that a single nucleation event is able to produce a much larger amount of plastic deformation than that in FCC pillars. This self-multiplication mechanism suggests a need for a different explanation of the size dependence of yield stress in FCC and BCC pillars. read less

Abstract: The mechanical action of femtosecond laser pulse on a target may result in ablation of the irradiated frontal layer and spallation of the target rear side as well. The dynamics of expansion of the solid Al film after laser heating is studied by means of molecular dynamics (MD) simulations with the two EAM potentials (ours and Mishin et al.) and our parallel auto-balancing MPD3 code. It is found that the rear side spallation threshold is half as much again the frontal ablation threshold. The experimental and evaluated characteristics agree well in both crater depth and incident fluence on the ablation threshold. read less

Abstract: A concurrent multiscale modeling methodology that embeds a molecular dynamics (MD) region within a finite element (FEM) domain has been enhanced. The concurrent MD-FEM coupling methodology uses statistical averaging of the deformation of the atomistic MD domain to provide interface displacement boundary conditions to the surrounding continuum FEM region, which, in turn, generates interface reaction forces that are applied as piecewise constant traction boundary conditions to the MD domain. The enhancement is based on the addition of molecular dynamicsbased cohesive zone model (CZM) elements near the MD-FEM interface. The CZM elements are a continuum interpretation of the tractiondisplacement relationships taken from MD simulations using Cohesive Zone Volume Elements (CZVE). The addition of CZM elements to the concurrent MD-FEM analysis provides a consistent set of atomistically-based cohesive properties within the finite element region near the growing crack. Another set of CZVEs are then used to extract revised CZM relationships from the enhanced ESCM simulation of an edge crack under uniaxial loading. read less

Abstract: Growth of Fe nanostripes on a vicinal Cu(111) surface is investigated on the atomic scale by performing molecular dynamics and kinetic Monte Carlo simulations. We involve in our study the kinetic mechanisms of atomic incorporation recently reported by Mo et al. [Phys. Rev. Lett. 94, 155503 (2005)]. The atomistic processes responsible for the interlayer mass transport and the formation of Fe stripes of 1 ML height are identified. We demonstrate that strain relaxations at steps have a strong impact on the self-assembly of one-dimensional Fe atomic structures on vicinal Cu(111). read less

Abstract: The indentation response of Ni thin films of thicknesses in the nano scale was studied using molecular dynamics simulations with embedded atom method (EAM) interatomic potentials. Simulations were performed in single crystal films in the [111] orientation with thicknesses of 7nm and 33nm. In the elastic regime, the loading curves observed start deviating from the Hertzian predictions for indentation depths greater than 2.5% of the film thickness. The observed loading curves are therefore dependent on the film thickness. The simulation results also show that the contact stress necessary to emit the first dislocation under the indenter is nearly independent of film thickness. The deformation mechanism consists of the emission of dislocation loops in the area underneath the indenter. The loops are emitted in multiple directions with more dislocations emitted as the films thickness increases. This effect is interpreted as due to the back stress created by the first dislocations emitted when they reach the film/substrate interface and cannot cross into the substrate. Keyword: Embedded atom method, molecular dynamics, indentation, Nickel, Thin films. read less

Abstract: The strain field of isolated screw and edge dislocation cores in aluminum are calculated using density-functional theory and a flexible boundary condition method. Nye tensor density contours and differential displacement fields are used to accurately bound Shockley partial separation distances. Our results of 5-7.5 A (screw) and 7.0-9.5 A (edge) eliminate uncertainties resulting from the wide range of previous results based on Peierls-Nabarro and atomistic methods. Favorable agreement of the predicted cores with limited experimental measurements demonstrates the need for quantum mechanical treatment of dislocation cores. read less

Abstract: In this computational study, we used molecular dynamics and the embedded atom method to successfully reproduce the growth of gold nanorod morphologies from starting spherical seeds in the presence of model surfactants. The surfactant model was developed through extensive systematic attempts aimed at inducing nonisotropic nanoparticle growth in strictly isotropic computational growth environments. The aim of this study was to identify key properties of the surfactants which were most important for the successful anisotropic growth of nanorods. The observed surface and collective dynamics of surfactants shed light on the likely growth phenomena of real nanoprods. These phenomena include the initial thermodynamically driven selective adsorption, segregation, and orientation of the surfactant groups on specific crystallographic surfaces of spherical nanoparticle seeds and the kinetic elongation of unstable surfaces due to growth inhibiting surfactants on those surfaces. Interestingly, the model not only reproduced the growth of nearly all known nanorod morphologies when starting from an initial fcc or fivefold seed but also reproduced the experimentally observed failure of nanorod growth when starting from spherical nanoparticles such as the I(h) morphology or morphologies containing a single twinning plane. Nanorod morphologies observed in this work included fivefold nanorods, fcc crystalline nanorods in the [100] direction and [112] directions and the more exotic "dumbell-like" nanorods. Non-nanorod morphologies observed included the I(h) and the nanoprism morphology. Some of the key properties of the most successful surfactants seemed to be suggestive of the important but little understood role played by silver ions in the growth process of real nanorods. read less

Abstract: In nanoscale cutting of silicon wafer, it has been found that under certain conditions ductile mode chip formation can be achieved. In order to understand the mechanism of the ductile chip formation, experiments and molecular dynamics (MD) simulations have been conducted in this study. The results of MD simulations of nanoscale cutting of silicon showed that because of the high hydrostatic pressure in the chip formation zone, there is a phase transformation of the monocrytslline silicon from diamond cubic structure to both β silicon and amorphous phase in the chip formation zone, which results in plastic deformation of the workpiece material in the chip formation zone, as observed in experiments. The results further showed that although from experimental observation the plastic deformation in the ductile mode cutting of silicon is similar to that in cutting of ductile materials, such as aluminium, in ductile mode cutting of silicon it is the phase transformation of silicon rather than atomic dislocation that results in the plastic deformation. read less

Abstract: We propose, for the fcc structured Ag, Au, Cu, Ni, Pd, and Pt metals, an N-body potential with a simple power-function form, which significantly simplifies the fitting procedure and computation. The proposed potentials are able to correctly reproduce the lattice constants, cohesion energies, elastic constants, relative stabilities of different structures, formation energies of vacancy, and surface energies. In addition, the thermal properties, such as melting points and heat capacities, etc., are also satisfactorily determined from the proposed potentials. Moreover, the proposed potential is applied to calculate the trigonal and tetragonal paths between the fcc and bcc structures, and the calculated paths match well with those obtained from the first principles calculations. read less

Abstract: The interactions between edge dislocations and the grain boundary have been studied by using quasicontinuum simulations. With an increase in the shear strain, dislocation pile-up is created and local stress concentration occurs at the head of the pile-up. The relationship between the stress concentration and the number of dislocations in the pile-up is discussed. 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: Kinetic Monte Carlo (KMC) method realizes the millisecond or second order atomistic thin film growth. Twenty five kinds of events which may occur on Al (111) surface were classified. An attempt frequency and an activation energy of each event were defined using vibration analyses and nudged elastic band (NEB) method by which the minimum energy path (MEP) can be reasonably predicted. Temperature and deposition rate dependences of Al (111) film growth were intensively investigated in the present paper. The higher temperature and the lower rate drive the layer-by-layer film structural change. Two types of islands (fcc and hcp) were seen by modeling without considering the events of diffusion of dimer and trimer, while only fcc islands remain with considering such events. Thus, we find that the primitive events of diffusion of dimer and trimer take important roles in determination of surface morphology. read less

Abstract: Atomistic simulations of 001 , 110 , and 111 nanoindentation are performed to investigate anisotropic effects in elastic and incipient plastic behavior under nanoindentation. We compared two materials, singlecrystalline Al and Cu, focusing on the large difference between their anisotropic properties. The indent loaddepth behavior of Al during elastic deformation exhibits slight anisotropy, while that of Cu varies greatly according to the indentation axis. In addition, incipient plastic deformation, that is, dislocation nucleation, depends largely on the predicted slip system. While dislocations are emitted on the surface when indented by a spherical indenter with small radius, collective dislocation emission occurs from within the material. In the case of dislocation emission within the material, the critical mean pressure, which is an important indicator of dislocation nucleation, is inherent to the indentation axis in both materials. read less

Abstract: Voter and Chen version of an Embedded Atom Model has been applied to study the locally stable structures, energies, melting, isomerization and growth patterns of small aluminum clusters, Aln, in the size range of n = 2 - 13. Using molecular dynamics and thermal quenching simulations, the global minima and the other locally stable structures have been distinguished from those stationary structures that correspond to saddle points of the potential energy surface. A large number (10000) of independent initial configurations generated at high temperatures has been used to obtain the stable isomers, and the probabilities of sampling different basins of attractions, for each size of the clusters. Their energy spectra have been determined and melting, and isomerization dynamics are investigated. read less

Abstract: Grain boundary sliding is often the picture that explains computer simulation results and experiments on plasticity of nanophase materials. Using atomistic computer simulations we perform a detailed study of the effects of high hydrostatic pressure on nanophase Cu plasticity and find that it can be understood in terms of pressure dependent grain boundary sliding controlled by a Mohr-Coulomb law. This result explains recent findings on pressure-induced ultrahigh strength observed in computer simulations of shocks in nanophase Cu reported by Bringa et al. [Science 309, 1838 (2005)]. read less

Abstract: We report the discovery of a novel shape memory effect (SME) in Cu, Au, and Ni nanowires that have single-crystalline face-centered-cubic (FCC) structures and lateral dimensions of 1.5-5 nm. Under tensile loading and unloading, these wires are capable of recovering elongations of up to 51%, well beyond the recoverable strains of 5-8% typical for most bulk shape memory alloys (SMAs). Results of atomistic simulations and evidences from experiments show that this phenomenon only exists at the nanometer scale and is associated with a reversible crystallographic lattice reorientation driven by the high surface-stressinduced internal stresses at the nanoscale. This understanding also explains why these metals do not show an SME at macroscopic scales. The finding here has important implications for a wide range of nanodevices. read less

Abstract: We report a study of dynamic cracking in a silicon single crystal in which the ReaxFF reactive force field is used for several thousand atoms near the crack tip, while more than 100,000 atoms are described with a nonreactive force field. ReaxFF is completely derived from quantum mechanical calculations of simple silicon systems without any empirical parameters. Our results reproduce experimental observations of fracture in silicon including changes in crack dynamics for different crack orientations. read less

Abstract: Molecular dynamics simulations are carried out to analyze the structure and mechanical behavior of Cu nanowires with lateral dimensions of 1.45-2.89 nm. The calculations simulate the formation of nanowires through a top-down fabrication process by slicing square columns of atoms from single-crystalline bulk Cu along the [001], [010], and [100] directions and by allowing them to undergo controlled relaxation which involves the reorientation of the initial configuration with a (001) axis and {001} surfaces into a new configuration with a axis and {111} lateral surfaces. The propagation of twin planes is primarily responsible for the lattice rotation. The transformed structure is the same as what has been observed experimentally in Cu nanowires. A pseudoelastic behavior driven by the high surface-to-volume ratio and surface stress at the nanoscale is observed for the transformed wires. Specifically, the relaxed wires undergo a reverse transformation to recover the configuration it possessed as part of the bulk crystal prior to relaxation when tensile loading with sufficient magnitude is applied. The reverse transformation progresses with the propagation of a single twin boundary in reverse to that observed during relaxation. This process has the diffusion less nature and the invariant-plane strain of a martensitic transformation and is similar to those in shape memory alloys in phenomenology. The reversibility of the relaxation and loading processes endows the nanowires with the ability for pseudoelastic elongations of up to 41% in reversible axial strain which is well beyond the yield strain of the approximately 0.25% of bulk Cu and the recoverable strains on the order of 8% of most bulk shape memory materials. The existence of the pseudoelasticity observed in the single-crystalline, metallic nanowires here is size and temperature dependent. At 300 K, this effect is observed in wires with lateral dimensions equal to or smaller than 1.81 X 1.81 nm. As temperature increases, the critical wire size for observing this effect increases. This temperature dependence gives rise to a novel shape memory effect to Cu nanowires not seen in bulk Cu. read less

Abstract: Mechanical properties of solids are governed by crystal imperfections. Computational materials science is largely concerned with the modelling of such defects, e.g. their formation, migration, and interaction energies. Atomistic simulations of systems containing lattice defects are inherently difficult because of the generally complicated geometrical structure of the defects, the need for large simulation cells, etc. In this thesis, the role of lattice defects in the mechanism behind homogeneous melting is demonstrated. Also, a generic calculational scheme for studying atomic vibrations close to extended defects (applied to a dislocation) has been considered. Furthermore, heat capacities in the solid and liquid phases of aluminium have been calculated, as well as various thermophysical defect properties. The work was carried out using classical atomistic simulations, mainly molecular dynamics, of aluminium and copper. The interatomic forces were modelled with effective interactions of the embedded-atom type. The main results of this thesis are the following: • The thermal fluctuation initiating melting is an aggregate typically with 6-7 interstitials and 3-4 vacancies. • In the initial stage of melting, no signs of a shear modulus melting mechanism, or the presence of line-like defects (dislocations), can be seen. • The typical time interval from when melting initiates to the time at which the liquid phase is fully developed is of the order of 1000τ, where the period τ corresponds to the maximum vibrational frequency in the solid. • The solid-liquid boundary advances at a pace comparable to that of thermal transport by vibrating atoms in the crystal at high temperatures. • The seemingly small anharmonic effect in the heat capacity of aluminium is caused by a partial cancellation of the low-order term linear in the temperature and anharmonic terms of higher order in the temperature. • The core region of an edge dislocation in face-centred cubic aluminium has compressed and expanded regions. The excess volume associated with the dislocation core is small, about 6 percent of the atomic volume, as a result of a partial cancellation between the volume changes of the compressed and expanded regions. • The compressed and expanded regions of the edge dislocation core give negative and positive contributions, respectively, to the excess vibrational entropy. The overall effect is a positive vibrational excess entropy of the dislocation core which is about 2kB per atomic repeat length along the dislocation core. • The atomic vibrations near the dislocation core are modelled by considering an atomic cluster with about 500-1000 atoms containing the core of dislocation, embedded in a large discrete, but relaxed, lattice of about 23 000 atoms. An atomic region that is four atomic layers thick and about 18 atomic diameters long in the direction parallel to the Burgers vector, accounts for most of the excess entropy. • The constant-pressure heat capacity of aluminium shows a minimum as a function of temperature in the liquid phase. read less

Abstract: An analytic embedded-atom potentials was developed. It was applied to calculating mono-vacancy formation energy, divacancy binding energy, elastic constants, energy difference of different structures, the surface energy, and the phonon spectra of iron and europium. The formation enthalpies of Fe-Eu binary alloy were also calculated. The calculated physical properties are in agreement with the experiments available or other theoretical results. The formation enthalpies are in good agreement with the results obtained by Miedema’s theory. read less

Abstract: The structure of the $\ensuremath{\gamma}∕{\ensuremath{\gamma}}^{\ensuremath{'}}$ phase interface in a Ni-based single-crystal superalloy is simulated by molecular dynamics (MD) using an embedded atom method potential. From the calculated results we find that three dislocation network patterns, namely square, rectangle, and equilateral triangle, appear on {100}, {110}, and {111} interphase interface, respectively. The dislocation networks consist of four edge dislocations (⟨011⟩ {100}, $⟨\overline{1}10⟩$ {110}, ⟨001⟩ {110}, and ⟨112⟩ {111}). The energy of the $\ensuremath{\gamma}∕{\ensuremath{\gamma}}^{\ensuremath{'}}$ phase interface for {100}, {110}, and {111} plane is $271\phantom{\rule{0.3em}{0ex}}\mathrm{mJ}∕{\mathrm{m}}^{2}$, $240\phantom{\rule{0.3em}{0ex}}\mathrm{mJ}∕{\mathrm{m}}^{2}$, and $32\phantom{\rule{0.3em}{0ex}}\mathrm{mJ}∕{\mathrm{m}}^{2}$. The side length of network is $166.8\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ for the square, $166.8\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ and $235.8\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ for the rectangle and $166.8\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ for the equilateral triangle. The relationship between the size of network and mismatch is presented quantitatively. The calculated results can be supported by very recent experiments. Based on the MD simulation and the energy analysis we have revealed the basic characteristic of structure on $\ensuremath{\gamma}∕{\ensuremath{\gamma}}^{\ensuremath{'}}$ phase interface. The related mechanism of the stability of the interphase interface is also discussed. read less

Abstract: The interplay between structure and magnetic properties of small cobalt clusters embedded in a Cu(001) surface is studied performing ab initio and tight-binding calculations in a fully relaxed geometry. We reveal that, despite the small macroscopic mismatch between Co and Cu, the strain relaxations at the interface have a profound effect on the structure of the clusters and the substrate. The physical mechanism responsible for the strain relaxations in embedded clusters is related to the size-dependent mesoscopic mismatch which has been recently introduced to understand homo- and heteroepitaxial growth at the mesoscale [O. V. Lysenko et al., Phys. Rev. Lett. 89, 126102 (2002)]. We show that the atomic relaxations strongly reduce the magnetic anisotropy energy (MAE) and the orbital magnetic moments of embedded clusters. The largest MAE of about 1.8 meV is found for a single Co atom in the Cu(001) surface. A strong enhancement of the spin magnetic moments in embedded clusters as compared to a single atom of Co incorporated in the Cu(001) surface is found. Magnetic properties of embedded and supported clusters are compared. While in supported clusters the MAE is strongly enhanced at the edge atoms, the immersion of the cluster into the surface and atomic relaxations make the distribution of the local MAE contributions and orbital-moment values almost homogeneous. read less

Abstract: We thank J. J. Saenz for helpful discussions, and Cecalcula (Venezuela) for computer facilities. This work has been partially supported by the CSIC-IVIC researchers exchange program and the Spanish DGICyT (MEC) through Project No. BFM2003-01167/FISI. read less

Abstract: A molecular dynamics simulation has been performed to studied the structure of Al and Fe at different temperatures in a frame of a microcanonical ensemble. The interaction among atoms has been modelled with the embedded atom method. The short range order has been study using the radial distribution function, the bond angle distribution function and the HoneycuttAndersen indexing method was used for analyse the cluster type formed. Different kinds of clusters were found, although it was observed that icosahedral or quasi-icosahedral clusters increase as the temperature decreases. read less

Abstract: Mechanical properties of solids are governed by crystal imperfections. Computational materials science is largely concerned with the modelling of such defects, e.g. their formation, migration, and interaction energies. Atomistic simulations of systems containing lattice defects are inherently difficult because of the generally complicated geometrical structure of the defects, the need for large simulation cells, etc. In this thesis, the role of lattice defects in the mechanism behind homogeneous melting is demonstrated. Also, a generic calculational scheme for studying atomic vibrations close to extended defects (applied to a dislocation) has been considered. Furthermore, heat capacities in the solid and liquid phases of aluminium have been calculated, as well as various thermophysical defect properties. The work was carried out using classical atomistic simulations, mainly molecular dynamics, of aluminium and copper. The interatomic forces were modelled with effective interactions of the embedded-atom type. The main results of this thesis are the following: • The thermal fluctuation initiating melting is an aggregate typically with 6-7 interstitials and 3-4 vacancies. • In the initial stage of melting, no signs of a shear modulus melting mechanism, or the presence of line-like defects (dislocations), can be seen. • The typical time interval from when melting initiates to the time at which the liquid phase is fully developed is of the order of 1000τ, where the period τ corresponds to the maximum vibrational frequency in the solid. • The solid-liquid boundary advances at a pace comparable to that of thermal transport by vibrating atoms in the crystal at high temperatures. • The seemingly small anharmonic effect in the heat capacity of aluminium is caused by a partial cancellation of the low-order term linear in the temperature and anharmonic terms of higher order in the temperature. • The core region of an edge dislocation in face-centred cubic aluminium has compressed and expanded regions. The excess volume associated with the dislocation core is small, about 6 percent of the atomic volume, as a result of a partial cancellation between the volume changes of the compressed and expanded regions. • The compressed and expanded regions of the edge dislocation core give negative and positive contributions, respectively, to the excess vibrational entropy. The overall effect is a positive vibrational excess entropy of the dislocation core which is about 2kB per atomic repeat length along the dislocation core. • The atomic vibrations near the dislocation core are modelled by considering an atomic cluster with about 500-1000 atoms containing the core of dislocation, embedded in a large discrete, but relaxed, lattice of about 23 000 atoms. An atomic region that is four atomic layers thick and about 18 atomic diameters long in the direction parallel to the Burgers vector, accounts for most of the excess entropy. • The constant-pressure heat capacity of aluminium shows a minimum as a function of temperature in the liquid phase. read less

Abstract: : We explore asymmetries in the plastic flow of idealized nanocrystalline nickel through static molecular simulations, We find that both the yield and flow stresses of these materials are higher in compression than in tension. This result is discussed in the context of earlier work on metallic glasses, and it is suggested that very similar atomic-level mechanisms control yield in both of these materials classes. read less

Abstract: Semiempirical interatomic potentials have been developed for Al, $\ensuremath{\alpha}\ensuremath{-}\mathrm{Ti},$ and $\ensuremath{\gamma}\ensuremath{-}\mathrm{TiAl}$ within the embedded atom method (EAM) formalism by fitting to a large database of experimental as well as ab initio data. The ab initio calculations were performed by the linearized augmented plane wave (LAPW) method within the density functional theory to obtain the equations of state for a number of crystal structures of the Ti-Al system. Some of the calculated LAPW energies were used for fitting the potentials while others for examining their quality. The potentials correctly predict the equilibrium crystal structures of the phases and accurately reproduce their basic lattice properties. The potentials are applied to calculate the energies of point defects, surfaces, and planar faults in the equilibrium structures. Unlike earlier EAM potentials for the Ti-Al system, the proposed potentials provide a reasonable description of the lattice thermal expansion, demonstrating their usefulness for molecular-dynamics and Monte Carlo simulations at high temperatures. The energy along the tetragonal deformation path (Bain transformation) in $\ensuremath{\gamma}\ensuremath{-}\mathrm{TiAl}$ calculated with the EAM potential is in fairly good agreement with LAPW calculations. Equilibrium point defect concentrations in $\ensuremath{\gamma}\ensuremath{-}\mathrm{TiAl}$ are studied using the EAM potential. It is found that antisite defects strongly dominate over vacancies at all compositions around stoichiometry, indicating that $\ensuremath{\gamma}\ensuremath{-}\mathrm{TiAl}$ is an antisite disorder compound, in agreement with experimental data. read less

Abstract: An embedded-atom-method potential for tantalum (Ta) has been carefully constructed by fitting to a combination of experimental and density-functional theory (DFT) data. The fitted data include the elastic constants, lattice constant, cohesive energy, unrelaxed vacancy formation energy, and hundreds of force data calculated by DFT for a variety of structures such as liquids, surfaces, clusters, interstitials, vacancies, and stacking faults. We also fit to the cohesive energy vs volume data from the equation of state for the body-centered-cubic (bcc) Ta and to the calculated cohesive energy using DFT for the face-centered-cubic (fcc) Ta structure. We assess the accuracy of the new potential by comparing several calculated Ta properties with those obtained from other potentials previously reported in the literature. In many cases, the new potential yields superior accuracy at a comparable or lower computational cost. read less

Abstract: We introduce an approach in which results from atomistic simulations are combined with discrete dislocation dynamics simulations of crack-tip plasticity. The method is used to study the effects of dislocation pinning due to grain boundaries or secondary particles on the fracture behavior of aluminum. We find that the fracture resistance is reduced with decreasing pinning distance. The results show that the pinning of the dislocations causes a net decrease in the shear stress projected on the slip plane, preventing further dislocation emission. Semibrittle cleavage occurs after a certain number of dislocations is emitted. read less

Abstract: Crack propagation studies in nanocrystalline Ni samples with mean grain sizes ranging from 5 to 12 nm are reported using atomistic simulations. For all grain sizes pure intergranular fracture is observed. Intergranular fracture is shown to proceed by the coalescence of microvoids formed at the grain boundaries ahead of the crack. The energy released during propagation is higher than the Griffith value, indicating an additional grain-boundary accommodation mechanism. read less

Abstract: We develop a many-body force field (FF) for tantalum based on extensive ab initio quantum mechanical (QM) calculations and illustrate its application with molecular dynamics (MD). As input data to the FF we use ab initio methods (LAPW-GGA) to calculate: (i) the zero temperature equation of state (EOS) of Ta for bcc, fcc, and hcp crystal structures for pressures up to ∼500 GPa, and (ii) elastic constants. We use a mixed-basis pseudopotential code to calculate: (iii) volume-relaxed vacancy formation energy also as a function of pressure. In developing the Ta FF we also use previous QM calculations of: (iv) the EOS for the A15 structure; (v) the surface energy bcc (100); (vi) energetics for shear twinning of the bcc crystal. We find that, with appropriate parameters, an embedded atom model FF (denoted as qEAM FF) is able to reproduce all this QM data. We illustrate the use of the qEAM FF with MD to calculate such finite temperature properties as the melting curve up to 300 GPa and thermal expansivity in a wide temperature range. Both our predictions agree well with experimental values. read less

Abstract: In this paper we have developed empirical Embedded Atom Model potentials, following the fitting scheme proposed by Chantasiriwan and Milstein, in order to describe the stacking fault behaviour of copper, gold, nickel and aluminium. We show that the potentials based on this scheme can be modified to provide reasonable stacking-fault energy values and consequently a better description of the plastic properties. Modifications were done by changing the cut-off distance in case of aluminium and nickel, and in case of gold and copper by also modifying the functional form of the pair-potential. In order to validate these modified potentials we have tested them by studying various properties, such as structural, defect, and surface energies, and phonon spectra and comparing results with those from experiments and other model potentials. read less

Abstract: A computationally efficient and accurate description of interatomic interactions is indispensable to the fidelity of atomistic simulations. In the development of popular empirical potentials, it is assumed that atoms separated beyond a certain cut-off distance have negligible interatomic forces and hence may be safely ignored in the force calculations. This arbitrary, and yet common, practice of force truncation is undoubtedly ad hoc and is not grounded in the physics of the interactions. With the advent of fast computers and accurate first-principles calculations, it is now feasible to determine what this cut-off distance should be. In this work, employing a first-principles calculation based on density functional theory and the local density approximation (LDA) we probe the extent of interatomic forces in aluminium caused by a variety of defect types. The forces on neighbours to these defects, obtained from first-principles calculations, were then compared with the corresponding values from many short- and long-range semiempirical literature potentials. It is clear that none of these semiempirical potentials can reproduce the LDA results, although the newest potentials that use LDA force data for potential determination come close. The results also indicate that nearest-neighbour forces are dominant for zero- and one-dimensional defects. Only for a free surface did we find forces at more distant neighbours to be comparable in magnitude. Using the new LDA force data for the single vacancy, we modify a literature potential to improve significantly the agreement with the first-principles calculations. read less

Abstract: Experimental data on and theoretical models for the viscosity of various types of liquids and melts under pressure are reviewed. Experimentally, the least studied melts are those of metals, whose viscosity is considered to be virtually constant along the melting curve. The authors' new approach to the viscosity of melts involves the measurement of the grain size in solidified samples. Measurements on liquid metals at pressures up to 10 GPa using this method show, contrary to the empirical approach, that the melt viscosity grows considerably along the melting curves. Based on the experimental data and on the critical analysis of current theories, a hypothesis of a universal viscosity behavior is introduced for liquids under pressure. Extrapolating the liquid iron results to the pressures and temperatures at the Earth's core reveals that the Earth's outer core is a very viscous melt with viscosity values ranging from 102 Pa s to 1011 Pa s depending on the depth. The Earth's inner core is presumably an ultraviscous (>1011 Pa s) glass-like liquid — in disagreement with the current idea of a crystalline inner core. The notion of the highly viscous interior of celestial bodies sheds light on many mysteries of planetary geophysics and astronomy. From the analysis of the pressure variation of the melting and glass-transition temperatures, an entirely new concept of a stable metallic vitreous state arises, calling for further experimental and theoretical study. read less

Abstract: This dissertation explores Random Neural Networks (RNNs) in several aspects and their applications. First, Novel RNNs have been proposed for dimensionality reduction and visualization. Based on Extreme Learning Machines (ELMs) and Self-Organizing Maps (SOMs) a new method is created to identify the important variables and visualize the data. This technique reduces the curse of dimensionality and improves furthermore the interpretability of the visualization and is tested on real nursing survey datasets. ELM-SOM+ is an autoencoder created to preserves the intrinsic quality of SOM and also brings continuity to the projection using two ELMs. This new methodology shows considerable improvement over SOM on real datasets. Second, as a Supervised Learning method, ELMs has been applied to the hierarchical multiscale method to bridge the the molecular dynamics to continua. The method is tested on simulation data and proven to be efficient for passing the information from one scale to another. Lastly, the regularization of ELMs has been studied and a new regularization algorithm for ELMs is created using a modified Lanczos Algorithm. The Lanczos ELM on average divide computational time by 20 and reduce the Normalized MSE by 14% comparing with regular ELMs. read less

Abstract: Grain boundaries (GBs) make a significant contribution to the kinetics of plastic deformation of polycrystalline and nanocrystalline metals. The motion of GB during shear deformation applied along the GB attracts much attention in the literature. GBs can have an arbitrary orientation with respect to the direction of shear in the conditions of real deformation of polycrystals. In this work, we study the relaxation of shear stresses in the case, when the shear is applied across the low-angle tilt GB, using the molecular dynamics (MD) method. Under these conditions, the initial stage of stress relaxation is associated with the motion of GBs as walls of perfect dislocations. The subsequent interaction of GBs moving towards each other with opposite misorientation angles leads to either their annihilation or stopping with the subsequent emission of other types of dislocations. The model of the low-angle tilt GB motion, where GB is walls of periodically located perfect edge dislocations, is proposed basing on the equation of motion of a solitary dislocation in a single crystal for a theoretical description of the first stage. The proposed model accurately describes the motion of GBs before the annihilation or other dislocation reactions. Also, the nucleation of partial dislocations from GBs is considered, and the reactions of the splitting of boundary dislocations into partial dislocations, which then penetrate into the crystal grains, are investigated on the basis of MD simulation data. read less

Abstract: Molecular dynamic simulations of the generation and propagation of shock pulses of picosecond duration initiated by nanoscale impactors, and their interaction with the rear surface is carried out for aluminum and copper. It is shown that the presence of deposited nanoparticles on the rear surface increases the threshold value of the impact intensity leading to the rear spallation. The interaction of a shock wave with nanoparticles leads to severe plastic deformation in the surface layer of the metal including nanoparticles. A part of the compression pulse energy is expended on the plastic deformation, which suppresses the spall fracture. Spallation threshold substantially increases at large diameters of deposited nanoparticles, but instability develops on the rear surface of the target, which is accompanied by ejection of droplets. The instability disrupts the integrity of the rear surface, though the loss of integrity occurs through the ejection of mass, rather than a spallation. read less

Abstract: Intermittent crystalline plasticity with power-law behaviors, which is reminiscent of self-organized criticality, has been reported in recent experimental and numerical studies. In this study, we show that the compressive loading condition can provide different statistical features of intermittent plasticity in metals from those under tensile loading. Employing an embedded atom method potential for aluminum we performed molecular dynamics simulations for uniaxial tensile and compressive deformation. It is shown that the power spectra of tensile stress have power-law decay region. Powerlaw distribution of stress drop and waiting time of plastic deformation events are also observed. However, under the compressive loading large-scale deformation events which result in a plateau and larger cutoff on the stress drop distribution are observed. This difference is originated from the geometrical feature of slip systems that dominate interaction between dislocations in crystals. read less

Abstract: The simple method to analyze atomistic strain in molecular dynamics (MD) simulation is studied. The proposed method, here called atomic strain measure (ASM), is based on Green-Lagrangian strain measure which is traditionally defined in continuum mechanics. The ASM is formulated for the use in atomic system with some adequate assumptions. In our formulation, pairwise interatomic vectors of finite length are approximately substituted for infinitesimal continuum line segments between material's points. The obtained expression of ASM is very simple and easy to use. The authors are checking the validity, limitation and usefulness of ASM by actual MD models of aluminum. A perfect-crystal model results in qualitatively good results for strain evaluation, as for the response to homogenous deformation. The ASM is also applied to a polycrystal model, which has a lot of inhomogeneous nanostructures, i.e. crystalline defects such as grain boundaries (GBs) and triple junctions (TJs) among them. It is confirmed that a certain concentration of strain onto the vicinity of GB plane or TJ point is able to be clearly captured by using ASM. It is concluded that approach of the present ASM analysis for atomic simulation is very effective to obtain change of nanostructure. read less

Abstract: Based on experiments and first-principles calculations, a Ni–Al–Re system embedded atom method (EAM) potential is constructed for the γ(Ni)/γ′(Ni3Al) superalloy. The contribution of the inner elastic constants is considered in the fitting of Re with a hexagonal close-packed structure. Using this potential, point defects, planar defects and lattice misfit of γ(Ni) and γ′(Ni3Al) are investigated. The interaction between Re and the misfit dislocation of the γ(Ni)/γ′(Ni3Al) system is also calculated. We conclude that the embedding energy has an important effect on the properties of the alloys, such as the planar fault energies of Ni3Al, by considering the relationship between the charge transfer calculated from first-principles, the elastic constants of Ni3Al and the host electron density of the EAM potential. The multi-element potential predicts that Re does not form clusters in γ(Ni), which is consistent with recent experiments and first-principles calculations. read less

Abstract: Influence of temperature on ideal shear strength (ISS), τc, of two fcc metals (Al and Cu) was studied by means of molecular dynamics simulations. To get reliable results we investigated influence of parameters of the applied Parrinello-Rahman stress control method and implemented damping of simulation cell fluctuations to avoid occurrence of structural instability assisted by too high fluctuations. The damping successfully reduces strain and stress fluctuations during simulations if the damping factor is specified properly. We also investigated simulation cell size effect to evaluate minimal number of atoms providing reliable results in order to reduce computational efforts and estimate the possibility of applying ab initio calculations. Recently developed embedded atom method (EAM) interatomic potentials for both metals were also examined to find most appropriate for our study. EAM potential developed by Zope et al. and Mishin et al. were revealed to be most suitable for Al and Cu, respectively. It is essential to choose appropriate simulation parameters and interatomic potentials for the valid evaluation of ISS at elevated temperatures. We find almost linear decrease in ideal strength with increasing temperature for [112](111) shear deformation, while critical strain decreases in a nonlinear manner. At room temperature, reduction of shear strength for Al(Cu) is less than 35%(25%) compared to that at 0 K. read less

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Abstract: Abstract: This chapter surveys the computer-based multi-scale modelling approaches currently being used to develop physical models of the effects of radiation on nuclear materials. The focus is on the problem of radiation-induced hardening (and embrittlement) in steels, limited to the scales ranging from the nucleus to the single crystal. First, the multi-scale nature of radiation effects is illustrated, including examples of microstructural and mechanical property changes observed in steels used in nuclear reactors. Then the chapter discusses the fundamental ideas upon which the multi-scale modelling approach is based. Next, an overview of the techniques of use in a multi-scale modelling framework is given, with an example of how these can be integrated. A discussion of the state-of-the-art and other general remarks conclude the chapter. read less

Abstract: The grain boundary has been recognized for one of the major defect structures in determining the material strength. It is increasingly important to understand the individual characteristics of various types of grain boundaries due to the recent advances in material miniaturization technique. In the present study three types of grain boundaries of coincidence site lattice (CSL), small angle (SA), and random types are considered as the representative example of grain boundaries. The grain boundary energies and atomic configurations of CSL are first evaluated by first-principle density functional theory (DFT) and the embedded atom method (EAM) calculations. SA and random grain boundaries are subsequently constructed by the same EAM and the fundamental characteristics are investigated by the discrete dislocation mechanics models and the Voronoi polyhedral computational geometric method. As the result, it is found that the local structures are well accorded with the previously reported high resolution-transmission electron microscope (HR-TEM) observations, and that stress distributions of CSL and SA grain boundaries are localized around the grain boundary core. The random grain boundary shows extremely heterogeneous core structures including a lot of pentagon-shaped Voronoi polyhedral resulting from the amorphous-like structure. read less

Abstract: Mechanical properties of nanocrystalline metals of which the grainsize is below about 100nm are affected by the relationship betweenthe intergranular and intragranular deformation. Intragranulardeformation in nanocrystalline metals could be caused by the movementof perfect dislocations or Shockley's partial dislocations, hencestacking fault energy is one of the important value to control theinternal structure. Moreover, the proportion of the grain boundaryregion dramatically increases with grain size decreasing, hence it isalso important to investigate the effects of individual grain boundarystructures and the distribution of grain boundary characteristics onthe macroscopic mechanical properties of polycrystalline materials. Inthis study, we investigate deformation mechanism and mechanicalproperties of nanocrystalline Al and Cu that show the differentstacking fault energy using molecular dynamics simulations, and wealso consider the effects of grain boundary misorientationdistributions on the mechanical properties of such materials. read less

Abstract: Innovative thin film technology to realize the finer electric devices needs to understand the atomic level process of film growth and its relationship to the film characterization. In this paper, the long film growth phenomena for a few micro-second order with the short severe collisions by incident particles are analyzed by the proposed hybridized atomistic modeling. This method combined molecular dynamics (MD) with kinetic Monte Carlo (KMC) can directly treat two types of events of deposition and diffusion, which have quite different time scales. The solutions suggest that the large incident kinetic energy of deposited atoms compatible to the realistic physical vapor deposition (PVD) impels to fluctuate the equilibrium on Al (111) surface very drastically and affects the atomic level surface morphology. It is found that the faster incident atoms with 1.0 × 104 m/s can make the smoother surface than those with the velocity of 1.0 × 103 m/s. This is due to much activated atomic migration, which can be realized only by MD. read less

Abstract: Atomic simulations of shear deformation of aluminum and coppernanocrystalline models are performed to determine the influences ofdefect plane energies on defect structures in nanocrystallinemetals. In aluminum models with high stacking fault energy, deformation twins can be observed in the case of the specific crystalorientation which is the same one as that expected analytically. Inthis case, extrinsic stacking faults always form in the early stage ofthe formation of intrinsic stacking faults. It is also observed thatnew crystal slip generates in an extended stacking fault in anothercrystal orientation in aluminum. In order to clarify the appearancemechanism of these defects in aluminum, we use the dislocation theoryand nudged elastic band method, and necessary conditions to formdeformation twin in aluminum are discussed. read less

Abstract: For metallic alloys, the amorphous state is often regarded as the limiting structure as grain size is reduced towards zero. One interesting consequence of this limit is that the properties of the finest nanocrystalline metals must begin to resemble those of metallic glasses. In this work we focus upon the nature of the plastic yield mechanisms in these material classes, and seek to identify commonalities and disparities in the nature of plastic yield in glasses and nanocrystals. The discussion is presented with reference to static atomistic simulations of (i) an amorphous binary alloy, and (ii) a nanocrystalline Ni specimen with grain size of 3 nm. We show that both these materials deform by the operation of fine atomic shearing events, and both exhibit asymmetric yielding as a consequence. read less

Abstract: Atomistic dislocation models were used to determine the properties of dislocation core fields in Al using an EAM potential. Equilibrium atom configurations were compared with initial configurations displaced according to the Volterra field to determine core displacement fields for edge, screw, and mixed (60? and 30?) geometries. The core field was approximated by a line force defect field lying parallel to the dislocation line direction. Best-fit parameters for the core fields were obtained in terms of the anisotropic elastic solution for a line force defect, from which the line force strengths and the origin of the line forces were determined. The line force stress fields were then used to compute the forces between dislocations for several dislocation configurations. The Volterra field dominates beyond 50b but core field forces modify the equilibrium angle of edge dislocation dipoles and determine the force between otherwise non-interacting edge and screw dislocations at distances out to 50b compared to the Volterra-only forces. read less