Abstract: Intense research efforts are focused on the development of advanced high-entropy alloys intended for premium aerospace components and other applications, where high strength and good formability are crucial. The mechanical properties of these alloys are closely related to the phase transformation, dislocation evolution, and grain size, and these factors are affected by the deformation temperature. The response of the retained austenite to strain-induced martensitic transformation at various temperatures was studied in an advanced Ti68Nb7Ta3Zr4Mo18 (at.%) high-entropy alloy via molecular dynamics simulation. It was found that the Ti68Nb7Ta3Zr4Mo18 alloy changes from a single crystal to a polycrystal during the tensile process, and the transition of the Ti68Nb7Ta3Zr4Mo18 (at.%) high-entropy alloy from the BCC phase to the FCC phase occurs. At high temperatures and low strain rates, grain boundary slip is the main deformation mechanism, and at low temperatures and high strain rates, dislocation slip replaces grain boundary slip as the dominant deformation mechanism, which improves the strength of the alloy. Moreover, when the grain size is too small, the strength of the alloy decreases, which does not satisfy the fine grain strengthening theory and shows an inverse Hall–Petch relationship. This study offers a new compositional window for the additive manufactured lightweight high-strength material categories for various applications including the aerospace industry. read less

Abstract: Machine learning of the quantitative relationship between local environment descriptors and the potential energy surface of a system of atoms has emerged as a new frontier in the development of interatomic potentials (IAPs). Here, we present a comprehensive evaluation of ML-IAPs based on four local environment descriptors --- atom-centered symmetry functions (ACSF), smooth overlap of atomic positions (SOAP), the Spectral Neighbor Analysis Potential (SNAP) bispectrum components, and moment tensors --- using a diverse data set generated using high-throughput density functional theory (DFT) calculations. The data set comprising bcc (Li, Mo) and fcc (Cu, Ni) metals and diamond group IV semiconductors (Si, Ge) is chosen to span a range of crystal structures and bonding. All descriptors studied show excellent performance in predicting energies and forces far surpassing that of classical IAPs, as well as predicting properties such as elastic constants and phonon dispersion curves. We observe a general trade-off between accuracy and the degrees of freedom of each model, and consequently computational cost. We will discuss these trade-offs in the context of model selection for molecular dynamics and other applications. read less

Abstract: Fe-cluster-based crystal structures are predicted for chalcogenides Fe3X4 (X = S, Se, Te) using an adaptive genetic algorithm. Topologically different from the well-studied layered structures of iron chalcogenides, the newly predicted structures consist of Fe clusters that are either separated by the chalcogen atoms or connected via sharing of the vertex Fe atoms. Using first-principles calculations, we demonstrate that these structures have competitive or even lower formation energies than the experimentally synthesized Fe3X4 compounds and exhibit interesting magnetic and electronic properties. In particular, we show that Fe3Te4 can be a good candidate as a rare-earth-free permanent magnet and Fe3S4 can be a magnetic nodal-line topological material. read less

Abstract: The Materials Genome Initiative seeks to significantly decrease the cost and time of development and integration of new materials. Within the domain of atomistic simulations, several roadblocks stand in the way of reaching this goal. While the NIST Interatomic Potentials Repository hosts numerous interatomic potentials (force fields), researchers cannot immediately determine the best choice(s) for their use case. Researchers developing new potentials, specifically those in restricted environments, lack a comprehensive portfolio of efficient tools capable of calculating and archiving the properties of their potentials. This paper elucidates one solution to these problems, which uses Python-based scripts that are suitable for rapid property evaluation and human knowledge transfer. Calculation results are visible on the repository website, which reduces the time required to select an interatomic potential for a specific use case. Furthermore, property evaluation scripts are being integrated with modern platforms to improve discoverability and access of materials property data. To demonstrate these scripts and features, we will discuss the automation of stacking fault energy calculations and their application to additional elements. While the calculation methodology was developed previously, we are using it here as a case study in simulation automation and property calculations. We demonstrate how the use of Python scripts allows for rapid calculation in a more easily managed way where the calculations can be modified, and the results presented in user-friendly and concise ways. Additionally, the methods can be incorporated into other efforts, such as openKIM. read less

Abstract: Controlling the electronic structure and surface strain of a nanoparticle catalyst has become an important strategy to tune and to optimize its catalytic efficiency for a chemical reaction. Using density functional theory (DFT) calculations, we predicted that core/shell M/CuPd (M = Ag, Au) NPs with a 0.8 or 1.2 nm CuPd2 shell have similar but optimal surface strain and composition and may surpass Pt in catalyzing oxygen reduction reactions. We synthesized monodisperse M/CuPd NPs by the coreduction of palladium acetylacetonate and copper acetylacetonate in the presence of Ag (or Au) nanoparticles with controlled shell thicknesses of 0.4, 0.75, and 1.1 nm and CuPd compositions and evaluated their catalysis for the oxygen reduction reaction in 0.1 M KOH solution. As predicted, our Ag/Cu37Pd63 and Au/Cu40Pd60 catalysts with 0.75 and 1.1 nm shells were more efficient catalysts than the commercial Pt catalyst (Fuel Cells Store), with their mass activity reaching 0.20 A/mg of noble metal at -0.1 V vs Ag/AgCl (4 M KCl); this was over 3 times higher than that (0.06 A/mg Pt) from the commercial Pt. These Ag(Au)/CuPd nanoparticles are promising non-Pt catalysts for oxygen reduction reactions. read less

Abstract: Magnesium (Mg) is one of the most abundant metallic elements in nature and presents attractive mechanical properties in the industry. Particularly, it has a low density and relatively high strength/weight and stiffness/weight ratios, which make it one of the most attractive lightweight metals. However, the huge potential of Mg is restricted by its low ductility, associated with its hexagonal close packed (hcp) structure. This problem can be solved if Mg adopts the body centered cubic (bcc) structure, which is stable at high pressure or in confinement with stiff bcc metals like Nb. Molecular dynamics method is a magnificent tool to study material’s structure and deformation mechanisms at the atomic level, however, requiring accurate interatomic potentials. The majority of the interatomic potentials available in the literature for Mg have only been fitted to the properties of its stable hcp phase. In the present work, we perform systematic study of applicability of currently available Mg potentials to modeling the properties of metastable bcc polymorph of Mg, taking into account cohesive energy curves, elastic constants, stacking fault energies, and phonon dispersion curves. We conclude that the modified embedded atom method (MEAM) potentials are the most suitable for investigating bcc Mg in Mg/Nb nano-composites, while the properties of high-pressure bcc Mg would be better modeled by neural network interatomic potentials after different local atomic environments corresponding to bcc Mg being included into the fitting database. read less

Abstract: Получена: 18 мая 2018 г. Принята: 25 июня 2018 г. Опубликована: 29 июня 2018 г. В статье представлена вторая часть обзора современных подходов и работ, посвященных построению потенциалов межатомного взаимодействия с использованием методологии погруженного атома (EAM-потенциалы). Эта часть обзора посвящена одной из наиболее остро стоящих проблем в молекулярной динамике – вопросам построения потенциалов, которые были бы пригодны для описания структуры и физико-механических свойств многокомпонентных (в первую очередь – бинарных и тернарных) материалов. Отмечены первые работы, в которых предлагались подходы к построению функций перекрестного взаимодействия для сплавов никеля и меди – как с использованием методологии EAM, так и несколько отличающийся по процедуре построения потенциал типа Финисса-Синклера. Рассматриваются работы, в которых производится сопоставление различных подходов к построению потенциалов, а также к процедуре идентификации их параметров на примере одних и тех же многокомпонентных систем (типа Al-Ni или Cu-Au). Кроме того, особый интерес представляют некоторые тернарные системы, например Fe–Ni–Cr, W–H– He или U–Mo–Xe, которые являются ключевыми для материалов атомной энергетики и которые в последние годы активно изучаются как возможные материалы для использования в термоядерных ректорах. Приведены примеры работ, в которых предлагаются и исследуются потенциалы для описания многокомпонентных систем, пригодных для использования в аэрокосмической промышленности и изготовленных прежде всего на основе никеля. Рассмотрены результаты исследований различных интерметаллических соединений, отмечены работы, в которых при помощи построенного EAM потенциала удалось количественно точно описать фазовые диаграммы соединений и вычислить характеристики фазовых переходов. read less

Abstract: Работа посвящена трехмерному моделированию течений газов в микроканалах сложных технических систем. Предложен многомасштабный подход, сочетающий решения уравнений квазигазодинамики (КГД) и молекулярной динамики (МД). Представлена параллельная реализация подхода, основанная на методах расщепления по физическим процессам и разделения областей. Реализация ориентирована на использование вычислительных систем с центральной и гибридной архитектурами. Расчеты показали устойчивость численного алгоритма. С его помощью методами МД были получены основные коэффициентные зависимости для КГД системы, проверен переход от МД к КГД и обратно, произведен расчет трехмерного течения. Полученные результаты подтвердили эффективность разработанного подхода. read less

Abstract: A Gaussian approximation machine learning interatomic potential for platinum is presented. It has been trained on density-functional theory (DFT) data computed for bulk, surfaces, and nanostructured platinum, in particular nanoparticles. Across the range of tested properties, which include bulk elasticity, surface energetics, and nanoparticle stability, this potential shows excellent transferability and agreement with DFT, providing state-of-the-art accuracy at a low computational cost. We showcase the possibilities for modeling of Pt systems enabled by this potential with two examples: the pressure-temperature phase diagram of Pt calculated using nested sampling and a study of the spontaneous crystallization of a large Pt nanoparticle based on classical dynamics simulations over several nanoseconds. read less

Abstract: Body-centered-cubic (BCC) high entropy alloys (HEAs) can show exceptionally high strength up to high temperatures. Mechanistic theories are needed to guide alloy discovery within the immense multicomponent HEA compositional space. Here, two new theories for strengthening as controlled by screw and edge dislocations, respectively, are applied to predict the yield stresses of a range of BCC alloys over a wide range of temperatures. Results show that the screw theory, with one fitting parameter, can capture experiments in many dilute and non-dilute alloys while the parameter-free edge theory agrees with experiments in non-dilute alloys having a sufficiently large misfit parameter. These results indicate a transition in single-phase alloy strengthening from traditional screw dominance to edge dominance with increasing misfit that is enabled in complex non-dilute alloys. These results point to the use of the edge theory to guide design of high-temperature alloys in the non-dilute range. Graphical abstract read less

Abstract: In this article, the compression simulation of AZ31 magnesium alloy is simulated by the molecular dynamics method. The effects of loading mode, temperature, and strain rate on the compression behavior are analyzed. The lattice distortion, mechanical behavior, structural evolution, and dislocation evolution in the compression process are deeply analyzed, and the results of different loading modes are obtained. The hexagonal close-packed (HCP) → face-centered cubic (FCC) phase transformation mechanism of AZ31 magnesium alloy during compression at temperature and strain rate, which is related to the mechanical behavior, has been studied completely. This article perfects the research on the compression behavior of magnesium alloys, excavates the application potential of magnesium alloys, and provides a new idea for improving the processing technology and developing high-performance magnesium alloys. read less

Abstract: The behavior of Mo in contact with molten Al was modelled by classical molecular dynamics (CMD) simulation of a pure Mo solid in contact with molten Al at 1200 K using the Materials Studio®. Results showed that no reaction or cross diffusion of atoms occurs at the Mo(s)–Al(l) interface, and that molten Al atoms exhibit an epitaxial alignment with the exposed solid Mo crystal morphology. Furthermore, the two phases {Mo(s) and Al(l)} are predicted to interact with weak van der Waals forces and give interfacial energy of about 203 mJ/m2. Surface energy measurements by the sessile drop experiment using the van Oss–Chaudhury–Good (VCG) theory established a Mo(s)–Al(l) interface energy equivalent to 54 mJ/m2, which supports the weak van der Waals interaction. The corrosion resistance of a high purity (99.97%) Mo block was then tested in a molten alloy of 5% Mg mixed in Al (Al-5 wt.%Mg) at 1123 K for 96 h, using the ALCAN’s standard “immersion” test, and the results are presented. No Mo was found to be dissolved in the molten Al-Mg alloy. However, a 20% mass loss in the Mo block was due to intergranular corrosion scissoring the Mo block in the ALCAN test, but not as a result of the reaction of pure Mo with the molten Al-Mg alloy. It was observed that the Al-Mg alloy did not stick to the Mo block. read less

Abstract: Сonstant search for new mechanisms of energy transport associated with a decrease in losses results in the active investigation of the possibilities of generating mobile excitations at frequencies outside the phonon spectrum. The effect of energy transfer at frequencies outside the band, possible in case if the excitation amplitude exceeds the critical value, was previously actively studied in reduced dimension systems. In this work, for the first time, the possibility and features of energy transfer at frequencies outside the phonon spectrum of a crystal are studied on the Pt3Al biatomic lattice, the corresponding excitations in which are created through the implementation of harmonic vibrations at frequencies lying in the phonon spectrum gap. It was found that as a result of the initial impact, the parameters of which lie in the band gap, at the first stage, discrete breathers are formed on aluminum atoms lying in the immediate vicinity of the impact zone. The threshold value of the energy required to excite breathers, determined by the amplitude and frequency of exposure, decreases as the frequency approaches the boundary of the slit in the spectrum. At the second stage, after the end of the harmonic action, a soliton-like wave is formed, moving along the crystal with an average speed of 4.5 km / s. The wave velocity is practically independent of the initial disturbance amplitude. The shape and amplitude of the moving wave are weakly dependent on the initial frequency. Significant differences in the profile and a tendency to bimodality were revealed only when exposed to frequencies close to the lower limit of the spectrum. The results obtained contribute to a fundamental understanding of the features of the propagation of elastic disturbances in nonlinear media in case of forbidden zones region impacts. read less

Abstract: We propose an interpolation scheme to describe pair correlations in crystals with many-body interactions that requires only information on relative displacements for the nearest neighbours and in the long range. Using crystalline Ni as a test case, the scheme is shown to deliver the functional form for the radial distribution function at least as well as molecular dynamics simulations. The results provide a fast route for verification of interatomic potentials and study of many-body interactions using a combination of x-ray scattering and x-ray absorption spectroscopy. read less

Abstract: In the study, the structural and some physical properties of pure calcium (Ca) during the heating process were investigated by classical molecular dynamic (MD) simulations method by using the embedded atom method (EAM) and tight-binding (TB) many body potentials. During this process, energy-, lattice-parameter and densitytemperature curves were used to see the changes in physical parameters depending on temperature. In addition, the evolution of the atomic structure of the system was investigated using different analysis methods such as the pair distribution function, the structure factor and the Honeycutt-Andersen (HA) method. The results obtained for both potentials were compared with appropriate experimental and other MD simulation results reported in the literature and discussed together. It has been observed that the EAM potential in a wide temperature range produces more successful results than the TB potential. HA results showed that especially 1541 and 1551 type quintet clusters and 1431 type quartet clusters play more effective roles in the melting process of the system. read less

Abstract: The tensile characteristics and deformation mechanisms of Cu-Ta metallic glasses with the insertion of Cu crystalline cores are investigated using molecular dynamics (MD) simulations. The effects of different Cu crystalline core diameters (D Cu ), experiment temperatures (T), and Cu crystalline core numbers (N) are studied. The results show that the plasticity of the Cu-Ta MGs is significantly improved by inserting Cu crystalline cores. The Shockley dislocations (<112>) make up the majority, and the FCC structures mainly transform into the HCP structures in the Cu crystalline cores. As increasing D Cu , the shear transformation zones (STZs) form more severely, the fraction of atoms with the high shear strain increases, and the tensile strength reduces. As increasing T, the STZs formation is fainter and most intense at 100 K, the fraction of atoms with the shear strain greater than 0.5 (f0.5) and the tensile strength reduce, while the fraction of atoms with the shear strain greater than 0.3 (f0.3) increases. As changing N, the STZs formations in the samples with the N = 2 and 8 are more pronounced, the f0.5 of the samples with the N = 1 and 8 are lower than those in the other cases, and the tensile strength reduces as the N increases. read less

Abstract: In this work, we have developed a temperature‐dependent higher‐order Cauchy–Born (THCB) rule for multiscale crystal defect dynamics (MCDD) of crystalline solids based on the harmonic approximation. As a template, we employed the THCB rule to develop an atomistic‐informed constitutive model for the body‐centered cubic (BCC) single crystal tantalum ( α ‐Ta). Considering the effect of strain gradients in different process zone elements, the corresponding higher order stress are used to model crystal plasticity of single crystal α ‐Ta. Different from face‐centered cubic crystals, BCC crystals are strongly influenced by temperature. It is shown in this article that the developed finite temperature atomistic‐informed crystal plasticity finite element method is able to capture the temperature‐dependent dislocation substructure and hence crystal plastic deformation. The main contributions and novelties of the present work are highlighted by following findings: (1) A THCB rule and an atomistic‐informed strain gradient theory have been developed, and the corresponding temperature‐related higher‐order stress and elastic tensor formulations are derived; (2) The finite temperature MCDD provides an atomistic‐informed crystal plasticity finite element method that can simulate anisotropic crystal plasticity in any orientation within stereographic triangle at micron scale and above; (3) The developed MCDD is able to capture the non‐Schmid effects of BCC single crystal tantalum ( α ‐Ta); (4) The developed MCDD is able to capture the size effect of single crystal plasticity; and (5) The finite temperature MCDD can simulate the temperature dependent dislocation substructure, and it captures cross‐slip in single crystal tantalum at low temperature (∼20 K) and captures dislocation cell structure at high temperature (∼500 K). read less

Abstract: The coalescence and melting process of different sizes and arrangements of Ag and Cu nanoparticles is studied through the molecular dynamics (MD) method. The results show that the twin boundary or stacking fault formation and atomic diffusion of the nanoparticles play an important role in the different stages of the heating process. At the beginning of the simulation, Cu and Ag nanoparticles will contact to each other in a very short time. As the temperature goes up, Cu and Ag nanoparticles may generate stacking fault or twin boundary to stabilize the interface structure. When the temperature reaches a critical value, the atoms gain a strong ability to diffuse and eventually melt into one liquid sphere. The coalescence point and melting temperature increase as cluster diameter increases. Moreover, the arrangement of Cu and Ag nanoparticles has a certain effect on the stability of the initial joint interface, which will affect subsequent coalescence and melting behavior. read less

Abstract: Propagation of mode I crack along bi-metal (001) interfaces of Fe/W, Fe/Ni, Fe/Co and Ti/Mg is simulated by molecular dynamics and discussed with the eigenvalue/vector of the atomic elastic stiffness, Bija=Δσia/Δεj, and surface energy. The crack does not propagate at the interface but in the adjacent phase of smaller surface energy, except in Fe/Ni. The 1st eigenvalue ηa(1), or the solution of BijaΔεj=ηaΔεi of each atom, clarifies the difference of ‘soft/hard’ of both phases at the onset of crack propagation. In the case of Fe/Ni, the ηa(1) of Ni atoms remarkably decreases in the Fe/Ni bi-metal structure, even though Ni has higher ηa(1) than Fe at no-load perfect lattices. Thus the rupture occurs in the Ni side even though the Ni has slightly higher (001) surface energy than Fe. Deformation modes at the crack propagation are also visualized by the eigenvector of ηa(1) < 0 unstable atoms. This article is part of the theme issue ‘Fracture dynamics of solid materials: from particles to the globe’. read less

Abstract: Ion thruster is a revolution technology with potential applications in space mission but the thruster’s operation lifetime is limited by the sputtering from thruster components. In this work, molecular dynamic simulations are performed to explore the dependence of deformation characteristics of an aluminum surface on incident angle and kinetic energy under low-energy xenon-ion impact. The fraction of non-12-coordinated atoms is used to quantitatively characterize the microstructural evolution and defect density levels. It is found that defect density level has a linear relation with incident energy, and there exists a critical incident angle around 20°, at which the aluminum surface has the maximum defect density level. In addition, a collision model is developed to theoretically reveal the physical mechanisms behind the dependence. Our findings may helpful in developing long endurance electric propulsion devices for practical applications. read less

Abstract: Fe, Mg, and O are among the most abundant elements in terrestrial planets. While the behavior of the Fe–O, Mg–O, and Fe–Mg binary systems under pressure have been investigated, there are still very few studies of the Fe–Mg–O ternary system at relevant Earth’s core and super-Earth’s mantle pressures. Here, we use the adaptive genetic algorithm (AGA) to study ternary Fe x Mg y O z phases in a wide range of stoichiometries at 200 GPa and 350 GPa. We discovered three dynamically stable phases with stoichiometries FeMg2O4, Fe2MgO4, and FeMg3O4 with lower enthalpy than any known combination of Fe–Mg–O high-pressure compounds at 350 GPa. With the discovery of these phases, we construct the Fe–Mg–O ternary convex hull. We further clarify the composition- and pressure-dependence of structural motifs with the analysis of the AGA-found stable and metastable structures. Analysis of binary and ternary stable phases suggest that O, Mg, or both could stabilize a BCC iron alloy at inner core pressures. read less

Abstract: We have studied the interfacial thermal conductance, G, of the flat Au(111)-water interface using non-equilibrium molecular dynamics simulations. We utilized two metal models, one based on the embedded atom method (EAM) and the other including metallic polarizability via a density readjusting EAM. These were combined with three popular water models, SPC/E, TIP4P, and TIP4P-FQ, to understand the role of polarizability in the thermal transport process. A thermal flux was introduced using velocity shearing and scaling reverse non-equilibrium molecular dynamics, and transport coefficients were measured by calculating the resulting thermal gradients and temperature differences at the interface. Our primary finding is that the computed interfacial thermal conductance between a bare metal interface and water increases when polarizability is taken into account in the metal model. Additional work to understand the origin of the conductance difference points to changes in the local ordering of the water molecules in the first two layers of water above the metal surface. Vibrational densities of states on both sides of the interface exhibit interesting frequency modulation close to the surface but no obvious differences due to metal polarizability. read less

Abstract: Coherent precipitation of ordered phases is responsible for providing exceptional high-temperature mechanical properties in a wide range of compositionally complex alloys. Ordered phases are also essential to enhance the magnetic or catalytic properties of alloyed nanoparticles. The present work aims to demonstrate the relevance of Bragg coherent diffraction imaging (BCDI) for studying bulk and thin-film samples or isolated nanoparticles containing coherent nanoprecipitates/ordered phases. The structures of crystals of a few tens of nanometres in size are modelled with realistic interatomic potentials and are relaxed after introduction of coherent ordered nanoprecipitates. Diffraction patterns from fundamental and superstructure reflections are calculated in the kinematic approximation and used as input to retrieve the strain fields using algorithmic inversion. First, the case of single nanoprecipitates is tackled and it is shown that the strain field distribution from the ordered phase is retrieved very accurately. Then, the influence of the order parameter S on the strain field retrieved from the superstructure reflections is investigated. A very accurate strain distribution can be retrieved for partially ordered phases with large and inhomogeneous strains. Subsequently, the relevance of BCDI is evaluated for the study of systems containing many precipitates, and it is demonstrated that the technique is relevant for such systems. Finally, the experimental feasibility of using BCDI to image ordered phases is discussed in the light of the new possibilities offered by fourth-generation synchrotron sources. read less

Abstract: While combining two metals in the same nanoparticle can lead to remarkable novel applications, the resulting structure in terms of crystallinity and shape remains difficult to predict. It is thus essential to provide a detailed atomistic picture of the underlying growth processes. In the present work we address the case of core-shell Fe-Au and Fe-Ag nanoparticles. Interface properties between Fe and the noble metals Au and Ag, computed using DFT, were used to parameterize Fe-Au and Fe-Ag pairwise interactions in combination with available many-body potentials for the pure elements. The growth of Au or Ag shells on nanometric Fe cores with prescribed shapes was then modelled by means of Monte Carlo simulations. The shape of the obtained Fe-Au nanoparticles is found to strongly evolve with the amount of metal deposited on the Fe core, a transition from the polyhedral Wulff shape of bare iron to a cubic shape taking place as the amount of deposited gold exceeds two monolayers. In striking contrast, the growth of silver proceeds in a much more anisotropic, Janus-like way and with a lesser dependence on the iron core shape. In both cases, the predicted morphologies are found to be in good agreement with experimental observations in which the nanoparticles are grown by physical deposition methods. Understanding the origin of these differences, which can be traced back to subtle variations in the electronic structure of the Au/Fe and Ag/Fe interfaces, should further contribute to the better design of core-shell bimetallic nanoparticles. read less

Abstract: Combining first-principles accuracy and empirical-potential efficiency for the description of the potential energy surface (PES) is the philosopher's stone for unraveling the nature of matter via atomistic simulation. This has been particularly challenging for multi-component alloy systems due to the complex and non-linear nature of the associated PES. In this work, we develop an accurate PES model for the Al-Cu-Mg system by employing Deep Potential (DP), a neural network based representation of the PES, and DP Generator (DP-GEN), a concurrent-learning scheme that generates a compact set of ab initio data for training. The resulting DP model gives predictions consistent with first-principles calculations for various binary and ternary systems on their fundamental energetic and mechanical properties, including formation energy, equilibrium volume, equation of state, interstitial energy, vacancy and surface formation energy, as well as elastic moduli. Extensive benchmark shows that the DP model is ready and will be useful for atomistic modeling of the Al-Cu-Mg system within the full range of concentration. read less

Abstract: Recently, machine learning methods have become easy-to-use tools for constructing high-dimensional interatomic potentials with ab initio accuracy. Although machine-learned interatomic potentials are generally orders of magnitude faster than first-principles calculations, they remain much slower than classical force fields, at the price of using more complex structural descriptors. To bridge this efficiency gap, we propose an embedded atom neural network approach with simple piecewise switching function-based descriptors, resulting in a favorable linear scaling with the number of neighbor atoms. Numerical examples validate that this piecewise machine-learning model can be over an order of magnitude faster than various popular machine-learned potentials with comparable accuracy for both metallic and covalent materials, approaching the speed of the fastest embedded atom method (i.e. several μs per atom per CPU core). The extreme efficiency of this approach promises its potential in first-principles atomistic simulations of very large systems and/or in a long timescale. read less

Abstract: Selective laser melting (SLM) provides flexibility to manufacture components with complex structures. However, the unexpected crystallization and the dynamical microstructural evolution at the atomic scale still remains unknown during the SLM process. Here, the crystallization and formation mechanism of an equimolar FeCrNi medium-entropy alloy (MEA) with excellent mechanical properties prepared by SLM is studied via molecular dynamics (MD) simulations. The surface morphology, atomic microstructural evolution, and Cr elemental segregation are investigated during the crystallization process. The crystallization and microstructural characteristics are clearly observed. A large number of stacking faults take place at the boundary of the melting pool, but some stacking faults extend to the center region due to the thermal gradient effect. A segregation phenomenon of a nanoscale Cr-rich cluster occurs in the FeCrNi MEA to reveal the dynamic precipitation mechanism during the SLM process. Furthermore, higher energy density and lower scanning speed would promote the segregation and even form a mesh connected structure. The current result provides an insight into the crystallization and formation mechanism of microstructures to develop advanced alloys with high strength and toughness via the reasonable choice of SLM parameters. read less

Abstract: Over the last few decades, computational tools have been instrumental in understanding the behavior of materials at the nano-meter length scale. Until recently, these tools have been dominated by two levels of theory: quantum mechanics (QM) based methods and semi-empirical/classical methods. The former are time-intensive but accurate and versatile, while the latter methods are fast but are significantly limited in veracity, versatility, and transferability. Recently, machine learning (ML) methods have shown the potential to bridge the gap between these two chasms due to their (i) low cost, (ii) accuracy, (iii) transferability, and (iv) ability to be iteratively improved. In this work, we further extend the scope of ML for atomistic simulations by capturing the temperature dependence of the mechanical and structural properties of bulk platinum through molecular dynamics simulations. We compare our results directly with experiments, showcasing that ML methods can be used to accurately capture large-scale materials phenomena that are out of reach of QM calculations. We also compare our predictions with those of a reliable embedded atom method potential. We conclude this work by discussing how ML methods can be used to push the boundaries of nano-scale materials research by bridging the gap between QM and experimental methods. read less

Abstract: Multiscale simulation study results of multilayer structures consisting of graphene sheets with embedded Pt nanoparticles is reported. Density functional theory is used to understand the energetics of Pt–graphene interfaces and provide reference data for the parameterization of a Pt–graphene interaction potential. Molecular dynamics simulations then provide the conformation and energetics of graphene sheets with embedded Pt nanoparticles of varying density, form, and size. These results are interpreted using a continuum mechanical model of sheet deformation, and serve to parameterize a meso‐scale Monte Carlo model to investigate the question under which conditions the free volume around the Pt nanoparticles forms a percolating cluster, such that the structures can be used in catalytic applications. This article is concluded with a discussion of potential applications of such multilayer structures. read less

Abstract: Building on our previously introduced multicell Monte Carlo (MC)^{2} method for modeling phase coexistence, this paper provides important improvements for efficient determination of phase equilibria in solids. The (MC)^{2} method uses multiple cells, representing possible phases. Mass transfer between cells is modeled virtually by solving the mass balance equation after the composition of each cell is changed arbitrarily. However, searching for the minimum free energy during this process poses a practical problem. The solution to the mass balance equation is not unique away from equilibrium, and consequently the algorithm is in risk of getting trapped in nonequilibrium solutions. Therefore, a proper stopping condition for (MC)^{2} is currently lacking. In this work, we introduce a consistency check via a predictor-corrector algorithm to penalize solutions that do not satisfy a necessary condition for equivalence of chemical potentials and steer the system toward finding equilibrium. The most general acceptance criteria for (MC)^{2} is derived starting from the isothermal-isobaric Gibbs ensemble for mixtures. Using this ensemble, translational MC moves are added to include vibrational excitations as well as volume MC moves to ensure the condition of constant pressure and temperature entirely with a MC approach, without relying on any other method for relaxation of these degrees of freedom. As a proof of concept the method is applied to two binary alloys with miscibility gaps and a model quaternary alloy, using classical interatomic potentials. read less

Abstract: Using large-scale molecular dynamics (MD) simulations, the effects of interface and layer number in the nanoindentation response of experimentally observed nanotwinned Cu/high entropy alloy (HEA) FeCoCrNi nanolaminate are studied. The dislocations are nucleated and emitted, which are more limited to the first twinning layer > second twinning layer > HEA layer. The stacking fault strengthening is pronounced due to the obvious difference of stacking fault energy between Cu and HEA, which can be rarely observed from the previous work in traditional alloys and metals. After the indentation induced deformation, the nanotwinned Cu/HEA FeCoCrNi nanolaminates for different layer numbers generate a mass of Shockley partial dislocations to cause the good plasticity, attributed to the strong strain gradient effect. The strong layer number and interface structure effects found here can provide insight for the design of advanced nanolaminate with high strength and good plasticity. read less

Abstract: The structural diversity of rare-earth and transition metal borides indicates that alkali-transition metal borides (A-T-B) show tremendous promise in exhibiting a variety of crystal structures with different dimensionalities of T-B frameworks. On the other hand, the A-T-B ternary systems are severely underexplored because of the synthetic challenges associated with their preparation. Accurate and efficient computational predictions of low-energy stable and metastable phases can identify the optimal compositions of the hypothetical compounds in the A-T-B systems to guide the synthesis. In this work, we have computationally discovered several new phases in the Li–Ni–B ternary system. The newly discovered LiNiB, Li2Ni3B, and Li2NiB phases expand the existing theoretical database, and the convex-hull surface of Li–Ni–B has been re-constructed. The lowest energy structure of the LiNiB compound has been found by an adaptive genetic algorithm with layered motif, which matches with the experimentally determined structure. According to our electrochemical calculations, LiNiB and another predicted layered Li2NiB compounds have great potential as anode materials for lithium batteries. The Li2Ni3B compound with the space group P4332 was predicted to crystallize in a cubic structure composed of distorted octahedral units of BNi6, which is isostructural to two noncentrosymmetric superconductors Li2Pd3B and Li2Pt3B. While we were unable to experimentally confirm the Li2Ni3B compound utilizing the hydride synthetic route, attempts to synthesize this compound by alternate methods remain highly desirable, considering its potential superconducting properties. read less

Abstract: By means of the embedded-atom method, a Fe–Cu potential has been constructed through a newly mathematic form of cross potential. The newly constructed Fe–Cu potential has demonstrated to be more reliable than the five reported Fe–Cu potentials. Based on the Fe–Cu potential, the mechanical and thermodynamic properties and the structural stability of Fe–Cu solid solutions in the whole composition range are derived by molecular dynamics simulation. It is found that the heat of formation curves of the FexCu100 − x solid solutions with body-centered-cubic (BCC) and face-centered-cubic (FCC) structures intersect at the point of x = 65, implying that FexCu100 − x solid solutions with FCC and BCC structures are thermodynamically stable when 0 ≤ x ≤ 65 and 65 < x ≤ 100, respectively. In addition, the derived lattice constants, structural stability, elastic constants, elastic moduli, heat capacity, and coefficients of thermal expansion of Fe–Cu solid solutions from the new Fe–Cu potential agree well with the data of the experiments, first-principles calculation, and the Miedema model.By means of the embedded-atom method, a Fe–Cu potential has been constructed through a newly mathematic form of cross potential. The newly constructed Fe–Cu potential has demonstrated to be more reliable than the five reported Fe–Cu potentials. Based on the Fe–Cu potential, the mechanical and thermodynamic properties and the structural stability of Fe–Cu solid solutions in the whole composition range are derived by molecular dynamics simulation. It is found that the heat of formation curves of the FexCu100 − x solid solutions with body-centered-cubic (BCC) and face-centered-cubic (FCC) structures intersect at the point of x = 65, implying that FexCu100 − x solid solutions with FCC and BCC structures are thermodynamically stable when 0 ≤ x ≤ 65 and 65 < x ≤ 100, respectively. In addition, the derived lattice constants, structural stability, elastic constants, elastic moduli, heat capacity, and coefficients of thermal expansion of Fe–Cu solid solutions from the new Fe–Cu potential agree well with the data o... read less

Abstract: Molecular dynamics simulation of the solidification behavior of AlCoCuFeNi nanowire was carried out basing on the embedded atom potential with different cooling rates (1∙10, 1∙10, and 1∙10 K/s). To simulate an infinite nanowire, a periodical boundary condition along the nanowire axis direction was applied. The crystallization of the nanowire was characterized by studying the temperature dependence of the potential energy. The adaptive common neighbor analysis (CNA) was performed and the radial distribution function (RDF) was calculated to determine the structure and lattice parameters of phases of the AlCoCuFeNi nanowire. It has been shown that the final structure of investigated nanoparticle changes from amorphous to crystalline with decreasing of the rate of cooling. read less

Abstract: Calismamizda Zr 48 Cu 36 Al 8 Ag 8 dortlu alasiminin atomik yapisi ve cam olusturma sureci molekuler dinamik simulasyon ile gomulu atom metodu kullanilarak arastirildi. Cam gecis surecini ve sicakliga bagli atomik yapi gelisimini arastirmak icin ortalama hacim-sicaklik egrisi, ciftler dagilim fonksiyonu (PDF) ve cift analiz metodu kullanildi. 300 K’de hesaplanan toplam PDF, g(r ), daha once rapor edilen deneysel g(r) ile iyi bir uyum saglamaktadir. Ote yandan ortalama hacim-sicaklik grafiginden yararlanilarak belirlenen cam gecis sicakligi da deneysel degerlerle birbirlerine yakindir. Zr-Zr ve Cu-Cu ciftlerinin kismi PDF'lerin pikleri sicaklik dususune bagli olarak normal bir artis egilimi gosterirken, Al-Al ve Ag-Ag ciftleri ise cok yuksek pikler ureterek anormal bir davranis sergilemektedir. Bu davranisin nedeninin simulasyon hucresindeki Al ve Ag atomlarinin topaklanmasi oldugu gorulmustur. Sistemin mikro yapisi incelendiginde ise kisa menzil duzenin gostergesi olan 1431, 1541 ve 1551 bagli ciftlerinin butun sicakliklarda baskin oldugu gozlenmistir. Azalan sicaklikla birlikte ozellikle ideal ikosahedral duzeni temsil eden 1551 bagli ciftlerinin oraninin artmasi sistemin kisa menzil duzeninin geliserek artmaya devam ettigini gostermektedir. read less

Abstract: Comprehensive molecular dynamics simulations are conducted to unravel the mechanics and mechanisms associated with the strength and fracture behavior of a highly ordered gold nanowire (Au-NW) array of a pair of nanofasteners (nanoconnectors) under externally applied shear strain. Large-scale atomic/molecular massively parallel simulator (LAMMPS and embedded atom method were adopted to model the atomic interactions of a number of neighboring nanofasteners. This was affected via the use of a periodic simulation box around a pair of highly ordered nanotube arrays to minimize the cost of the computations. Energy minimization using a conjugate gradient algorithm was first performed and followed by atomic relaxation to achieve an equilibrated configuration under the canonical ensemble of constant temperature and volume. The relaxed equilibrated configuration of the nanofastener was then subjected to an externally applied shear strain γ x y at a rate of γ ̇ x y = 0.1 per nanosecond under the canonical ensemble. Our results reveal the importance of the morphology and the overlap depth of the mating nanowire arrays upon the mechanical and fracture behavior of the nanofastener under shear loading. Our work also disclosed the phenomenon of multiple contacts of some displaced nanowires with their neighbors even after their fracture leading to multiple cold-welds with added redundancy to the nanofastener. Finally, in this research, we identified the locations of dislocation emissions and the resulting fracture processes that govern the mechanical integrity and ultimately the functionality of the Au-NW connector. The proposed highly ordered alignment, as conceived numerically herein, can yield a peak stress two to three times higher than that corresponding to a random alignment reported in a previous study. The Au-NW connector also exhibited resistance to fracture, even in cases where small overlap depth is considered in joint bonding. The nanoconnector was also tested at high temperatures (up to 450 K). Our results show that the rising temperature only leads to a minor reduction in the load transmitted by the nanoconnector. read less

Abstract: The formation of the ternary Pt3NiAu and Pt3CuAu nanostructures is examined using molecular dynamics simulations in the context of free (i.e., unsupported) cluster growth in an unreactive atmospher... read less

Abstract: We have investigated using first principles the occurrence and tunability of the spin Seebeck effect in Pt-based bimetallic wires of noble metals (viz. XPt, X = Cu, Ag, and Au) modelled in linear, ladder, and double zigzag (DZZ) topologies. The spin figure of merit ZsT and its charge counterpart ZcT are calculated by considering both electronic and phononic contributions to the thermal conductance. For this endeavour, we have employed the Landauer-Büttiker approach, with the requisite electron τel(E) and phonon τph(E) transmission functions obtained using the non-equilibrium Green's function approach, based on density functional theory and the general utility lattice program, respectively. We find that alloying and/or topological tailoring bring in quantitative as well as qualitative changes in the transport properties. Unlike the pristine wires, τel(E) now depends (except for the CuPt wire in the ladder topology) markedly on spin, thus resulting in an unequal current in the two spin channels and hence a non-zero spin Seebeck coefficient Ss. Remarkably, the AgPt wire in the linear topology and all wires in the DZZ1 and DZZ3 configurations of the DZZ topology exhibit half-metallic conduction, with a sizable gap in the density of ↑-spin states at the Fermi level. Alloying also introduces energy gap(s) in the phonon density of states. Consequently, we find a significant reduction in the electronic and phononic thermal conductance. Interestingly, Ss is of the same order as its charge counterpart Sc, and both can be tuned via the topology and chemical potential μ of the electrodes. This together with the reduced thermal conductance results in a significantly high room-temperature ZcT (∼33) and ZsT (∼25) at μ ∼ 0.24 eV in the AgPt wire in the DZZ3 topology, with about a ten-fold increase in ZcT as compared to the pristine wires. Furthermore, there exists a characteristic μ in some bimetallic wires for which Sc approaches zero, but Ss remains quite appreciable. This result arises due to the emergence of bipolar thermal conduction, which under criticality makes S↑ + S↓ = 0. Importantly, such a situation may be exploited to generate a pure thermal spin voltage. Our results can be explicated on the basis of changes in the electronic band structure and phononic spectra due to alloying and topological effects. read less

Abstract: An Fe-W potential has been constructed by means of the embedded-atom method and has proven to be more realistic than the three published Fe-W potentials in the literature. Based on the constructed Fe-W potential, molecular dynamic simulation has been used to reveal structural stability, thermodynamic properties, and mechanical properties of BCC Fe-W solid solutions within the entire composition range. It is found that the Fe-W interaction in BCC Fe-W solid solutions should be weak and attractive with small and negative heats of formation, which agree well with those from the thermodynamic Miedema model and could clarify the controversy regarding heats of formation of Fe-W solid solutions in the literature. In addition, the derived coefficient of thermal expansion, heat capacity, elastic constants, and elastic moduli of Fe-W solid solutions from the present Fe-W potential are in good agreement with the corresponding data from ab initio calculation or experiments in the literature.An Fe-W potential has been constructed by means of the embedded-atom method and has proven to be more realistic than the three published Fe-W potentials in the literature. Based on the constructed Fe-W potential, molecular dynamic simulation has been used to reveal structural stability, thermodynamic properties, and mechanical properties of BCC Fe-W solid solutions within the entire composition range. It is found that the Fe-W interaction in BCC Fe-W solid solutions should be weak and attractive with small and negative heats of formation, which agree well with those from the thermodynamic Miedema model and could clarify the controversy regarding heats of formation of Fe-W solid solutions in the literature. In addition, the derived coefficient of thermal expansion, heat capacity, elastic constants, and elastic moduli of Fe-W solid solutions from the present Fe-W potential are in good agreement with the corresponding data from ab initio calculation or experiments in the literature. read less

Abstract: In this work, we use density functional theory to calculate the entire generalized stacking fault energy (GSFE) surface for eight transition metals with a face-centered cubic structure: Ag, Au, Cu, Ir, Ni, Pd, Pt, and Rh. Analysis of the ⟨ 112 ⟩ GSFE curves finds that the displacements corresponding to the unstable stacking fault energy are larger than the ideal value for all eight metals except Ag and Cu. Over the entire surface, Pt is found to not possess well-defined local maxima or minima, suggesting spreading in favor of dissociation of the dislocation core, unlike the other seven metals. Our calculations also reveal that at a large ⟨ 112 ⟩ displacement, where atoms on two {111} adjacent planes are aligned, an anomalous local minimum occurs for Ir and Rh. The oddity is explained by relatively large, localized atomic displacements that take place in the two metals to accommodate the alignment that do not occur in the other six metals. In addition to the fully calculated surfaces, we characterize a continuous 11-term Fourier-series function, which provides a particularly excellent representation of the GSFE surfaces for Ag, Au, Cu, Ni, and Pd. read less

Abstract: Обзор посвящен исследованию дискретных бризеров в кристаллах стехиометрии A3B, AB и в моноатомных кристаллах. Дана краткая история развития концепции дискретных бризеров в кристаллах, рассмотрены возможные пути дальнейших исследований. Приведены данные, свидетельствующие о том, что для существования дискретных бризеров необходимо наличие свойств дискретности и нелинейности исследуемой системы. В рассматриваемых работах исследование дискретных бризеров осуществлялось методом молекулярной динамики с использованием как парных, так и многочастичных потенциалов. На примере кристаллов Pt3Al и CuAu показано, что дискретные бризеры с жестким типом нелинейности способны сосредоточивать энергию порядка нескольких электронвольт, при этом они являются мобильными и могут перемещаться по кристаллу вдоль плотноупакованных направлений. Выявлены два механизма возбуждения дискретных бризеров с мягким типом нелинейности в кристалле стехиометрии А3В потоком частиц. Продемонстрировано, что внешние поля, осциллирующие с частотой вне фононного спектра кристалла А3В, могут являться причиной возбуждения бризеров с мягким типом нелинейности вблизи его поверхности. Для металлов Pt, Au, Ni, Pd, Cu показана зависимость продолжительности жизни ДБ от их коэффициента Пуассона. Приведены амплитудно-частотные характеристики и зависимости времени жизни дискретных бризеров от начальных параметров возбуждения. read less

Abstract: Three-dimensional integration technology using TSV interconnections has emerged as a promising solution to improve the performance of microelectronic devices. It is necessary to study the effect of strain rate and work temperature on the interfacial strength of Cu/Ta interface for microelectronic devices reliability. In this work, to investigate the effect of the temperature and strain rate on the Cu/Ta interfacial properties, a series of large-scale molecular dynamic (MD) simulations were performed. The stress-strain curves and the deformation processes of Cu/Ta interface were obtained. The results showed that the interfacial tensile strength and interfacial shear strength of Cu(010)/Ta(010) interface are 5.56 GPa and 0.465 GPa, respectively, when the strain rate is 109/s and temperature is 300 K. And the location of failure for Cu/Ta interface is closer to the monocrystalline Cu parts. Then, the effects of strain rate and temperature on the interfacial tensile and shear strength were discussed, and the results indicated that the interfacial strength decrease with the decrease of strain rate from 109/s to 107/s. Similarly, the strongest correlation was seen to be between the interfacial strength and temperature, it can be seen that the interfacial tensile and shear strength decrease with increasing of the temperature. read less

Abstract: Bu calismada, farkli atomlar-arasi potansiyeller ve klasik molekuler dinamik (MD) benzetimleri kullanilarak, yuzey merkezli kubik (fcc) yapiya sahip platin (Pt) elementinin taban durumdan baslayip erime noktasinin hemen uzerindeki bir sicaklik araligindaki fiziksel ozellikleri ve atomik yapisinin devinimi detayli bir sekilde incelenmistir. Artan sicakliga bagli olarak kati sistemin yapisal gelisimini analiz etmek ve erime noktasini belirlemek icin ciftler dagilim fonksiyonu (PDF), enerji-sicaklik (E-T), orgu parametresi-sicaklik (a-T), dogrusal termal genlesme katsayi-sicaklik (CLTE-T) egrileri ve cift analiz yontemi gibi analiz yontemleri kullanildi. Tum potansiyeller icin MD benzetiminin sonuclarinin analizinden elde edilen veriler daha once rapor edilen deneysel veya teorik verilerle karsilastirilmis ve tartisilmistir. Farkli atomlar-arasi potansiyellerle elde edilen sonuclar cogunlukla birbirleri ile tutarli olmasina ragmen, farklilik gosterdikleri noktalar da bulunmaktadir. Ozellikle, sistemin erime noktasinin belirlenmesi konusunda, her bir potansiyelin farkli erime sicakliklari urettigi gozlenmistir. Tum potansiyel enerji fonksiyonlarinda ortak olarak, artan sicaklikla birlikte fcc yapiyi temsil eden 1421 bagli ciftlerinin sayisi azalmis ve bu ciftlerin buyuk bir bolumunun ozellikle kusurlu icosahedra ( deficos ) ve kusurlu fcc yapiyi temsil eden 1541 ve 1431 bagli ciftlerine donustugu gorulmustur. Pt elementi icin burada ele alinan potansiyellerin bazilari dusuk bazilari ise yuksek sicaklik araligindaki fiziksel ozellikleri aciklamada basarili olurken, Sheng ve arkadaslari tarafindan one surulen gomulu atom metot potansiyeli (EAM1) ve siki-bagli (TB) potansiyelinin saf Pt elementinin genis sicaklik olceginde ele alinan tum ozelliklerini aciklamada digerlerine gore daha basarili oldugu gorulmustur. read less

Abstract: Basic characteristics of 6 × 6 matrix of atomic elastic stiffness (AES), Bija=Δσia/Δej, or the deformation resistance at each atom point, are discussed first in static analyses of generalized stacking fault (GSF) energy surface for 8 fcc, 4 bcc and 4 hcp metals with Zhou’s EAM potential. For hcp metals, the stress–strain peak along the GSF path exactly coincides with the point where the AES loses the resistance showing negative 1st eigenvalue ηa(1), or the solution of BijaΔej=ηaΔei=Δσia; however, all fcc and 2 bcc (Mo and W) never have negative ηa(1) along the GSF path. Fe and Ta transiently show ηa(1) < 0 while they also have positive ηa(1) at the GSF energy peak. Then we performed MD simulations of edge and screw dislocation dipoles in a periodic slab cell of typical elements of fcc, bcc and hcp; and discussed the eigenvalue and the corresponding eigenvector {Δexx, Δeyy, Δezz, Δγyz, Δγzx, Δγxy} of the dislocation cores. As expected from the results of the GSF analyses, dislocation cores in fcc Ni have no ηa(1) < 0 atoms, even in their glide process under external shear loading. Bcc Fe and hcp Co definitely have ηa(1) < 0 atoms in the dislocation cores and their migration direction can be visualized by the maximum shear direction of the strain tensor of the corresponding eigenvector.Basic characteristics of 6 × 6 matrix of atomic elastic stiffness (AES), Bija=Δσia/Δej, or the deformation resistance at each atom point, are discussed first in static analyses of generalized stacking fault (GSF) energy surface for 8 fcc, 4 bcc and 4 hcp metals with Zhou’s EAM potential. For hcp metals, the stress–strain peak along the GSF path exactly coincides with the point where the AES loses the resistance showing negative 1st eigenvalue ηa(1), or the solution of BijaΔej=ηaΔei=Δσia; however, all fcc and 2 bcc (Mo and W) never have negative ηa(1) along the GSF path. Fe and Ta transiently show ηa(1) < 0 while they also have positive ηa(1) at the GSF energy peak. Then we performed MD simulations of edge and screw dislocation dipoles in a periodic slab cell of typical elements of fcc, bcc and hcp; and discussed the eigenvalue and the corresponding eigenvector {Δexx, Δeyy, Δezz, Δγyz, Δγzx, Δγxy} of the dislocation cores. As expected from the results of the GSF analyses, dislocation cores in fcc Ni have n... read less

Abstract: The molecular dynamics method is used to consider the effect of energy transport in an A3B stoichiometry crystal, using Pt3Al as an example, which consists in the transfer of energy at frequencies outside the phonon spectrum of the crystal. This effect is called the effect of nonlinear supratransmission. The model was a bulk face-centered cubic crystal, the atoms of which interacted through a multi-particle potential, obtained by the immersed atom method. Different forms of oscillation of the region of external influence are considered. The possibility of transporting energy from the crystal surface into the depths by means of excitation of quasi-breathers near the impact area and their subsequent destruction in a crystal and dissipation of energy, stored on them, is shown. The quasi-breathers most intensely occurred near the impact region with a sinusoidal waveform. The results obtained indicate that the contribution of quasi-breathers to the energy transfer through the crystal, increases with increasing exposure amplitude. The minimum amplitude of the external influence, at which this effect was observed, is established. The results of the study can be useful in creating materials with predetermined properties, through various intensive external influences. read less

Abstract: An n-body W–Cu potential is constructed under the framework of the embedded-atom method by means of a proposed function of the cross potential. This W–Cu potential is realistic to reproduce mechanical property and structural stability of WCu solid solutions within the entire composition range, and has better performances than the three W–Cu potentials already published in the literature. Based on this W–Cu potential, molecular dynamics simulation is conducted to reveal the mechanical property and dislocation evolution of the bilayer structure between pure W and W0.7Cu0.3 solid solution. It is found that the formation of the interface improves the strength of the W0.7Cu0.3 solid solutions along tensile loading perpendicular to the interface, as the interface impedes the evolution of the dislocation lines from the W0.7Cu0.3 solid solutions to the W part. Simulation also reveals that the interface has an important effect to significantly reduce the tensile strength and critical strain of W along the tensile loading parallel to the interface, which is intrinsically due to the slip of the edge or screw dislocations at low strains as a result of the lattice mismatch. read less

Abstract: Small concentrations of alloying elements can modify the α-γ phase transition temperature Tc of Fe. We study this effect using an atomistic model based on a set of many-body interaction potentials for iron and several alloying elements. Free-energy calculations based on perturbation theory allow us to determine the change in Tc introduced by the alloying element. The resulting changes are in semi-quantitative agreement with experiment. The effect is traced back to the shape of the pair potential describing the interaction between the Fe and the alloying atom. read less

Abstract: The atomic disorder and the vibrational properties of Pd nanocubes have been studied through a combined use of X-ray diffraction and molecular dynamics simulations. The latter show that the trend of the mean square relative displacement as a function of the radius of the coordination shells is characteristic of the nanoparticle shape and can be described by a combined model: A correlated Debye model for the thermal displacement and a parametric expression for the static disorder. This combined model, supplemented by results of line profile analysis of the diffraction patterns collected at different temperatures (100, 200, and 300 K) can explain the observed increase in the Debye–Waller coefficient, and shed light on the effect of the finite domain size and of the atomic disorder on the vibrational properties of metal nanocrystals. read less

Abstract: Production rates in long-term predictive radiation damage accumulation models are generally considered independent of the material's microstructure for reactor components. In this study, the effect of pre-existing microstructural elements on primary damage production in alpha-Zr -- and vice-versa -- is assessed by molecular dynamics (MD) simulations. a-type dislocation loops, c-component dislocation loops and a tilt grain boundary (GB) were considered. Primary damage production is reduced in the presence of all these microstructural elements, and clustering behavior is dependent on the microstructure. Collision cascades do not cause a-type loop growth or shrinkage, but they cause c-component loop shrinkage. Cascades in the presence of the GBs produce more vacancies than interstitials. This result, as well as other theoretical, MD and experimental evidence, confirm that vacancy loops will grow in the vacancy supersaturated environment near GBs. Distinct temperature-dependent growth regimes are identified. Also, MD reveals cascade-induced events where a-type vacancy loops are absorbed by GBs. Fe segregation at the loops inhibits this cascade-induced absorption. read less

Abstract: Lattice structures, defect structures, and deformation mechanisms of high-entropy alloys (HEAs) have been studied using atomistic simulations to explain their remarkable mechanical properties. These atomistic simulation techniques, such as first-principles calculations and molecular dynamics allow atomistic-level resolution of structure, defect configuration, and energetics. Following the structure–property paradigm, such understandings can be useful for guiding the design of high-performance HEAs. Although there have been a number of atomistic studies on HEAs, there is no comprehensive review on the state-of-the-art techniques and results of atomistic simulations of HEAs. This article is intended to fill the gap, providing an overview of the state-of-the-art atomistic simulations on HEAs. In particular, we discuss how atomistic simulations can elucidate the nanoscale mechanisms of plasticity underlying the outstanding properties of HEAs, and further present a list of interesting problems for forthcoming atomistic simulations of HEAs. read less

Abstract: The mechanical properties of core–shell bimetallic composite nanowires, forming the bases of nanoporous metallic foams, have been investigated and compared with pure metal nanowires using molecular dynamics simulations. In the current study, pure copper and gold nanowires under uniaxial loading were tested at room temperature and compared to composite nanowires of the same materials (core) with a nickel coating (shell). The core radius ranged from 1 to 15 nm, and the shell thickness ranged from 0.1 to 5 nm. The tension strain was performed along the [001] direction under room temperature. Both coherent and semi-coherent composite nanowires were studied, and the effect of coating layer thickness was investigated. The strengthening mechanisms of the core–shell structures due to the presence of the two different types of interfaces were investigated for various nickel thicknesses. The atomistic simulation results revealed that the addition of the nickel shell strengthens the structure when the layer thickness exceeds a critical value. read less

Abstract: Interatomic potentials are widely used in computational materials science, in particular for simulations that are too computationally expensive for density functional theory (DFT). Most interatomic potentials have a limited application range and often there is very limited information available regarding their performance for specific simulations. We carried out high-throughput calculations for molybdenum and silicon with DFT and a number of interatomic potentials. We compare the DFT reference calculations and experimental data to the predictions of the interatomic potentials. We focus on a large number of basic materials properties, including the cohesive energy, atomic volume, elastic coefficients, vibrational properties, thermodynamic properties, surface energies and vacancy formation energies, which enables a detailed discussion of the performance of the different potentials. We further analyze correlations between properties as obtained from DFT calculations and how interatomic potentials reproduce these correlations, and suggest a general measure for quantifying the accuracy and transferability of an interatomic potential. From our analysis we do not establish a clearcut ranking of the potentials as each potential has its strengths and weaknesses. It is therefore essential to assess the properties of a potential carefully before application of the potential in a specific simulation. The data presented here will be useful for selecting a potential for simulations of Mo or Si. read less

Abstract: ABSTRACT In this present study, deformation behaviour of Mo20W20Co20Ta20Zr20 high entropy alloy (HEA) coated single crystal (SC) nickel (Ni) subjected to nanoindentation test have been investigated to study the mechanical properties and underlying mechanism during nanoindentation test using molecular dynamics (MD) simulation with embedded atom method (EAM) potential. Centro-Symmetry Parameter (CSP) Analysis and Radial Distribution Function (RDF) plots are obtained to get insight of structural evolution during nanoindentation and thereby determine the underlying physics of deformation. During nanoindention test Stacking faults (SFs) formation, dislocation generation, dislocation loops, Lomer–Cottrell (LC) lock and Hirth lock formation due to dislocation-dislocation interaction are observed. At higher indentation depth, formation of dislocation loops is augmented, which indicates nanoindentation deformation is found to be Stacking Fault dominated deformation. The accumulation and relaxation of shear stress near indenter tip at the time of deformation process under nanoindentation test causes the variation of dislocation density, strain hardening, and plastic deformation, which is influenced by the formation of dislocation barriers (LC and Hirth locks) and dislocation loops (shear and prismatic loops). read less

Abstract: The molecular dynamics method is used to calculate and analyze the statistical characteristics of a quasi-breather with a hard type of nonlinearity in monoatomic FCC metals, for example, Cu, Au, Pt, Ni and Pd. Within the framework of this model, the following statistical characteristics and dependencies were calculated for quasi-breathers: a grouped statistical series of absolute and relative frequencies, a polygon of absolute and relative frequencies, a histogram of relative frequencies, an empirical distribution function, an estimate of the mathematical expectation and variance of the original sample. The densities of phonon states are calculated for all crystals. Statistics allow you to understand the causes of the destruction of breathers and more fully describe the process of their dissipation of energy.     ACKNOWLEDGMENTS For AME, PVZ, the study was carried out with the financial support of the Russian Federal Property Fund and the Altai Territory within the framework of the scientific project No. 18-42-220002; The MDS is grateful to the Ministry of Education and Science of the basic part of the state task, project No. 3.4820.2017/BC.   read less

Abstract: Local stress relaxation mechanisms of crystals are a long-standing interest in the field of materials physics. Constantly encountered inelastic deformation mechanisms in metals under dynamic loadings, such as dislocation, deformation twinning and phase transition, have been extensively discussed separately or as some of their combinations. Recently, virtual melting is found to be a dominant local stress relaxation mechanism under extreme strain rates. However, these deformation mechanisms have never been investigated in the same metal at an atomic level due to the lack of high pressure interatomic potentials. In this work, an embedded-atom model potential of Pb is developed and tested for high pressure applications. The developed potential of Pb could not only reproduce many energetic, elastic and defective properties at ambient conditions well, but also correctly describe face-centered cubic (fcc)-hexagonal close packed (hcp) and hcp-body-centered cubic phase transition of Pb under high pressures. Shock Hugoniot, as well as equation of states for fcc and hcp phase, also agrees well with the literature ones up to more than 100 GPa. With the developed potential, non-equilibrium molecular dynamic simulations are conducted to investigate dynamic behaviors of Pb single crystal under ramp-shock compressions. Depending on applied strain rates, dislocation-mediated plasticity, phase transition and virtual melting, constantly reported by experiments or theoretical investigations, are observed in our results. Additionally, a new phase transition mechanism of Pb subjected to the ramp compressions is uncovered. read less

Abstract: Molecular dynamics simulations using an embedded atom method (EAM) potential shows that melting behaviors of Ti nanoparticles are strongly dependent on their size. For the particles having the diameter of less than 2.5 nm, their structures are preferred with the icosahedron of geometric shell closures, and there are multi‐structures’ transitions. With the increase in size, while most atoms in the particles can hold their HCP packing patterns, there exist movements and structural rearrangements of the atoms in the surface. At a high temperature, the accumulation of structural disorder can quickly extend into the entire particle, which resembles the melting of bulk Ti. In the HCP particles with sizes of less than 4 nm, the surface atoms still have important influence on their melting. read less

Abstract: In this paper, four coarse-graining (CG) models are proposed to accelerate molecular dynamics simulations of FCC metals. To this aim, at first, a proper map between beads of the CG models and atoms of the all-atom (AA) system is assigned, afterwards mass of the beads and the parameters of the CG models are determined in a manner that the CG models and the original all-atom model have the same physical properties. To evaluate and compare precision of these four CG models, different static and dynamic simulations are conducted. The results show that these CG models are at least 4 times faster than the AA model, while their errors are less than 1 percent. read less

Abstract: Atomistic simulations using classical interatomic potentials are powerful investigative tools linking atomic structures to dynamic properties and behaviors. It is well known that different interatomic potentials produce different results, thus making it necessary to characterize potentials based on how they predict basic properties. Doing so makes it possible to compare existing interatomic models in order to select those best suited for specific use cases, and to identify any limitations of the models that may lead to unrealistic responses. While the methods for obtaining many of these properties are often thought of as simple calculations, there are many underlying aspects that can lead to variability in the reported property values. For instance, multiple methods may exist for computing the same property and values may be sensitive to certain simulation parameters. Here, we introduce a new high-throughput computational framework that encodes various simulation methodologies as Python calculation scripts. Three distinct methods for evaluating the lattice and elastic constants of bulk crystal structures are implemented and used to evaluate the properties across 120 interatomic potentials, 18 crystal prototypes, and all possible combinations of unique lattice site and elemental model pairings. Analysis of the results reveals which potentials and crystal prototypes are sensitive to the calculation methods and parameters, and it assists with the verification of potentials, methods, and molecular dynamics software. The results, calculation scripts, and computational infrastructure are self-contained and openly available to support researchers in performing meaningful simulations. read less

Abstract: MoS2 nanosheets can be used as solid lubricants or additives of lubricating oils to reduce friction and resist wear. However, the atomic scale mechanism still needs to be illustrated. Herein, molecular simulations on the indentation and scratching process of MoS2 monolayer supported by Pt(111) surface were conducted to study the anti-pressure and friction reduction mechanisms of the MoS2 monolayer. Three deformation stages of Pt-supported MoS2 monolayer were found during the indentation process: elastic deformation, plastic deformation and finally, complete rupture. The MoS2 monolayer showed an excellent friction reduction effect at the first two stages, as a result of enhanced load bearing capacity and reduced deformation degree of the substrate. Unlike graphene, rupture of the Pt-supported MoS2 monolayer was related primarily to out-of-plane compression of the monolayer. These results provide a new insight into the relationship between the mechanical properties and lubrication properties of 2D materials. read less

Abstract: We performed molecular dynamics simulations for uniaxial tension of Cu/Fe nano-multilayered films with different modulation periods (λ) using Kurdjumov-Sachs (K-S) relationship to investigate their interfacial morphologies and the effects of λ on their mechanical properties. It shows that the mismatch dislocation lines at the interfaces are periodic. At the Cu side of the interface, triangles distribute with the smallest period; while at the Fe side, parallelograms distribute with the smallest period. There are two yield points, except the case of λ=2.10 nm where there is only one yield point, in the stress-strain curve of Cu/Fe multilayer film under tension, corresponding to the sequential nucleation of dislocations in the Cu and Fe layers from interface, respectively. We further found that there is a critical modulation period, λc, and the flow stress decreases with the increases of λ if λ > λc while increases with the increases of λ if λ λc while increases with the increases of λ if λ < λc. The possible mechanisms are discussed. read less

Abstract: This paper recounts recent advances on the atomistic modeling of twinning in bodycentered cubic (bcc) and face-centered cubic (fcc) alloy. Specifically, we have reviewed: (i) the experimental evidence of twinning-dominated deformation in singleand multigrain microstructures, (ii) calculation of generalized planar fault energy (GPFE) landscapes, and (iii) the prediction of critical friction stresses to initiate twinning-governed plasticity (e.g., twin nucleation, twin–slip and twin–twin interactions). Possible avenues for further research are outlined. [DOI: 10.1115/1.4038673] read less

Abstract: Al-Based Al–Cu alloys have a very high strength to density ratio, and are therefore important materials for transportation systems including vehicles and aircrafts. These alloys also appear to have a high resistance to hydrogen embrittlement, and as a result, are being explored for hydrogen related applications. To enable fundamental studies of mechanical behavior of Al–Cu alloys under hydrogen environments, we have developed an Al–Cu–H bond-order potential according to the formalism implemented in the molecular dynamics code LAMMPS. Our potential not only fits well to properties of a variety of elemental and compound configurations (with coordination varying from 1 to 12) including small clusters, bulk lattices, defects, and surfaces, but also passes stringent molecular dynamics simulation tests that sample chaotic configurations. Careful studies verified that this Al–Cu–H potential predicts structural property trends close to experimental results and quantum-mechanical calculations; in addition, it properly captures Al–Cu, Al–H, and Cu–H phase diagrams and enables simulations of H2 dissociation, chemisorption, and absorption on Al–Cu surfaces. read less

Abstract: Size-selected clusters containing a handful of atoms may possess noble catalytic properties different from nano-sized or bulk catalysts. Size- and composition-selected clusters can also serve as models of the catalytic active site, where an addition or removal of a single atom can have a dramatic effect on their activity and selectivity. In this perspective, we provide an overview of studies performed under both ultra-high vacuum and realistic reaction conditions aimed at the interrogation, characterization, and understanding of the performance of supported size-selected clusters in heterogeneous and electrochemical reactions, which address the effects of cluster size, cluster composition, cluster-support interactions, and reaction conditions, the key parameters for the understanding and control of catalyst functionality. Computational modeling based on density functional theory sampling of local minima and energy barriers or ab initio molecular dynamics simulations is an integral part of this research by providing fundamental understanding of the catalytic processes at the atomic level, as well as by predicting new materials compositions which can be validated in experiments. Finally, we discuss approaches which aim at the scale up of the production of well-defined clusters for use in real world applications. read less

Abstract: Capturing segregation behavior in metal alloy nanoparticles accurately using computer simulations is contingent upon the availability of high-fidelity interatomic potentials. The embedded atom method (EAM) potential is a widely trusted interatomic potential form used with pure metals and their alloys. When limited experimental data is available, the A-B EAM cross-interaction potential for metal alloys AxB1−x are often constructed from pure metal A and B potentials by employing a pre-defined ‘mixing rule’ without any adjustable parameters. While this approach is convenient, we show that for AuPt, NiPt, AgAu, AgPd, AuNi, NiPd, PtPd and AuPd such mixing rules may not even yield the correct alloy properties, e.g., heats of mixing, that are closely related to the segregation behavior. A general theoretical formulation based on scaling invariance arguments is introduced that addresses this issue by tuning the mixing rule to better describe alloy properties. Starting with an existing pure metal EAM potential that is used extensively in literature, we find that the mixing rule fitted to heats of mixing for metal solutions usually provides good estimates of segregation energies, lattice parameters and cohesive energy, as well as equilibrium distribution of metals within a nanoparticle using Monte Carlo simulations. While the tunable mixing rule generally performs better than non-adjustable mixing rules, the use of the tunable mixing rule may still require some caution. For e.g., in Pt–Ni system we find that the segregation behavior can deviate from the experimentally observed one at Ni-rich compositions. Despite this the overall results suggest that the same approach may be useful for developing improved cross-potentials with other existing pure metal EAM potentials as well. As a further test of our approach, mixing rule estimated from binary data is used to calculate heat of mixing in AuPdPt, AuNiPd, AuPtNi, AgAuPd and NiPtPd. Excellent agreement with experiments is observed for AuPdPt. read less

Abstract: Machine-learning interatomic potential (MLIP) has been of growing interest as a useful method to describe the energetics of systems of interest. In the present study, we examine the accuracy of linearized pairwise MLIPs and angular-dependent MLIPs for 31 elemental metals. Using all of the optimal MLIPs for 31 elemental metals, we show the robustness of the linearized frameworks, the general trend of the predictive power of MLIPs, and the limitation of pairwise MLIPs. As a result, we obtain accurate MLIPs for all 31 elements using the same linearized framework. This indicates that the use of numerous descriptors is the most important practical feature for constructing MLIPs with high accuracy. An accurate MLIP can be constructed using only pairwise descriptors for most non-transition metals, whereas it is very important to consider angular-dependent descriptors when expressing interatomic interactions of transition metals. read less

Abstract: The research of quasi-breather statistical characteristics in the model crystal of A3B stoichiometry is conducted by means of molecular dynamics method in the paper by the example of Pt3Al. The phonon spectrum of this model crystal, the dependences of mean-square deviation, the coefficient of variation and the average frequency of the model quasi-breather on the time of its existence are obtained. The statistical data analysis allows for the conclusion that a quasi-breather model solution slightly differs from the exact breather corresponding to it in the model under consideration using the interatomic potential obtained by means of embedded atom method (EAM). read less

Abstract: In this work, we present a highly accurate spectral neighbor analysis potential (SNAP) model for molybdenum (Mo) developed through the rigorous application of machine learning techniques on large materials data sets. Despite Mo's importance as a structural metal, existing force fields for Mo based on the embedded atom and modified embedded atom methods still do not provide satisfactory accuracy on many properties. We will show that by fitting to the energies, forces and stress tensors of a large density functional theory (DFT)-computed dataset on a diverse set of Mo structures, a Mo SNAP model can be developed that achieves close to DFT accuracy in the prediction of a broad range of properties, including energies, forces, stresses, elastic constants, melting point, phonon spectra, surface energies, grain boundary energies, etc. We will outline a systematic model development process, which includes a rigorous approach to structural selection based on principal component analysis, as well as a differential evolution algorithm for optimizing the hyperparameters in the model fitting so that both the model error and the property prediction error can be simultaneously lowered. We expect that this newly developed Mo SNAP model will find broad applications in large-scale, long-time scale simulations. read less

Abstract: Empirical potential is vital to the classic atomic simulation, especially for the study of phase transitions, as well as the solid-interface. In this paper, we attempt to set up a uniform procedure for the validation among different potentials before the formal simulation study of phase transitions of metals. Two main steps are involved: (1) the prediction of the structures of both solid and liquid phases and their mutual transitions, i.e. melting and crystallization; (2) the prediction of vital thermodynamic (the equilibrium melting point at ambient pressure) and dynamic properties (the degrees of superheating and undercooling). We applied this procedure to the testing of seven published embedded-atom potentials (MKBA (Mendelev et al 2008 Philos. Mag. 88 1723), MFMP (Mishin et al 1999 Phys. Rev. B 59 3393), MDSL (Sturgeon and Laird 2000 Phys. Rev. B 62 14720), ZM (Zope and Mishin 2003 Phys. Rev. B 68 024102), LEA (Liu et al 2004 Model. Simul. Mater. Sci. Eng. 12 665), WKG (Winey et al 2009 Model. Simul. Mater. Sci. Eng. 17 055004) and ZJW (Zhou et al 2004 Phys. Rev. B 69 144113)) for the description of the solid–liquid transition of Al. All the predictions of structure, melting point and superheating/undercooling degrees were compared with the experiments or theoretical calculations. Then, two of them, MKBA and MDSL, were proven suitable for the study of the solid–liquid transition of Al while the residuals were unqualified. However, potential MKBA is more accurate to predict the structures of solid and liquid, while MDSL works a little better in the thermodynamic and dynamic predictions of solid–liquid transitions. read less

Abstract: Plasma etching process of single-crystal L10-FePt media [H. Wang et al., Appl. Phys. Lett. 102(5) (2013)] is studied using molecular dynamic simulation. Embedded-Atom Method [M. S. Daw and M. I. Baskes, Phy. Rev. B 29, 6443 (1984); X. W. Zhou, R. A. Johnson and H. N. G. Wadley, Phy. Rev. B 69, 144113 (2004)] is used to calculate the interatomic potential within atoms in FePt alloy, and ZBL potential [J.F. Ziegler, J. P. Biersack and U. Littmark, “The Stopping and Range of Ions in Matter,” Volume 1, Pergamon,1985] in comparison with conventional Lennard-Jones “12-6” potential is applied to interactions between etching gas ions and metal atoms. It is shown the post-etch structure defects can include amorphized surface layer and lattice interstitial point defects that caused by etchant ions passed through the surface layer. We show that the amorphized or damaged FePt lattice surface layer (or “magnetic dead-layer”) thickness after etching increases with ion energy for Ar ion impacts, but significantly small ... read less

Abstract: Core–shell nanoparticles made from iron embedded in gold have a strong potential interest in nanomedicine, the Au shell providing an efficient biocompatible coating for the magnetic Fe core. With the aim of determining theoretically the equilibrium morphologies of Fe–Au nanoparticles in a broad size range and with different compositions, a semiempirical many-body Fe–Au potential was designed combining well-established models for the pure metals and introducing dedicated contributions for the interactions involving mixed elements. The potential was parametrized against various energetic properties involving impurities, intermetallics, and finite clusters obtained using density functional calculations in the generalized gradient approximation. The potential was tested to investigate Fe–Au nanoparticles near equiconcentration containing about 1000–2000 atoms at finite temperature using parallel tempering Monte Carlo simulations initiated from the core–shell structure. The core–shell nanoparticles are found t... read less

Abstract: Bimetallic nanoparticles (BNPs) often possess peculiar segregation behavior as the particle size, composition, shape, and temperature are varied. However, a thermodynamic model for this phenomenon has been lacking thus far. We show for the first time that the distribution of metal species within a nanoparticle can be adequately captured in terms of distribution coefficients calculated for the facets, facet edges, and bulk regions. Thermodynamic relations for the distribution coefficients are derived. Only m distribution coefficients from the m(m – 1) distribution coefficients are independent, where m denotes the number of regions. The theory is applied to AuPt, NiPt, and AuAg BNPs. Distribution coefficients are calculated at 400 and 600 K using Monte Carlo (MC) simulations of varying BNP sizes and compositions. A wide range of mixing behavior from alloying to partial or full segregation and core–shell to onion-like structures can be observed. A key finding is that the distribution coefficients are indepen... read less

Abstract: Heterostructures of tantalum and its oxide are of tremendous technological interest for myriad technological applications, including electronics, thermal management, catalysis, and biochemistry. In particular, local oxygen stoichiometry variation in TaOx memristors comprising thermodynamically stable metallic (Ta) and insulating oxide (Ta2O5) have been shown to result in fast switching on the subnanosecond time scale over a billion cycles. This rapid switching opens up the potential for advanced functional platforms such as stateful logic operations and neuromorphic computation. Despite its broad importance, an atomistic scale understanding of oxygen stoichiometry variation across Ta/TaOx heterointerfaces, such as during early stages of oxidation and oxide growth, is not well understood. This is mainly due to the lack of a unified interatomic potential model for tantalum oxides that can accurately describe metallic (Ta) and ionic (TaOx) as well as mixed (Ta/TaOx interfaces) bonding environments simultaneo... read less

Abstract: Interatomic potentials for pure Ag and Mg–Ag alloy have been developed in the framework of the second nearest-neighbors modified embedded-atom method (MEAM). The validity and the transferability of the Ag potential were obtained by calculating physical, mechanical, thermal, and dislocation related properties. Since the {1 1 1}-generalized stacking fault energy curves obtained from first-principle calculations was used to develop the Ag potential, the critical resolved shear stress to move screw dislocations in Ag single crystal is in good agreement with the experimental data. By combining the ability of the potential to predict the surface energies with its accuracy in describing dislocation properties, the potential is thought to be a predictive model for analyzing the fracture properties of Ag. In addition, the performance of the potential was tested under dynamics conditions by predicting the melting temperature, where a good agreement with experimental value was found. The Ag-MEAM potential was then coupled to an existing Mg-MEAM potential to describe the properties of the binary system MgAg. While the heat of formation, the elastic constants, and the (1 1 0) γ-surface of the MgAg compound in the B2 phase were used to parameterize the potential, heat of formation for MgAg alloys with different stoichiometry, thermal properties of the B2-MgAg compound, as well as dislocation related properties in B2-MgAg compound were tested to validate the transferability of the potential. The heat of formation of Mg5Ag2, MgAg, and MgAg3, the elastic constants and the thermal properties of B2-MgAg obtained with the proposed potential align with first-principles and experimental data. In addition, the core structure of both 〈0 0 1〉 and 〈1 1 1〉 dislocations in {1 1 0} are in agreement with theoretical predictions, and the magnitudes of the critical resolved shear stress obtained at 0 K for both slip systems partially validate the slip behavior of B2-MgAg. Furthermore, the interaction between silver solute element and dislocations from the basal plane is correctly captured by the potential. read less

Abstract: Molecular dynamics (MD) simulations were performed to investigate the influence of nickel (Ni) composition and nanowire thickness on the thermal properties of Al-x%Ni (at%) nanowires using the embedded atom model (EAM) potential. The melting of the nanowire was characterised by studying the temperature dependence of the cohesive energy and mean square displacement. The effect of the nanowire thickness on the cohesive energy, melting temperature, heat capacity as well as latent heat was studied in canonical ensemble. Moreover, the crystal stability of Al, Al-20%Ni, Al-40%Ni, Al-60%Ni, Al-80%Ni, Al3Ni, Ni3Al and Ni nanowires was studied at different temperatures using mean square displacement and cohesive energy. read less

Abstract: The microstructures of titanium (Ti), an attractive tritium (T) storage material, will affect the evolution process of the retained helium (He). Understanding the diffusion behavior of He at the atomic scale is crucial for the mechanism of material degradation. The novel diffusion behavior of He has been reported by molecular dynamics (MD) simulation for the bulk hcp-Ti system and the system with grain boundary (GB). It is observed that the diffusion of He in the bulk hcp-Ti is significantly anisotropic (the diffusion coefficient of the [0001] direction is higher than that of the basal plane), as represented by the different migration energies. Different from convention, the GB accelerates the diffusion of He in one direction but not in the other. It is observed that a twin boundary (TB) can serve as an effective trapped region for He. The TB accelerates diffusion of He in the direction perpendicular to the twinning direction (TD), while it decelerates the diffusion in the TD. This finding is attributable to the change of diffusion path caused by the distortion of the local favorable site for He and the change of its number in the TB region. read less

Abstract: Recently a new class of metal alloys, of single-phase multicomponent composition at roughly equal atomic concentrations ("equiatomic"), have been shown to exhibit promising mechanical, magnetic, and corrosion resistance properties, in particular, at high temperatures. These features make them potential candidates for components of next-generation nuclear reactors and other high-radiation environments that will involve high temperatures combined with corrosive environments and extreme radiation exposure. In spite of a wide range of recent studies of many important properties of these alloys, their radiation tolerance at high doses remains unexplored. In this work, a combination of experimental and modeling efforts reveals a substantial reduction of damage accumulation under prolonged irradiation in single-phase NiFe and NiCoCr alloys compared to elemental Ni. This effect is explained by reduced dislocation mobility, which leads to slower growth of large dislocation structures. Moreover, there is no observable phase separation, ordering, or amorphization, pointing to a high phase stability of this class of alloys. read less

Abstract: Rhenium alloys exhibit a unique combination of chemical, physical, and mechanical properties that makes them attractive for a variety of applications. Herein, we present atomic‐scale structural and atomic part‐per‐million level three‐dimensional (3D) chemical characterization of a Re–Ni coating, combining aberration‐corrected scanning transmission electron microscopy (STEM) and atom‐probe tomography (APT). A unique combination of a columnar and multilayer structure is formed by single‐bath dc‐electroplating and is reported here for the first time. Alternating thicker Re‐rich and thinner Ni‐rich layers support a mechanism in which Ni acts as a reducing agent. The multilayers exhibit hetero‐epitaxial growth resulting in high residual shear stresses that lead to formation of corrugated interfaces and an outer layer with mud‐cracks. read less

Abstract: Miniaturization of microelectronic device components and the development of nano– electro– mechanical systems require advanced understanding of thermal transport in nano-materials and devices, where the atomic nature of matter becomes important and the validity of well-known continuum approximations becomes questionable [1]. In the case of semiconductors and insulators, heat is carried primarily by vibrations in the crystal lattice known as phonons. Phonon transport is classically studied by lattice dynamics based on harmonic wave theory in the frequency space. However, the anharmonic behaviors forming in a crystal structure cannot be described with this theory. Alternatively, the coupled motions of the atoms in real space can be modeled by molecular dynamics (MD), which provides the natural formation and transport of phonons via vibrations in the crystal lattice. Hence, MD has been widely employed to model phonon transfer in nanostructures and channels [2,3]. The performance and reliability of aforementioned devices strongly depend on the removal of heat either to the ambient or to a coolant. In such cases, phonon transport observed at the interfaces of nanoscale device components and surrounding/confined fluid, or at the interfaces of suspended nanoparticles and fluid medium in nano-fluidic coolants plays a critical role. At such interfaces, heat transfer is interrupted with a temperature jump due to the deficiency in overlap between phonon dispersions of dissimilar materials. Classical theories considering specular or diffuse phonon scattering predict the upper or lower limits of interface thermal resistance (ITR), while a detailed investigation of intermolecular interactions is needed to resolve interface phonon scattering mechanisms. In this chapter, we present interface phonon transfer at the molecular level, and investigate the validity of continuum hypothesis and Fourier’s law in nano-channels. First, we focus on the conventional ways of using MD for heat transport problems. Most of the previous MD research sandwiched a liquid domain between two solid walls and induced heat flux by fixing the wall CONTENTS 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: Surface strains in core-shell nanoparticles modify catalytic activity. Here, a continuum-based strategy enables accurate surface-strain-based screening and design of core-shell systems using minimal input as a means to enhance catalytic activity. The approach is validated here for Pt shells on Cu(x)Pt(1-x) cores and used to interpret experimental results on the oxygen reduction reaction in the same system. The analysis shows that precise control of particle sizes and shell thicknesses is required to achieve peak activity, rationalizing the limited increases in activity observed in experiments. The method is also applied to core-shell nanorods to demonstrate its wide applicability. read less

Abstract: The interaction of shear bands with crystalline nanoprecipitates in Cu-Zr-based metallic glasses is investigated by a combination of high-resolution TEM imaging and molecular-dynamics computer simulations. Our results reveal different interaction mechanisms: Shear bands can dissolve precipitates, can wrap around crystalline obstacles, or can be blocked depending on size and density of the precipitates. If the crystalline phase has a low yield strength, we also observe slip transfer through the precipitate. Based on the computational results and experimental findings, a qualitative mechanism map is proposed that categorizes the various processes as a function of the critical stress for dislocation nucleation, precipitate size, and distance. read less

Abstract: The molecular structure of the electrical double layer determines the chemistry in all electrochemical processes. Using x-ray absorption spectroscopy (XAS), we probed the structure of water near gold electrodes and its bias dependence. Electron yield XAS detected at the gold electrode revealed that the interfacial water molecules have a different structure from those in the bulk. First principles calculations revealed that ~50% of the molecules lie flat on the surface with saturated hydrogen bonds and another substantial fraction with broken hydrogen bonds that do not contribute to the XAS spectrum because their core-excited states are delocalized by coupling with the gold substrate. At negative bias, the population of flat-lying molecules with broken hydrogen bonds increases, producing a spectrum similar to that of bulk water. The water double-layer structure at an electrode changed from ordered to disordered when the applied bias was switched. Dissecting the electrical double layer The structure of water within a nanometer of an electrode surface is known as the electrical double layer. This layer creates a strong electrical field that can affect electrochemical reactions. Velasco-Velez et al. explored the structure of the electrical double layer at a bare gold electrode. With no applied potential and at positive potentials, the layer is highly structured (resembling ice) with few dangling hydrogen bonds. However, at negative potentials, the layer was more like bulk water, and half of the water molecules lie flat on the surface. Science, this issue p. 831 read less

Abstract: Monte Carlo simulations of heterogeneous systems of tin at liquid–vapour equilibrium have been performed at several temperatures from 600 to 1500 K, using a modified embedded atom model potential. Surface tension of the corresponding planar interfaces has been evaluated using the test area method. Calculation results are in good agreement with experiments presenting a maximum deviation of 10% from experiments. In addition, the Monte Carlo simulations provide a temperature coefficient (the derivative of the surface tension in regard with temperature) in reasonable agreement with the experimental coefficient. read less

Abstract: Microscopic Monte Carlo simulations of liquid sheets of copper and tin have been performed in order to study the dependence of the surface tension on the thickness of the sheet. It results that the surface tension is constant with the thickness as long as the sheet remains in one piece. When the sheet is getting thinner, holes start to appear, and the calculated surface tension rapidly decreases with thickness until the sheet becomes totally unstable and forms a cylinder. We assume here that this decrease is not due to a confinement effect as proposed by Werth et al. [Physica A 392, 2359 (2013)] on Lennard-Jones systems, but to the appearance of holes that reduces the energy cost of the surface modification. We also show in this work that a link can be established between the stability of the sheet and the local fluctuations of the surface position, which directly depends on the value of the surface tension. Finally, we complete this study by investigating systems interacting through different forms of Lennard-Jones potentials to check if similar conclusions can be drawn. read less

Abstract: Summary Highly monodispersed Cu–Pt bimetallic nanoclusters were synthesized by a facile synthesis approach. Analysis of transmission electron microscopy (TEM) and spherical aberration (C s)-corrected scanning transmission electron microscopy (STEM) images shows that the average diameter of the Cu–Pt nanoclusters is 3.0 ± 1.0 nm. The high angle annular dark field (HAADF-STEM) images, intensity profiles, and energy dispersive X-ray spectroscopy (EDX) line scans, allowed us to study the distribution of Cu and Pt with atomistic resolution, finding that Pt is embedded randomly in the Cu lattice. A novel simulation method is applied to study the growth mechanism, which shows the formation of alloy structures in good agreement with the experimental evidence. The findings give insight into the formation mechanism of the nanosized Cu–Pt bimetallic catalysts. read less

Abstract: Interfacial energy calculated with an atomistic model is very sensitive to adding atoms to or removing atoms from the interface, especially when the interface has a general orientation. Therefore, it is crucial to construct an appropriate initial atomic configuration in order to obtain a reliable value for the interfacial energy from atomistic simulations. In this work, a simple method is proposed for constructing the atomic configuration of a general interface under the condition that the interface is virtually free of interstitial and vacancy. The validity of the method is demonstrated by using it to calculate the equilibrium morphology of precipitates with interfaces in irrational orientations, which shows good agreement with experimental observations. read less

Abstract: We have developed a semi-empirical and many-body type model potential using a modified charge density profile for Cu–Ni alloys based on the embedded-atom method (EAM) formalism with an improved optimization technique. The potential is determined by fitting to experimental and first-principles data for Cu, Ni and Cu–Ni binary compounds, such as lattice constants, cohesive energies, bulk modulus, elastic constants, diatomic bond lengths and bond energies. The generated potentials were tested by computing a variety of properties of pure elements and the alloy of Cu, Ni: the melting points, alloy mixing enthalpy, lattice specific heat, equilibrium lattice structures, vacancy formation and interstitial formation energies, and various diffusion barriers on the (100) and (111) surfaces of Cu and Ni. read less

Abstract: Habit planes between face-centered cubic (fcc)/body-centered cubic (bcc) phases usually exhibit irrational orientations, which often agree with the O-line criterion. Previously, energy calculation was made to test whether the habit planes were energetically favorable, but the values of the energy were found very sensitive to the initial atomic configuration in an irrationally orientated interface. In this paper, under the O-line condition, simple selection criteria are proposed to define and remove interfacial interstitials and vacancies in the initial atomic configuration. The criteria are proved to be effective in obtaining robust energy results. Interfacial energies of two types of O-line interfaces in fcc/bcc systems are calculated following the criteria. The observed transformation crystallography of precipitates in Ni–Cr and Cu–Cr systems can be explained consistently as the irrational habit plane in each system is associated with the lowest energy O-line interface. read less

Abstract: Short-period laser ablation of a platinum solid target was investigated through three-dimensional classical molecular dynamics simulations using the embedded atom method potential. The platinum target was ablated by an ultrashort-pulse laser with three different fluences near the ablation threshold and single 100-fs pulse. Although each laser fluence causes melting and evaporation of the target surface, ablation processes are morphologically different. When the laser fluence is just above the ablation threshold, the surface layer of the solid target breaks away, and so-called spallation occurs. With the moderate laser fluence, homogeneous nucleation of nano-sized clusters takes place in the liquidized layer at the surface, resulting in the homogenization in the emitted cluster size, while the surface layer fragments and vaporizes with the higher fluence. Moreover, in the spallation regime, the recreated surface has nano-sized roughness and is formed after the surface oscillates with a rv20-ns period. This inherent roughness formation may be a seed of the nano-sized regular structure observed by past experiments with repetitive pulses. read less

Abstract: As a potential first-wall fusion reactor material, tungsten will be subjected to high radiation flux and extreme mechanical stress. We propose that under these conditions, coupled grain boundary (GB) motion, in some cases enhanced by interstitial loading, can lead to a radiation-damage healing mechanism, in which a large stress activates coupled GB motion, and the GB sweeps up the defects, such as voids and vacancies, as it passes through the material. The stress-induced mobility characteristics of a number of GBs in tungsten are examined to investigate the likelihood of this scenario. read less

Abstract: Magnesium alloys containing long-period-stacking ordered (LPSO) phases have attracted considerable attention because they have been reported to exhibit excellent mechanical properties, including high strength and reasonable ductility. It is thought that the LPSO phase plays a critical role in producing these favorable mechanical properties. We analyze the deformation behavior of the LPSO phases with different stacking sequences using molecular dynamics simulations. To highlight the specific deformation behavior of the LPSO phases, we also perform deformation analyses of hexagonal-close-packed and face-centered-cubic (FCC) structures. We focus on the influence of the stacking order rather than the segregated atoms around the FCC-structured layers, and we model an LPSO structure by single element composition where the interatomic interaction is described by a smoothed Lennard-Jones potential. Our simulations indicate that an LPSO structure with a shorter stacking sequence tends to exhibit a higher compressive flow stress, because FCC-structured layers inhibit twinning deformations and non-basal slips. Kinking deformation is observed for an LPSO structure when both compression and shear deformation are present. It is shown that the first-order pyramidal-hcþ ai dislocation disarranges the stacking of an LPSO structure and leaves behind many lattice defects. In addition, those lattice defects activate numerous basal slips. Finally, basal dislocations arrange in a line and generate a misorientation angle. Furthermore, this angle originates the compressive deformation. We also observed some prismatic-hai dislocations and cross slips to the basal plane. These results suggest the importance of non-basal slips for kinking deformation. [doi:10.2320/matertrans.MI201211] read less

Abstract: New embedded-atom method potentials for the ternary palladium–silver–hydrogen system are developed by extending a previously developed palladium–hydrogen potential. The ternary potentials accurately capture the heat of mixing and structural properties associated with solid solution alloys of palladium–silver. Stable hydrides are produced with properties that smoothly transition across the compositions. Additions of silver to palladium are predicted to alter the properties of the hydrides by decreasing the miscibility gap and increasing the likelihood of hydrogen atoms occupying tetrahedral interstitial sites over octahedral interstitial sites. read less

Abstract: We examine the ramified islands observed in submonolayer Cu/Ni(100) growth. Our results indicate that the strain-energy contribution to the dependence of island energy on shape is surprisingly weak. In contrast, our accelerated dynamics simulations indicate that unexpected concerted popout processes occurring at step edges may be responsible. Kinetic Monte Carlo (KMC) simulations which include these processes produce island shapes which are very similar to those observed in experiment. These results suggest that the shape transition is of kinetic origin but is strongly mediated by strain. read less

Abstract: We present ab initio calculations of the melting temperatures for bcc metals Nb, Ta and W. The calculations combine phase coexistence molecular dynamics (MD) simulations using classical embedded-atom method potentials and ab initio density functional theory free energy corrections. The calculated melting temperatures for Nb, Ta and W are, respectively, within 3%, 4%, and 7% of the experimental values. We compare the melting temperatures to those obtained from direct ab initio molecular dynamics simulations and see if they are in excellent agreement with each other. The small remaining discrepancies with experiment are thus likely due to inherent limitations associated with exchange-correlation energy approximations within density-functional theory. read less

Abstract: A set of modified embedded-atom method (MEAM) potentials for the interactions between Al, Si, Mg, Cu, and Fe was developed from a combination of each element's MEAM potential in order to study metal alloying. Previously published MEAM parameters of single elements have been improved for better agreement to the generalized stacking fault energy (GSFE) curves when compared with ab initio generated GSFE curves. The MEAM parameters for element pairs were constructed based on the structural and elastic properties of element pairs in the NaCl reference structure garnered from ab initio calculations, with adjustment to reproduce the ab initio heat of formation of the most stable binary compounds. The new MEAM potentials were validated by comparing the formation energies of defects, equilibrium volumes, elastic moduli, and heat of formation for several binary compounds with ab initio simulations and experiments. Single elements in their ground-state crystal structure were subjected to heating to test the potentials at elevated temperatures. An Al potential was modified to avoid formation of an unphysical solid structure at high temperatures. The thermal expansion coefficient of a compound with the composition of AA 6061 alloy was evaluated and compared with experimental values. MEAM potential tests performed in this work, utilizing the universal atomistic simulation environment (ASE), are distributed to facilitate reproducibility of the results. read less

Abstract: The melting curve of NiTi alloy was predicted by using molecular dynamics simulations combining with the embedded atom model potential. The calculated thermal equation of state consists well with our previous results obtained from quasiharmonic Debye approximation. Fitting the well-known Simon form to our Tm data yields the melting curves for NiTi: 1850(1 + P/21.938)0.328 (for one-phase method) and 1575(1 + P/7.476)0.305 (for two-phase method). The two-phase simulations can effectively eliminate the superheating in one-phase simulations. At 1 bar, the melting temperature of NiTi is 1575 ± 25 K and the corresponding melting slope is 64 K/GPa. read less

Abstract: Compliant substrate technology offers an effective approach to grow high-quality multilayered films, of importance to microelectronics and microelectromechanical systems devices. By using a thin, soft substrate to relieve the mismatch strain of an epitaxial film, the critical thickness of misfit dislocation formation in the overlayer is effectively increased. Experiments have indicated that stiff films deposited onto Si substrates can delaminate at the interface. However, the atomic mechanisms of the deformation and the fracture of the films have not been well studied. Here, we have applied molecular dynamics simulations to study the delamination of a stiff body-centered-cubic crystalline film from a compliant Si substrate due to tensile loading. The observed mechanical behavior is shown to be relatively independent of small changes in temperature, loading rate, and system size. Fracture occurs at the interface between the two materials resulting in nearly atomically clean surfaces. Dislocations are seen ... read less

Abstract: One of the key problems in studying alloy nanoparticle catalysis is their surface morphology and segregation behavior. We have developed an accurate embedded atom method (EAM) potential and employed it in the simulation of PdAu metal alloy nanoparticles. The potential was parameterized based on an extensive set of density-functional-theory (DFT) calculations of metal clusters in addition to bulk-alloy properties. The EAM potential accurately reproduces DFT energies of both bulk PdAu alloys and small nanoparticles. We utilized the developed EAM potential in a Monte Carlo simulation of PdAu nanoparticles ranging from 55-atom $(\ensuremath{\sim}1\text{ }\text{nm})$ to 5083-atom particles $(\ensuremath{\sim}4.5\text{ }\text{nm})$. The effects of different factors (particle size, temperature, and composition ratios) on the segregation behavior of PdAu alloy are examined. Our simulation results quantitatively reveal the extent of surface segregation and a strong dependence of surface morphology on the nanoparticle size. read less

Abstract: Palladium hydrides have important applications. However, the complex Pd–H alloy system presents a formidable challenge to developing accurate computational models. In particular, the separation of a Pd–H system to dilute (α) and concentrated (β) phases is a central phenomenon, but the capability of interatomic potentials to display this phase miscibility gap has been lacking. We have extended an existing palladium embedded-atom method potential to construct a new Pd–H embedded-atom method potential by normalizing the elemental embedding energy and electron density functions. The developed Pd–H potential reasonably well predicts the lattice constants, cohesive energies, and elastic constants for palladium, hydrogen, and PdH_x phases with a variety of compositions. It ensures the correct hydrogen interstitial sites within the hydrides and predicts the phase miscibility gap. Preliminary molecular dynamics simulations using this potential show the correct phase stability, hydrogen diffusion mechanism, and mechanical response of the Pd–H system. read less

Abstract: This report summarizes the major research and development accomplishments for the late start LDRD project (investment area: Enable Predictive Simulation) entitled 'Atomically Engineering Cu/Ta Interfaces'. Two ultimate goals of the project are: (a) use atomistic simulation to explore important atomistic assembly mechanisms during growth of Cu/Ta multilayers; and (b) develop a non-continuum model that has sufficient fidelity and computational efficiency for use as a design tool. Chapters 2 and 3 are essentially two papers that address respectively these two goals. In chapter 2, molecular dynamics simulations were used to study the growth of Cu films on (010) bcc Ta and Cu{sub x}Ta{sub 1-x} alloy films on (111) fcc Cu. The results indicated that fcc crystalline Cu films with a (111) texture are always formed when Cu is grown on Ta. The Cu films are always polycrystalline even when the Ta substrate is single crystalline. These polycrystalline films are composed of grains with only two different orientations, which are separated by either orientational grain boundaries or misfit dislocations. Periodic misfit dislocations and stacking fault bands are observed. The Cu film surface roughness was found to decrease with increasing adatom energy. Due to a Cu surface segregation effect, the Cu{sub x}Ta{sub 1-x}more » films deposited on Cu always have a higher Cu composition than that used in the vapor mixture. When Cu and Ta compositions in the films are comparable, amorphous structures may form. The fundamental origins for all these phenomena have been studied in terms of crystallography and interatomic interactions. In chapter 3, a simplified computational method, diffusional Monte Carlo (dMC) method, was developed to address long time kinetic processes of materials. Long time kinetic processes usually involve material transport by diffusion. The corresponding microstructural evolution of materials can be analyzed by kinetic Monte Carlo simulation methods, which essentially simulate structural evolution by tracing each atomic jump. However, if the simulation is carried out at a high temperature, or a jump mechanism with a very low energy barrier is encountered, the jump frequency may approach the atom vibration frequency, and the computational efficiency of the kinetic Monte Carlo method rapidly decreases to that of a molecular dynamics simulation. The diffusional Monte Carlo method addresses the net effects of many atom jumps over a finite duration, kinetically controlled process. First, atom migration due to both random and non-random jumps is discussed. The concept of dMC is then introduced for random jump diffusion. The validity of the method is demonstrated using several diffusion cases in one-, two- and three-dimensional spaces, including the dissolution of spinodal structures. The application of the non-random diffusion theory to spinodal decomposition is also demonstrated.« less read less

Abstract: Molecular dynamics simulations and selected experiments have been carried out to study the growth of Cu films on (010) bcc Ta and the deposition of CuxTa1−x alloy films on (111) fcc Cu. They indicate that fcc Cu films with a (111) texture are always formed when Cu is deposited on Ta surfaces. These films are polycrystalline even when the Ta substrate is single crystalline. The grains have one of two different orientations and are separated by either orientational or misfit dislocations. Periodic misfit dislocations and stacking faults develop within these grains to release structure difference induced misfit strain energy. The Cu film surface roughness was found to decrease with increase in the adatom energy for deposition. When CuxTa1−x is deposited on Ta, the films always have a higher Cu composition than that of the vapor mixture. This arises from a surface segregation phenomenon. When the Cu and Ta fractions in the films are comparable, amorphous structures form. The fundamental origins for the segreg... read less

Abstract: The atomic-scale structures and properties of thin films are critically determined by the various kinetic processes activated during their atomic assembly. Molecular dynamics simulations of growth allow these kinetic processes to be realistically addressed at a timescale that is difficult to reach using ab initio calculations. The newest approaches have begun to enable the growth simulation to be applied for a wide range of materials. Embedded atom method potentials can be successfully used to simulate the growth of closely packed metal multilayers. Modified charge transfer ionic + embedded atom method potentials are transferable between metallic and ionic materials and have been used to simulate the growth of metal oxides on metals. New analytical bond order potentials are now enabling significantly improved molecular dynamics simulations of semiconductor growth. Selected simulations are used to demonstrate the insights that can be gained about growth processes at surfaces. read less

Abstract: Magnetic tunnel junctions (MTJs) require the growth of a thin (∼20 Å) dielectric metal oxide layer, such as Al2O3, on a ferromagnetic metal layer, such as Co, CoFe, or CoNiFe. The atomic assembly mechanisms that combine to form a uniformly thin metal oxide layer on these metal surfaces are not well understood. The application of molecular dynamics simulations to the growth of metal and metal oxide multilayers that involve more than one metal element has not been possible using the conventional interatomic potentials. A recently proposed modified charge transfer ionic–embedded atom method potential appears to correctly enable the charge transfer between oxygen and numerous metal elements to be modelled in a format amenable for molecular dynamics studies. Here we parametrize this charge transfer ionic–embedded atom method potential for the quinternary O–Al–Ni–Co–Fe system so that a direct molecular dynamics simulation of the growth of the tunnelling magnetoresistive multilayers can be realized. read less

Abstract: The tunneling magnetoresistance sTMRd of thin dielectric tunnel barriers that are sandwiched between pairs of ferromagnetic metal thin films is highly sensitive to the barrier layer atomic scale thickness, the uniformity in thickness, and the composition. Widely used AlO x barriers are formed by the oxidation of 1–2 nm thick aluminum layers vapor deposited onto one of the ferromagnetic metal electrodes. The device is completed by vapor depositing the second ferromagnetic layer upon the oxide. Efforts to increase the TMR and tunneling conductance by reducing the thickness of the barrier have been successful until the aluminum layer thickness is decreased below,1 nm whereupon the TMR disappears. The TMR loss is thought to occur because the oxide layer becomes discontinuous leading to regions of metal contact across the barrier layer in the completed device. Using a molecular dynamics simulation technique combined with a recently developed charge transfer potential for metal alloy oxides, we have investigated the atomistic scale phenomena responsible for the disruption of the oxide film’s continuity. We show that discontinuous oxides always form during the oxidation of ,0.6 nm thick crystalline aluminum films ons111d Ni65Co20Fe15 single-crystal layers even when the precursor aluminum layer is continuous and of uniform thickness. The discontinuous mechanism of oxidation is shown to result from a surface-tension-driven dewetting as aluminum is converted to an amorphous oxide. The phenomenon establishes a lower limit of about 1 nm for the thickness of an aluminum oxide tunnel barrier fabricated by oxidation ons111d single-crystal Ni 65Co20Fe15 surfaces. read less

Abstract: Der Fokus dieser Arbeit lag zunachst auf einer simulationsgestutzen Untersuchung uber die Entsteh- ungsmechanismen von Oxidteilchen in ODS-Stahlen. Hierbei bilden empirische Wechselwirkungs- potenziale von Eisen-Yttrium-Sauerstoff (Fe-Y-O) die Grundlage fur eine Beschreibung dieser Oxid- teilchen-Bildungs-Prozesse in Molekulardynamik (MD) Simulationen, die auch Eigenschaften von Versetzungen und anderen Bestrahlungs-Panomenen detailiert zur weiteren Aufklarung behandeln konnen. Zu diesem Zweck ist das speziell auf die Simulation zugeschnittene Anfitten der o.g. MD Potenziale (hier fur Fe-Y-O) notwendig. Hierzu dienen die zuvor durchgefuhrten ab-initio (DFT) Rechnungen als Daten- referenzgrundlage (z.B. von Phasen oder Defekten) zur Optimierung der Potenzialparameter wahrend des Anfittens, um ein moglichst exaktes MD Potenzial zu erzeugen, dass die ab-initio Daten auf groseren MD Skalen detailgetreu abbildet. Im ersten Drittel dieses Projektes wurden mehrere Potenziale fur die einzelnen Metall-Komponenten, Fe-Fe und Y-Y, erzeugt. Dabei stellte sich heraus, dass etablierte Standardmethoden nicht in der Lage sind genaue Fe-Y Potenziale als Teillosung fur das Fe-Y-O Problem zu erzeugen. Dabei wurde eine Kombination aus dem (M)EAM Modell und zur Optimierung eine LSM gestutzte Software (POTFIT) genutzt. Die Komplexitat des Problems liegt in den richtungsabhangigen Atombindungen, die die hier entwickelten fortgeschrittenen Simulations- und Fitmethoden benotigen. Im ersten Schritt von drei Schritten (chapter 3) wurden zunachst einmal die Defizite der Standard-Fittechniken evaluiert, indem die wahrend des Fitting-Prozesses gefundenen Parametersets im EAM Formalismus mit der flexiblen Software POTFIT auf ihre Eignung hin grundlich untersucht worden sind. Die hierfur genutzten Fitfunktionen wurden ursprunglich Anfang 2000 von Zhou und Wadley entwickelt. Hierbei liegt die Ursache fur die dann entdeckte Parameterset-Problematik darin, dass zur Beschreibung des Fe-Y Systems das Model aus drei Potentialkomponenten besteht: Fe-Fe, Y-Y und Fe-Y. Fur diese einzelnen Komponenten sind die Potentialparameter erfolgreich angefittet worden mit Bezug zur Gitterkonstante und Bindungs- bzw. Kohasionsenergie (beides mit 1% Genauigkeit bezgl. DFT Rechnungen) sowie zu allen elastischen Konstanten (5% Genauigkeit bezgl. Experimente). All dies unter Zuhilfenahme von Parametersuchraum-beschrankenden Techniken, die zur Einhaltung der oben genannten Eigenschaften dienen und urspurnglich von Johnson & Oh sind. Selbst kompliziertere Defekteigenschaften, wie Zwischengitter- und Leerstellenbildungsenergien wurden erfolgreich angefittet. Das hier entwickelte EAM Potenzial fur Y-Y ist z.B. in der Lage bei Eigenzwischengitteratomen die basal oktaedrische Position von Zwischengitteratomen (ZA) im Yttrium hcp-Gitter als Grundzustand und die Transition eines jeden ZAs aus einer anderen Position, wie zuvor in DFT berechnet, zu reproduzieren. Zur Bildung des angestrebten Fe-Y Potenzials wurden diese beiden Komponenten, Fe-Fe und Y-Y, zum weiteren Fitten in dem weitgefacherten und komplexen Fe-Y Potzenzialsuchraum genutzt. Die Parametersets wurden mit sogenannten hier entwickelten Hauptparameter (Key Driver) systematisch untersucht. Ein flexibleres Konzept statt der starreren Universal Binding Relations in Abhangigkeit von der Rose Gleichung. Dieser Hauptparameter zeigte eindeutig, dass die Nutzung der Rose Gleichung zur Parametersuchraum-Minimierung den Suchraum dahingehend einschrankt, sodass ein akkurates Anfitten der hier genutzten 900 DFT Datensets nicht mehr moglich ist. Allerdings ist die Orientierung im Parametersuchraum mit dieser Rose Gleichung bei standardmasigen Optimierungsmethoden (wie LSM) unabdingbar, da ohne diese die benotigten globalen Optima fur die Parameter nicht auffindbar sind. Als aufklarendes Testverfahren zur weiteren Ergrundung dieser Problematik und Prufung zur Eignung fur Fe-Y Potenziale und den anschliesenden Simulationen diente der Versuch, 9 verschiedene Bindungs-energien von Yttrium-Leerstellenclustern mit ansteigender Leerstellenzahl zu reproduzieren. Dieser Test konnte von diesen Potenzialen nur teilweise erfullt werden und wurde auf die fehlende Beschreibung der Bindungswinkelabhangigkeit im Modell zuruckgefuhrt. Die Erweiterung von EAM durch MEAM mit Winkelabhangigkeit ist jedoch keineswegs eine zufriedenstellende Losung, da MEAM alternativlos auf der irrefuhrenden Rose Gleichung beruht. Daher war die Benutzung des ubersichtlicheren EAM Typs aus zwei Grunden nutzlich: 1. MEAM braucht die Rose Gleichung um diesen komplexen Formalismus zu beherrschen mit denselben Problemen wie in EAM, aber dieses grundlegende Problem ist in MEAM deutlich schwerer zu identifizieren als in EAM. 2. Die mit EAM gefundenen, angefitteten Parameter sind eine hervorragende Startparameter-Grundlage fur den verbesserten darauffolgenden RF-MEAM Typ. Im zweiten Schritt wurde das Problem aus dem ersten Schritt gelost, indem ein modifizierter MEAM Spezialtyp im referenzlosen Format (RF-MEAM) angewandt worden ist. Im Gegensatz zum herkommlichen MEAM wird hier die Rose Gleichung durch mehr DFT Daten und insbesondere einer intelligenteren Machine Learning ahnlichen Genetic Algorithmus (GA) Optimiertechnik ersetzt, die allerdings eine bedachte Startparameterwahl vorraussetzt, womit Schritt 1 wieder ins Spiel kommt. Die genutzte fortgeschrittene MEAMfit Software, die per GA funktioniert, wurde zwischen 2016 und 2017 funktionierend eigens dafur implementiert. Mit den in Schritt 1 gefitteten Parametern und Set-Auswahltechniken konnten die weiterfuhrenden Fits mit optimalen Startparametern durchgefuhrt werden. Auf dieser Stufe waren diese Fits mit der speziell verbesserten Technik in der Lage ein detailgetreues Fe-Y Potenzial zu generieren, das sowohl alle Phasen (Fe2Y, Fe3Y, Fe5Y, Fe23Y6 und Fe17Y2 sowohl als auch reines Fe und Y) als auch die gesamte Defektdatenbasis mit einer durchschnittlichen Abweichung von ≈11% erfolgreich abbildet. Zusatzlich bestatigend zu dieser allgemeinen Ubereinstimmung wurde konsequenterweise der in Schritt 1 entwickelte Test hervorragend mit einmaliger Genauigkeit bestanden, mit max. 5% Abweichung von den komlexen o.g. Y-Leerstellen Bildungsenergien. Allerdings konnte ein systematischer Fehlertrend aufgespurt werden, der Schwachen in der Fe-Fe Komponente offenbarte. Als Folge dessen wurde umgehend diese Komponente durch ein anderes etabliertes Fe-Fe Potenzial von G. Ackland mit einer extrem genauen Schmelztemperatur (nur 3% Abweichung vom Exp.) ausgetauscht. Mit diesem genauen Potenzial konnte zum ersten Mal die Clusterbildung von gelosten Yttrium Atomen in einer Eisenschmelze erfolgreich per MD Simulation auf atomarer Ebene nachgestellt werden oberhalb von 1750 K. Temperaturen darunter hatten eine Ausscheidungsbildung von Y mit sehr geringer Y-Loslichkeit (<0.1%) in Ubereinstimmung mit den Experimenten zur Folge. Dies wurde durch den Pot. Typ A ermoglicht, der aber die energetische Reihenfolge bei den Fe-Y Phasen nicht ganz genau einhalt. Typ B hingegen halt diese ein, dort fehlt aber die Y-Clusterbildungsneigung. Durch den gebotenen Praxisbezug zur Metallurgie mussen die Loslichkeit und Clusterbildung gleichzeitig in der Simulation genau reproduzierbar sein, was aber weder Typ A noch B kann, was zum Typ A/B Dilemma fuhrt. Dieses Typ A/B Dilemma (Phasen oder Defekt Genauigkeit) fuhrt zum letzten dritten Schritt (chapter 5). Darin ist zusatzlich die Strukturaufklarung von der Fe17Y2 Phase mit Vergleichen zu exp. EXAFS Spektren unserer Kollaborationspartner vom ISSP (Riga) enthalten. Diese Aufklarung dient auch dazu die fehlenden magnetischen Abhangigkeiten im Potenzial zu kompensieren, da die Phasenreihenfolge mit sehr feinen Energieunterschieden wohl stark von magnetischen Wechselwirkungen gepragt ist. Obwohl Potenzial Typ B diesen (Magnetismus) nicht direkt beachtet, ist es in der Lage das tatsachlich gemessene EXAFS Spektrum grostenteils genau wiederzugeben. Allerdings offenbart eine einzige ausgepragte Phasenverschiebung, dass die angenommene hcp Struktur durch eine unterschwellige rhombohedrale Komponente, die sporadisch in der c-Gitterrichtung auftritt, korrigiert werden muss. AIMD (DFT) Berechnungen in Kooperation mit der University of Edinburgh bestatigen dies und zeigen sogar, dass magnetische Wechselwirkungen diese Strukturmischung stabilisieren. Endgultig bestatigt werden konnte dies mit der genauen EXAFS Spektren Reproduktion mit dem durch AIMD verbesserten nochmals gefitteten Potenzialtyp B, der als neuer Typ C durch AIMD indirekt den Einfluss der magnetischen Wechselwirkungen mit einschliest. Diese erstmalige nahezu deckungsgleiche MD Simulation eines EXAFS Spektrums von einem komplexen metallischen Alloy, hier Fe-Y, stellt eine bisher unerreichte Verbesserung dar. Schlieslich lost Typ C das Typ A/B Dilemma und ernoglicht eine genaue gleichzeitige MD Modellierung von Phasen- und Defekten in Fe-Y – ein Durchbruch in der MD-Potenzialentwicklung. read less

Abstract: The evolution characteristics of icosahedral clusters during the rapid solidification of Al10Cu50Zr40 alloy under cooling rates of 1.0 × 1011 K/s are investigated based on molecular dynamics simulations. The structural properties of short-range order and medium-range order of Al10Cu50Zr40 alloy are analyzed by several structural characterization methods. It is found that the icosahedral cluster is the dominant short range order structure. Simultaneously, the medium -range order structural evolutions are described in detail by quantitative method and visualization technology during the rapid solidification. The results reveal that the medium-range order icosahedral clusters are bonding with each other by chain, triangle, tetrahedral and their combination structures, and have excellent structural stability and configural continuity with the temperature decreasing. The medium-range order clusters that consist of , and interlocked with each other and built the densely packed network structures. read less

Abstract: A new method for guiding the motion of fullerene and fullerene-based nanocars is introduced in this paper. The effects of non-flat substrates on the motion of C60, a nanocar and a nanotruck are investigated at different conditions and temperatures. Their behavior is studied using two different approaches: analyzing the variation in potential energy and conducting all-atom classical molecular dynamics simulations. This paper proposes that the use of a stepped substrate will make their motion more predictable and controllable. The results of the simulations show that C60 stays on the top side of the step and cannot jump over the step at temperatures of 400 K and lower. However, at temperatures of 500 K and higher, C60 has sufficient energy to travel to the down side of the step. C60 attaches to the edge and moves just alongside of the edge when it is on the down side of the step. The edge also restricts the motion of C60 alongside the edge and reduces its range of motion. By considering the motion of C60, the general behavior of the nanocar and nanotruck is predictable. The nanocar stays on the top side of the step at temperatures of 400 K and less; at 500 K and higher temperatures, its wheels jump off the edge, and its range of motion is restricted. The relatively rigid chassis of the nanotruck does not allow the free individual motion of the wheels. As a result, the entire nanotruck stays on the top side of the step, even at 600 K. A pathway with the desired route can be fabricated for the motion of C60 and nanocars using the method presented in this paper. This represents a step towards the directional motion of C60 and nanocars. read less

Abstract: Abstract. The work is devoted to modeling the gas flows in microchannels of technical systems in conditions of multiscale computational domain. As an example, the problem of nitrogen flow in nickel microchannel for three-dimensional geometry is considered. General attention is paid to the calculation of gaseous medium macroparameters considering the molecular processes that occur in the gas flow and on the walls of the microchannel. The difference in the scales of the computational domain (the length of the channel, cross section of the channel, the free path of the molecules, the thickness of the boundary layer) and near-surface interaction of the gas with the metal lead to the necessity taking into account the relief and the properties of the microchannel at the molecular level. As a result, the mathematical model of the research flow can not be fully formulated within the framework of the macroscopic approach. For decision of the problem multiscale approach combining the solution of a quasigasdynamic (QGD) equations and correction of gasdynamic parameters by molecular dynamics method (MD), is used. QGD system of equations is solved by method of finite volumes. The MD system of equations is applied within each control volume and is solved by Verlet scheme. In MD calculations particles interactions are described by the potentials determining the basic properties of the components of the considered system. Parallel implementation of the approach is based on method of splitting into physical processes and separation of areas. Algorithms are focused on the use of computer systems with central and hybrid architectures. Calculations showed that the overall numerical algorithm is resistant to the use of data for the flow correction obtained by the MD calculations. With the help of MD methods basic coefficient relations for QGD system were obtained, the transitions from MD to QGD and back were checked, three-dimensional calculation of the nitrogen flow in the nickel microchannel was produced. The results confirmed the efficiency of the developed approach. read less

Abstract: The continuing search for broadly applicable, predictive, and unique potential functions led to the invention of the multi-state modified embedded atom method (MS-MEAM) (Baskes et al 2007 Phys. Rev. B 75 094113). MS-MEAM replaced almost all of the prior arbitrary choices of the MEAM electron densities, embedding energy, pair potential, and angular screening functions by using first-principles computations of energy/volume relationships for multiple reference crystal structures and transformation paths connecting those reference structures. This strategy reasonably captured diverse interactions between atoms with variable coordinations in a face-centered-cubic (fcc)-stable copper system. However, a straightforward application of the original MS-MEAM framework to model technologically useful hexagonal-close-packed (hcp) metals proved elusive. This work describes the development of an hcp-stable/fcc-metastable MS-MEAM to model titanium by introducing a new angular function within the background electron density description. This critical insight enables the titanium MS-MEAM potential to reproduce first principles computations of reference structures and transformation paths extremely well. Importantly, it predicts lattice and elastic constants, defect energetics, and dynamics of non-ideal hcp and liquid titanium in good agreement with first principles computations and corresponding experiments, and often better than the three well-known literature models used as a benchmark. The titanium MS-MEAM has been made available in the Knowledgebase of Interatomic Models (https://openkim.org/) (Tadmor et al 2011 JOM 63 17). read less

Abstract: In this paper, we propose an analytical method for estimating the thermal expansion coefficient (TEC) of metals at high-temperature ranges. Although the conventional method based on quasiharmonic approximation (QHA) shows good results at low temperatures, anharmonic effects caused by large-amplitude thermal vibrations reduces its accuracy at high temperatures. Molecular dynamics (MD) naturally includes the anharmonic effect. However, since the computational cost of MD is relatively high, in order to make an interatomic potential capable of reproducing TEC, an analytical method is essential. In our method, analytical formulation of the radial distribution function (RDF) at finite temperature realizes the estimation of the TEC. Each peak of the RDF is approximated by the Gaussian distribution. The average and variance of the Gaussian distribution are formulated by decomposing the fluctuation of interatomic distance into independent elastic waves. We incorporated two significant anharmonic effects into the method. One is the increase in the averaged interatomic distance caused by large amplitude vibration. The second is the variation in the frequency of elastic waves. As a result, the TECs of fcc and bcc crystals estimated by our method show good agreement with those of MD. Our method enables us to make an interatomic potential that reproduces the TEC at high temperature. We developed the GEAM potential for nickel. The TEC of the fitted potential showed good agreement with experimental data from room temperature to 1000 K. As compared with the original potential, it was found that the third derivative of the wide-range curve was modified, while the zeroth, first and second derivatives were unchanged. This result supports the conventional theory of solid state physics. We believe our analytical method and developed interatomic potential will contribute to future high-temperature material development. read less

Abstract: The melting process of Pd-Ni alloy nanobelts with different Ni atom content has been simulated by molecular dynamic (MD) method. The radial distribution function, the Lindemann index, and pair analysis method were used to characterize Pd-Ni nanobelt models in simulation. The results indicate that the melting temperature of Pd-Ni nanobelt with composition far from pure metal was lower than that of othermodels, and the breaking point of the nanobelt can be illustrated by the Lindemann index. Pair analysis indicates that the number of FCC pairs will decrease and almost disappear at melting point with increasing temperature. The melting points of Pd-Ni alloy nanobelts were also calculated by thermodynamic method, and the results were close to that obtained by MD simulation. read less