Abstract: The structural stability, mechanical properties, internal nucleation and growth mechanisms, and surface energies of Al2RE (RE = Sc, Y, La-Lu) second phases in Al alloys have been investigated by combining first-principles calculations with the Debye model. The results show that the lattice mismatch δ of Al2RE precipitates with Al matrix, which determines the strength of materials associated with dislocation slips, are closely related to the transferred electron e t between Al and RE atoms. Furthermore, the methods of cluster expansion (CE) and special quasi-random structures (SQS) have been adopted to calculate the mixing enthalpy H x of Al2RE11−xRE2x (RE1 = La, Ce, Pr, Nd; RE2 is the rest RE atoms) for studying the possibility of replacing in the Al2RE (RE = La, Ce, Pr, Nd) compounds, which is explored in experiments, and it is found that the mixing enthalpy H x increases with the substitute concentration. The calculated elastic constant C ij, bulk modulus B, shear modulus G, Young’s modulus E decrease with increasing atomic number from Sc to Y at first, and then increases to Lu; while the Cauchy pressure C 12-C 44 and pugh’s ratio B/G show the opposite trends. Moreover, the critical nucleation radius R* and coarsening rate K LSW of Al2RE are obtained from the classical nucleation theory (CNT) and LSW model, respectively. The results show that, the R* for all interfaces decreases from Y to La at first, and then increases linearly to Lu; whereas the K LSW of all models increases from Y to Ce firstly, and then changes very little. Finally, the calculated surface energy E sur of all Al2RE compounds is much higher than that of Al, and with the increase of atomic number, E sur of Al2RE intermetallic compounds decreases firstly from Sc to La, and then increases to Lu. The result is helpful for the further optimal designing high performance of Al alloys. read less

Abstract: In this paper, an in-depth theoretical study on some physical properties of Ti0.5Ta0.5 alloy with systematic symmetry under high pressure is conducted via first-principles calculations, and relevant physical parameters are calculated. The results demonstrate that the calculated parameters, including lattice parameter, elastic constants, and elastic moduli, fit well with available theoretical and experimental data when the Ti0.5Ta0.5 alloy is under T = 0 and P = 0 , indicating that the theoretical analysis method can effectively predict the physical properties of the Ti0.5Ta0.5 alloy. The microstructure and macroscopic physical properties of the alloy cannot be destroyed as the applied pressure ranges from 0 to 50GPa, but the phase transition of crystal structure may occur in the Ti0.5Ta0.5 alloy if the applied pressure continues to increase according to the TDOS curves and charge density diagram. The value of Young’s and shear modulus is maximized at P = 25 GPa . The anisotropy factors A ( 100 ) [ 001 ] and A ( 110 ) [ 001 ] are equal to 1, suggesting the Ti0.5Ta0.5 alloy is an isotropic material at 28 GPa, and the metallic bond is strengthened under high pressure. The present results provide helpful insights into the physical properties of Ti0.5Ta0.5 alloy. read less

Abstract: Based on exact muffin-tin orbitals (EMTO) and coherent potential approximation (CPA), we investigate the effects of Mo content on the mechanical properties of CrFeCoNiMox (0.1 ≤ x ≤ 0.3) high-entropy alloys (HEAs) with a face-centered-cubic (fcc) crystal structure; relevant physical parameters are calculated as a function of Mo content. The results indicate that the theoretical predictions of lattice constant, elastic constants, shear modulus, and Young's modulus are in good agreement with the available experimental data, which proves the validity of the applied approach. CrFeCoNiMo0.26 HEA has better ductility and plasticity with respect to other HEAs with different Mo contents because it has the minimum elastic moduli and Vickers hardness, and has the maximum Pugh's ratio and anisotropy factors, etc. CrFeCoNiMo0.2 HEA has better plasticity compared with CrFeCoNiMo0.1 and CrFeCoNiMo0.3 HEAs due to its minimum energy factor and maximum dislocation width. Screw dislocation is more likely to nucleate in CrFeCoNiMox (0.1 ≤ x ≤ 0.3) HEAs than edge dislocation. The present studies are helpful to explore the excellent mechanical properties of CrFeCoNiMox (0.1 ≤ x ≤ 0.3) HEAs during experiments. read less

Abstract: This study employs first principles method to run an investigation on structural, electronic, elastic, vibrational, thermodynamic and magnetic properties of RhMnX (X = Sb and Sn) half-Heusler compounds in C1b phase. The elastic constants (C11, C12 and C44) for RhMnX (X = Sb and Sn) half-Heusler compounds in C1b phase are computed using the energy-strain method. Directional changes of Young and Shear modulus, Poisson’s ratios and compressibility’s of RhMnX (X = Sb and Sn) compounds have been analysed. Evaluations of mechanical stabilities via obtained elastic constants are exhibited stable natures for these compounds. Moreover, the compounds are ductile based on the Pugh’s criteria. Electronic band structures and their partial density of states are presented for both compounds and used to interpret the electronic band profiles of these compounds in the C1b phase. Specifically, phonon dispersion curves of these compounds have been derived for the first time. Full phonon properties of these compounds suggest that both compounds are dynamically stable. Besides, the change in specific heat capacity and entropy of RhMnX (X = Sb and Sn) compounds have been also computed between 0–500 K. read less

Abstract: In this study, molecular dynamics (MD) are used to simulate the nanoimprinting behaviors of (001) Ni thin films using 3×3 array cuboid diamond punch. Different pitch of the punches was used to investigate the effect of the interaction among the forming pattern and the quality of the formed holes. Forming force was evaluated including normal and tractional force of the punch during pressing process. The dislocation behavior was also monitoring during the holes forming. The result showed that the pitch of the punch was 2w could get the best forming pattern. read less

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

Abstract: Molecular simulation is a powerful computational tool for a broad range of applications including the examination of materials properties and accelerating drug discovery. At the heart of molecular simulation is the analytic potential energy function. These functions span the range of complexity from very simple functions used to model generic phenomena to complex functions designed to model chemical reactions. The complexity of the mathematical function impacts the computational speed and is typically linked to the accuracy of the results obtained from simulations that utilize the function. One approach to improving accuracy is to simply add more parameters and additional complexity to the analytic function. This approach is typically used in non-reactive force fields where the functional form is not derived from quantum mechanical principles. The form of other types of potentials, such as the bond-order potentials, is based on quantum mechanics and has led to varying levels of accuracy and transferability. When selecting a potential energy function for use in molecular simulations, the accuracy, transferability, and computational speed must all be considered. In this focused review, some of the more commonly used potential energy functions for molecular simulations are reviewed with an eye toward presenting their general forms, strengths, and weaknesses.Molecular simulation is a powerful computational tool for a broad range of applications including the examination of materials properties and accelerating drug discovery. At the heart of molecular simulation is the analytic potential energy function. These functions span the range of complexity from very simple functions used to model generic phenomena to complex functions designed to model chemical reactions. The complexity of the mathematical function impacts the computational speed and is typically linked to the accuracy of the results obtained from simulations that utilize the function. One approach to improving accuracy is to simply add more parameters and additional complexity to the analytic function. This approach is typically used in non-reactive force fields where the functional form is not derived from quantum mechanical principles. The form of other types of potentials, such as the bond-order potentials, is based on quantum mechanics and has led to varying levels of accuracy and transferabilit... read less

Abstract: The pressure dependent behaviour of the structural, electronic, mechanical, vibrational, and thermodynamic properties of Pd2TiX (X=Ga, In) Heusler alloys was investigated by ab initio calculations. The lattice constant, the bulk modulus and its first pressure derivative, the electronic band structure and the density of states (DOS), mechanical properties such as elastic constants, anisotropy factor, Young’s modulus, etc., the phonon dispersion curves and phonon DOS, entropy, heat capacity, and free energy were obtained under pressure. It was determined that the calculated lattice parameters are in good agreement with the literature, the elastic constants obey the stability criterion, and the phonon dispersion curves have no negative frequency which shows that the compounds are stable. The band structures at 0, 50, and 70 GPa showed valence instability at the L point which explains the superconductivity in Pd2TiX (X=Ga, In). read less

Abstract: ABSTRACT The modified analytic embedded atom model (EAM) potentials considering farther neighbor atoms are improved for the noble metals (Ag, Au, Pt, Pd, Rh) and Cu. We not only adopt an end processing function and an enhanced smooth continuous condition for the pair potential, but also adjust the model parameters of multi-body potential by fitting a cohesive energy, a mono-vacancy formation energy, the Rose equation curve for the cohesive energy as a function of lattice parameter, a structure energy difference, elastic parameters and an equilibrium condition of crystal. The calculation results of structure energy differences misfit the experiment data for the noble metals and Cu in the unimproved EAM, because anyone of these differences have not been considered in the calculation of its model parameters. After the modification, the model showed better simulation results for the noble metals and Cu. read less

Abstract: The present paper addresses the temperature dependent elastic, mechanical and thermal properties of NaCl structure (B1 type) holmium monopnictides, HoX (X = N, P, As, Sb, Bi) computed using Coulomb and Born repulsive potentials extended up to second nearest neighbors. The second-order elastic constants (SOECs) of single crystals HoX are calculated as a function of temperature in the range 0–500K. The compounds under study are found to be brittle in nature. Beside these calculations, the theoretical hardness has been obtained for various rare-earth monopnictides using the elastic properties in the polycrystalline approach. The obtained hardness values indicate HoN to be hard, but cannot be considered super hard. The anisotropic nature of the chosen single crystal is an important physical quantity in studying the directional dependent thermal properties such as Debye temperature and thermal conductivity computed using ultrasonic velocities along different crystallographic directions. The obtained results are discussed in correlation with mechanical and thermophysical properties of similar materials. read less

Abstract: The sliding of three-dimensional clusters and two-dimensional islands adsorbed on crystal surfaces represents an important test case to understand friction. Even for the same material, monoatomic islands and thick clusters will not as a rule exhibit the same friction, but specific differences have not been explored. Through realistic molecular dynamics simulations of the static friction of gold on graphite, an experimentally relevant system, we uncover as a function of gold thickness a progressive drop of static friction from monolayer islands, that are easily pinned, towards clusters, that slide more readily. The main ingredient contributing to this thickness-induced lubricity appears to be the increased effective rigidity of the atomic contact, acting to reduce the cluster interdigitation with the substrate. A second element which plays a role is the lateral contact size, which can accommodate the solitons typical of the incommensurate interface only above a critical contact diameter, which is larger for monolayer islands than for thick clusters. The two effects concur to make clusters more lubric than islands, and large sizes more lubric than smaller ones. These conclusions are expected to be of broader applicability in diverse nanotribological systems, where the role played by static, and dynamic, friction is generally quite important. read less

Abstract: A modified embedded atom method (MEAM) potential model up to second neighbours has been used to calculate the phonon dispersions for Ni0.55Pd0.45 alloy in which Pd is introduced as substitutional impurity. Using the force-constants obtained from MEAM potential, the local vibrational density of states in host Ni and substitutional Pd atoms using Green’s function method has been calculated. The calculation of phonon dispersions of NiPd alloy shows a good agreement with the experimental results. Condition of resonance mode has also been investigated and resonance mode in the frequency spectrum of impurity atom at low frequency is observed. read less

Abstract: Purpose – The surface energy per unit area of material is known to be proportional to the thermal energy at the melting point of the material. The purpose of this paper is to employ the values of the melting points of metals to develop a model that estimates the average surface energies of metals. Average surface energy estimator (ASEE) was developed with the aid of computational intelligence technique on the platform of support vector regression (SVR) using the values of the melting point of the materials as the descriptor. Design/methodology/approach – The development of ASEE which involves 12 data set was conducted by training and testing SVR model using test-set-cross-validation technique. The developed model (ASEE) was used to estimate average surface energies of 3d, 4d, 5d and other selected metals in the periodic table. The average surface energies obtained from ASEE are in good agreement with the experimental values and with the values from other theoretical models. Findings – The accuracy of this... read less

Abstract: Restructuring process of vacancy voids under the influence of post-cascade shock waves is studied by molecular dynamics method. It is shown that depending on the time gaps, which generate waves, voids can either be combined into a single complex, or break up into separate parts. read less

Abstract: Solvation force oscillation in octamethylcyclotetrasiloxane (OMCTS) versus the distance between an atomic force microscope (AFM) tip and mica substrate has been studied through molecular dynamics simulations. A driving spring model in a liquid-vapor molecular ensemble is used to explore the force oscillation mechanism. It has been found that OMCTS fluid in tip-substrate contact has a strong tendency to form a layered structure, starting from n = 8 layers. The force profile obtained from simulation is qualitatively similar to those in contact mode AFM experiments. However, the bulk-like diffusion and rotation of OMCTS molecules underneath the AFM tip suggest that, under the tip-substrate confinement geometry, the layered OMCTS film cannot form a solidified structure except under n = 2 extreme contact-layer confinement. read less

Abstract: Multi-tool nanocutting single crystal copper processes are simulated by molecular dynamics. Then, nanomachining mechanism and removal processes are analyzed by atoms energy, and local atomic structures of the workpiece atoms are identified in the nanomachining process. The results show that local atomic structures of stacking atoms near the tool come to phase transition, which transforms from ideal FCC to icosahedra-like structure, closed-packed structure and defective FCC structure. In addition, the cutting force of two-side tools is high in the multi-tool nanocutting process during the preliminary phase of the cutting process. And then, the force of middle tool is higher than others. The removal atoms numbers increase with the increasing tools and the removal ratio of atoms increases when the distance between the tools increases. read less

Abstract: An end processing function of the pair-potential of modified analytical embedded atom method (MAEAM) was suggested for bcc metals. Through fitting the elastic constants, cohesive energy and an equilibrium condition of bcc metal crystals correctly, we changed the pair-potential parameters and the modification term parameter of the multi-body potential. The model calculations fully demonstrate the structure stabilities and the phonon dispersion curves of seven bcc transition metals: Cr, Fe, Mo, Nb, Ta, V and W. read less

Abstract: An end processing method of the pair-potential of modified analytical embedded atom method (MAEAM) was suggested for hcp metals with farther neighbor atoms. Through fitting the elastic constants, the cohesive energy and two equilibrium conditions of hcp metal crystals correctly, we changed the pair-potential parameters and the modification term parameters of the multi-body potential. The model calculations fully demonstrate the structure stability and the unrelaxed mono-vacancy properties of six hcp metals: Co, Mg, Re, Ru, Ti and Zr. read less

Abstract: We explored a new type alloy EAM potential (CLI-EAM) that the value of atomic electron density and pair potential between distinct atoms are obtained by Chen’s lattice inversion based on first-principles calculations. The alloy CLI-EAM potential acquired from NiAl alloy can also apply in Ni3Al successfully and the results of basic properties agreed with the experiments. The results of formation energy of point defects of NiAl and Ni3Al alloy indicate that the structural defects are anti-site defects of Al when enrichments of Al atoms. read less

Abstract: 1. Introduction Part I. Continuum Mechanics and Thermodynamics: 2. Essential continuum mechanics and thermodynamics Part II. Atomistics: 3. Lattices and crystal structures 4. Quantum mechanics of materials 5. Empirical atomistic models of materials 6. Molecular statics Part III. Atomistic Foundations of Continuum Concepts: 7. Classical equilibrium statistical mechanics 8. Microscopic expressions for continuum fields 9. Molecular dynamics Part IV. Multiscale Methods: 10. What is multiscale modeling? 11. Atomistic constitutive relations for multilattice crystals 12. Atomistic/continuum coupling: static methods 13. Atomistic/continuum coupling: finite temperature and dynamics Appendix References Index. read less

Abstract: The modified analytical embedded atom method is applied to calculate the phonon dispersion of body-centered cubic 3d transition metals Cr and Fe along five symmetry directions [q 0 0], [1 q q], [q q q], [q q 0] and [1/2 1/2 q]. Our results of phonon dispersion curves are in good agreement with the available experimental data. For the two transition metals Cr and Fe, along the same direction, a similar phonon dispersion curve is obtained in spite of the phonon frequency decreases for Cr and Fe due to the atom mass increases. There are no experimental results for comparison along the directions [1 q q] and [1/2 1/2 q], further experimental measurement are needed. read less

Abstract: We explored a new type EAM potential (CLI-EAM) that the value of atomic electron density and pair potential functions are obtained by Chen’s lattice inversion based on first-principles calculations. This EAM potential is applied to Cu, Ag, Cu and Pt metals successfully and the results of basic properties agreed with the experiments. For the same metal, the cohesive energy of fcc structures are the lower than bcc structures. read less

Abstract: A scheme is proposed in the present study to predict the phase stability of binary alloys in the fcc–fcc metal systems. Based on the long-range empirical potential model, the interatomic potentials are first constructed for 15 binary metal systems of Cu, Ag, Au, Ni, Pd, and Pt, and then molecular statics calculations are carried out to predict the heats of formation of the disordered solid solutions in these systems. It is found that the predicted results by the present scheme match well with those obtained by experiments or ab initio calculations, exhibiting a more precise feature than the Miedema's model and Johnson's calculations. According to the predicted heats of formation, the phase stability of the alloys in the 15 fcc–fcc metal systems are predicted in the present study, which are satisfactorily consistent with the corresponding experimental phase diagrams. read less

Abstract: The failure of the nanoscale metallic interface has raised concerns owing to the effect interfacial amalgamation has on its application in nanoelectronic devices. Single crystal copper [110] and [100], which are set as two components of [110]‖[100] nanocrystalline copper, are used to simulate the interfacial properties using molecular dynamics simulations. Repeated tension and compression cycles show that the two components of the interface can come into contact and separate without interfacial amalgamation. The [110]‖[100] interface could withstand momentary shocks of compression and heat produced by the momentary shocks. This property of the [110]‖[100] interface is dominated by crystalline orientations of interfacial structure, in comparison with [111]‖[100] and [111]‖[110] interfaces under the same conditions. read less

Abstract: An atomic force field for simulating copper clusters and nanoparticles is developed. More than 2000 cluster configurations of varying size and shape are used to constrain the parametrization of the copper force field. Binding energies for these training clusters were computed using density functional theory. Extensive testing shows that the copper force field is fast and reliable for near‐equilibrium structures of clusters, ranging from only a few atoms to large nanoparticles that approach bulk structure. Nonequilibrium dissociation and compression structures that are included in the training set are also well described by the force field. Implications for molecular dynamics simulations and extensions to other metallic and covalent systems are discussed. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009 read less

Abstract: Molecular dynamics (MD) simulations are carried out to study a three-pyramid tip nanoscratching single crystal copper. The surface integrity and subsurface damaged layers are investigated by the different scratching depths and crystal orientation. The effects show that nanoscratching process results in the extension of the high energy atoms in the subsurface from near the tool to the whole subsurface. The area of high energy atoms in subsurface is basically in consistence with that of the higher residual stress. The ordering degree of subsurface atoms decreases as the increase of scratching depths. The (100) plane body compared to the (111) plane body after scratching process, the numbers of defects in subsurface of the former are more than that of the latter, the surface integrity and the ordering degree of subsurface of the former is better than that of the latter, but the area of the subsurface damaged layers of the former is larger. It is noted that there exists the stack fault in subsurface for the (111) plane body. read less

Abstract: In this paper, molecular dynamics simulations have been conducted to study the mechanical stretching of copper nanowires which will finally lead to the formation of suspended liner atomic chains. A total of 2700 samples have been investigated to achieve a comprehensive understanding of the influence of temperature and orientation on the formation of linear atomic chains. Our results prove that linear atomic chains do exist for [100], [111] and [110] crystallographic directions. Stretching along the [111] direction exhibits a higher probability in forming the two-atom contact than that along the [110] and [100] directions. However, for longer linear atomic chains, there emerges a reversed trend. In addition, increasing temperature may decrease the formation probability for stretching along [111] and [110] directions, but this influence is less obvious for that along the [100] direction. read less

Abstract: A multiapproach modeling of surface diffusion driven decay of three dimensional high-aspect-ratio nanostructures is presented. Three different strategies are used: a continuous model, kinetic Monte Carlo simulations, and a molecular dynamics approach using an embedded atom model potential. All these models predict that an initially single-valued surface evolves into an intermediate multivalued state, in which features on the surface develop into a nonconvex shape. Moreover, such nanofeatures in the case of discrete models are quite well described in terms of the continuous modeling. Quantitative bounds for the aspect-ratios in which emergence of overhangs starts are estimated. read less

Abstract: We have simulated the vacuum deposition and subsequent growth of gold nanoparticles on various substrates in order to explore the effects that substrate morphology has on the resultant morphology of gold nanoparticles. The substrates and conditions explored included, the three low index faces, namely, (111), (100), and (110) for both fcc and bcc crystalline substrate structures, including various substrate lattice constants and temperatures. Firstly, we cataloged the major nanoparticle morphologies produced overall. While some substrates were found to produce a mixture of the main nanoparticle morphologies we were successful in identifying certain substrates and temperature conditions for which only I(h), D(h), or certain fcc crystalline nanoparticles can be grown almost exclusively. The substrate characteristics, temperature conditions, and governing growth dynamics are analyzed. We shed light on the balance between substrate influences and vacuum growth tendencies. From observations we can speculate that a substrate alters both the free energy stability of gold nanoparticles and/or the free energy barriers to transformation between certain morphologies. As such we find that substrates are an effective tool in templating the selective growth of desired nanoparticles or surface nanostructures. read less

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

Abstract: In molecular simulations, the frequencies of the low-frequency modes are many orders of magnitude lower than those of the highfrequency modes. Compared with the amplitudes of the low-frequency modes, the amplitudes of the high-frequency modes are often negligible and, thus, least interesting. As dictated by the period of the highest frequency mode, the critical time step for stable time integration can be significantly increased by suppressing the negligible high-frequency modes yet the solution remains virtually intact. In this light, a smoothed molecular dynamics (SMD) approach is proposed to eliminate the high-frequency modes from the dynamical system through the use of a regular background grid. By manipulating the grid size, it is possible to increase the critical time step significantly with respect to that of the conventional molecular dynamics (MD). The implementation of SMD is very similar to the conventional MD. Any time integrators and inter-atomic potentials used in the conventional MD can be equally adopted in SMD. The coupling of MD and SMD methods is briefly investigated, and the similarity between MD and SMD methods enables a simple and concise coupling. Examples on 1D atom chains and 3D tension/compression of single crystal show that the proposed SMD method and the conventional MD method yield close results yet the time step of the former can be one order higher than that of the latter. Tension of a cracked single crystal is examined to verify the coupling method, and the yield point can be captured precisely by the coupling method. Keyword: Molecular dynamics, Critical time 1 Department of Engineering Mechanics, Tsinghua University, Beijing 100084, P. R. China 2 Department of Mechanical Engineering, The University of Hong Kong, Hong Kong SAR, P. R. China step, Material point method, Background grid. read less

Abstract: In the harmonic approximation, the atomic force constants are derived and the phonon dispersion curves along four major symmetry directions [00ζ], [0ζζ], [ζζζ] and [0ζ1] (or Δ, Σ, Λ and Z in group-theory notation) are calculated for four noble metals Cu, Ag, Au and Pt by combining the modified analytic embedded atom method (MAEAM) with the theory of lattice dynamics. A good agreement between calculations and measurements, especially for lower frequencies, shows that the MAEAM provides a reasonable description of lattice dynamics in noble metals. read less

Abstract: We show that two quite recent treatments of dislocation-mediated melting transitions result in the thermal energy associated with the melting temperature, T m, being expressed as a product of a volume factor and a combination of elastic constants times a lattice structure-dependent factor. We further show that the result for the latent heat of fusion L m obtained in one of these studies leads to the ratio L m/GΩ, where G is the shear modulus at melting and Ω the atomic volume, being a constant. Since the ratio of the vacancy formation energy to GΩ is also found to be roughly constant, we suggest that the factor GΩ at melting is crucial in determining the melting temperature, the latent heat of fusion and the vacancy formation energy and we comment on the reasons why this should be so. read less

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

Abstract: The energies in three combinations of Ag(001)/Si(111), Ag(011)/Si(111) and Ag(111)/Si(111) twist boundaries have been calculated using the modified embedded atom method (MEAM). The results show that the interface energies corresponding to Ag(111)/Si(111), Ag(001)/Si(111) and Ag(011)/Si(111) increase successively and the lowest energies 365, 717.7 and 996.1 mJ m−2 corresponding to each interface appear at twist angles θ of 30, 0 and 25.24°, respectively. Considering minimization of interface energy, we can conclude that the Ag films deposited on Si(111) substrate will result in a (111) preferred orientation, especially at a twist angle of θ = 30°. Copyright © 2005 John Wiley & Sons, Ltd. read less

Abstract: Adsorbed atoms (adatoms) and vacancies have a significant role to play in the physics of surfaces and the mechanisms of film growth on a substrate. This paper investigates the effect of applied or residual strain on the energetic interaction between pairs of adatoms and vacancies. The analysis is based on a continuum-level point-defect model, where adatoms and vacancies have strain-dependent properties. Atomistic calculations are used to evaluate the defect properties for Si(111). The result is used as input for the defect model in order to investigate the strength and character of the interaction versus strain, separation distance, and relative orientation of the defects. It is found that strain may cause the defects to align in certain direction and modulate their interaction between repulsion and attraction, providing a mechanism for controlled building of nanostructures. read less

Abstract: The unrelaxed energies for Ag(001) twist grain boundaries (GBs) have been calculated using the modified analytical embedded atom method (MAEAM). The results show that, besides zero energy in a perfect crystal (corresponding to the twist angle 0°), the second smallest GB energy corresponds to the twist angle 36.87°. This is consistent with the experimental results. For other twist angles, the GB energies remain nearly constant even for a twist angle as small as 1.94°. Translation parallel to the boundary plane could result in a periodic change in GB energy. Copyright © 2004 John Wiley & Sons, Ltd. read less

Abstract: Pair distribution functions and constant-volume heat capacities of liquid copper, silver and nickel have been calculated by molecular dynamics simulations with four different versions of the embedded-atom method (EAM) model, namely, the versions of Johnson, Mei, Cai and Pohlong. The simulated structural properties with the four potential models show reasonable agreement with experiments and have little difference with each other, while the calculated heat capacities with the different EAM versions show remarkable discrepancies. Detailed analyses of the energy of the liquid metallic system show that, to predict successfully the heat capacity, an EAM model should match the state equation first proposed by Rose. read less

Abstract: The recent development of microscopes that allow for the examination of defects at the atomic scale has made possible a more direct connection between the defects and the macroscopic response they engender (see, e.g., the December 1999 issue of MRS Bulletin ). read less

Abstract: Molecular dynamics studies of the temperature dependence of self diffusion coefficient of palladium has been carried out using the many body potential generated by the Embedded Atom Method of Daw and Baskes. These values as well as the results for activation energy are compared with similar results for other fcc metals. read less

Abstract: We have calculated the Debye-Waller factor (DWF) of Cu from a model that was used successfully in earlier calculations of anharmonicity by Cowley and Shukla. The present calculation has been carried out using quasiharmonic theory, the lowest-order (λ2) anharmonic perturbation theory, and a Green's function (GF) method which sums an infinite series of the λ2−type anharmonic terms. The static approximation ω → 0 in the cubic contribution to the self-energy of the GF, introduced in the earlier work on the DWF by Shukla and Hubschle is further justified by showing that in the high-temperature limit the exact results for the λ2 anharmonic contributions (cubic and quartic) to the Helmholtz free energy are given in this approximation. Results for the DWF are also obtained for a modified version of the Morse potential with λ2 perturbation theory (PT) and the GF method. The GF results are in excellent agreement with the experimental Mossbauer and X-ray data in the entire temperature range, 300 K  T  120... read less

Abstract: Many-body empirical model potentials are formulated in the frame of the embedded-atom method (EAM) of Daw and Baskes for homonuclear cubic metals. The basic functions of the model (the pair interaction potential electron density function, and embedding energy function) are presented in analytical forms which do not impose limitations at least on the basic properties of the system like Cauchy discrepancy, equal anisotropy ratio imposed by the Johnson's EAM potential, etc. The developed potentials are convenient for computer simulations. read less

Abstract: The embedded-atom method (EAM) has been highly successful in predicting many properties of fcc metals. However, it is known to underestimate surface energies by as much as 40 to 50%. This suggests that it would be interesting to explore the possibility of obtaining a surface correction to the embedding energy. In this paper, the functional form for a surface embedding function, Fsurface, for the embedded-atom method (EAM) is proposed. The existence of a different F for surface atoms than for bulk atoms stems from the fact that the presence of a surface modifies the energy band structure of the solid. In order to study this change, we used the tight-binding method, that provides the ingredients needed to obtain an explicit expression for the relevant quantities. By comparing the energies of the EAM and tight binding for a surface-terminated bulk, we obtain a correction to the EAM embedding function and the EAM energy for the system. In order to quantify our result we apply it to the lower-index surface planes of Ag and Pd by adjusting our tight-binding parameters with known, available first-principles results for the (111) plane. We then predict the surface energies for the (100) and (110) planes with our method and show an improvement over using the bulk embedding function as compared with first-principles values. read less

Abstract: The determination of formation and migration energies of point and clustered defects in SiC is of critical importance to a proper understanding of atomic phenomena in a wide range of applications. We present here calculations of formation and migration energies of a number of point and clustered defect configurations. A newly developed set of parameters for the modified embedded-atom method (MEAM) is presented. Detailed molecular dynamics calculations of defect energetics using three representative potentials, namely the Pearson potential, the Tersoff potential and the MEAM, have been performed. Results of the calculations are compared to first-principles calculations and to available experimental data. The results are analysed in terms of developing a consistent empirical interatomic potential and are used to discuss various atomic migration processes. read less

Abstract: An effective interatomic potential suitable for all body‐centered‐cubic (bcc) metals is developed based on the embedded atom method. The potential predicts all major anomalies displayed in the phonon spectra of the bcc transition metals, as well as the large relaxation of the (100) surface of V, Nb, and Ta. read less

Abstract: The lattice inversion method is used to construct the pair potential and the hopping integral in Finnis-Sinclair model. In the approach, the lattice sum of square hopping integral is assumed to be an exponential function versus the nearest-neighbour distance, with two parameters determined from the Cauchy discrepancy and the difference of the unrelaxed vacancy-formation energy with the sublimation energy. The individual hopping integral is inverted from the exponential function and the pair potential is inverted from the remaining part of total cohesive energy. read less

Abstract: The effective interaction potential for tungsten is used to calculate the second moment of phonon spectrum, , Debye temperature, and cohesive properties. It is shown that the potentials obtained give good agreement of cohesive properties with the data calculated from the universal function of Rose et al. The applicability of the atomic sphere approximation to the calculation of the integral thermodynamic properties of tungsten is discussed. The convergence of in real space is studied; we find that the main contribution to is given by the first coordination shell. The Gibbs–Bogoliubov inequality and the variational procedure of Ross are used to calculate the temperature dependence of free energy in liquid tungsten. The thermodynamic functions obtained for solid and liquid phases are employed in determination of the melting temperature. © 1995 John Wiley & Sons, Inc. read less

Abstract: The lattice inversion method is used to construct the pair potential between a pair of unlike atoms for Cu-Au and Cu-Pd intermetallic compounds within the framework of Johnson's analytical model of the embedded-atom method. Compared with previous treatises, the alloy potential obtained by the present method is based on the Ll2 superstructures Cu3Au and Cu3Pd as references so that the usual assumption that the pair potential between distinct atoms is a function of monatomic pair potentials is cancelled. The alloy potentials from inversion fall in with those from the average schemes of Foiles el al. and Johnson in the short range but show deviation in the long range. The present method is used to solve the considerable disagreements of Johnson's calculations for the dilute-limit heats of solution and the phase stabilities of the intermetallic compounds of palladium with noble metals. While the overall degree of agreement is substantially improved, it is not good in some cases, nor is the phonon spectrum of gold. read less

Abstract: A model for the embedded-atom method with the use of the lattice inversion method is presented. The lattice sums of electron density and pair potential are assumed to be exponential functions of the lattice parameter. The individual functions of potential and density are inverted from the corresponding lattice sums by using the lattice inversion method. The model parameters are explicitly written by five physical inputs, i.e. the equilibrium lattice constant, the bulk modulus, the Voigt average shear modulus, the sublimation energy, and the unrelaxed vacancy-formation energy. As applications, the 〈100〉 uniaxial stress strain curves in the absence of lateral contraction and shear-mode failure for Cu, Ag, Au, Ni, Pd, and Pt are calculated. The predictions of tensile strengths and failure strains by the present method are found smaller than those by the pair-potential model. The results are in agreement with first-principles calculation of Esposito et al. for Cu and with empirical calculations of Milstein for Ni. read less

Abstract: As most technologically important metals will form oxides readily, any complete study of adhesion at real metal surfaces must include the metal-oxide interface. The role of this ubiquitous oxide layer cannot be overlooked, as the adhesive properties of the oxide or oxide-metal system can be expected to differ profoundly from the adhesive properties of a bare metal surface. We report on the development of a computational method for molecular-dynamics simulations, which explicitly includes variable charge transfer between anions and cations. This method is found to be capable of describing the elastic properties, surface energies, and surface relaxation of crystalline metal oxides accurately. We discuss in detail results using this method for \ensuremath{\alpha}-alumina and several of its low-index faces. read less

Abstract: Analytic expressions for the shear constants of sp-valent zincblende and f.c.c. structure types are obtained using a first-nearest-neighbour bond order potential. Novel expressions for the tetragonal and trigonal zincblende shear constant C' and C 44 are derived. ImportantlyC' is found to vary as the cube of the bond order. The angular character of the bond order potential is shown to remove the anisotropy constraint of C 44/C' = 2 for f.c.c. lattices within a nearest-neighbour model. read less

Abstract: The embedded atom method was used for the design of an atomic potential model for the simulation of the alpha - gamma iron interface. The model takes the form of three cubic expressions covering different ranges, and is normalized to give equilibrium to the two-body potential as well as to the embedding function at the perfect lattice configurations of both the alpha and gamma phases. The model yields good results for surface energies and the vacancy formation energy. Simulations were carried out for alpha - gamma interfaces with the Nishiyama-Wasserman orientation relationship. Interface energies were calculated for interfaces without structural ledges, with monoatomic ledges and with triatomic ledges with results close to the experimental values. The relaxation behaviour of the lattices from their initial configurations was also studied, and it was found that most of the relaxation occurs along the atomic habit planes instead of perpendicular to them. No major effect of different boundary conditions on the lattice relaxation and the interface energy was observed. read less

Abstract: A fast-converging transition-metal pair potential is used to interpret the structural phase transitions of Cr, Mo, and W under pressure. The bcc structure at equilibrium is found to be most stable for these three metals. The elastic constants, bulk modulus, binding energy, and phonon frequencies of these metals at the observed volume are also calculated. The calculated values and experimental results are found to agree within 15-20%, with a few exceptions read less

Abstract: The relation between atomic structure and elastic properties of grain boundaries is investigated theoretically from both atomistic and continuum points of view. A heterogeneous continuum model of the boundary is introduced where distinct phases are associated with individual atoms and possess their atomic level elastic moduli determined from the discrete model. The effective elastic moduli for sub-blocks from an infinite bicrystal are then calculated for a relatively small number of atom layers above and below the grain boundary. These effective moduli can be determined exactly for the discrete atomistic model, while estimates from upper and lower bounds are evaluated in the framework of the continuum model. The complete fourth-order elastic modulus tensor is calculated for both the local and the effective properties. Comparison between the discrete atomistic results and those for the continuum model establishes the validity of this model and leads to criteria to assess the stability of a given grain boundary structure. For stable structures the continuum estimates of the effective moduli agree well with the exact effective moduli for the discrete model. Metastable and unstable structures are associated with a significant fraction of atoms (phases) for which the atomic-level moduli lose positive definiteness or even strong ellipticity. In those cases, the agreement between the effective moduli of the discrete and continuum systems breaks down. read less

Abstract: Relationships between embedded-atom method parameters and the energies of fcc-hcp stability and intrinsic and extrinsic fcc stacking-faults were studied for Cu, Ag, Au, Ni, Pd, and Pt. It was found that the relative magnitudes of these energies for different metals are determined primarily by the physical input data and are almost independent of the cutoff distance and the functions used in the model. These energies increase with increasing vacancy formation energy, decrease with increasing atomic volume and shear modulus, and are almost independent of variations in the cohesive energy and the bulk modulus. However, the shape of the energy versus cutoff distance curve is almost the same for all six metals and is determined primarily by the cutoff distance and the functions used in the model. The shape for a given model is almost independent of the physical input parameters used for fitting to specific metals, can yield either positive or negative values (determined primarily by the cutoff distance), and is similar for all three energies. read less

Abstract: The authors use the method of Mobius transformation introduced by Chen (1990) to obtain pair potentials for FCC metals from first-principles total energy calculations. The derivation is exact for radial potentials and it converges much faster than the earlier reported method of Carlsson-Gelatt-Ehrenreich (1980). They have tested this formulation for Cu using the tight binding representation of the linear muffin-tin orbital method. Their results agree with those obtained by Carlsson and co-workers and qualitatively with the other Morse-type pair potentials derived from effective medium theories. read less

Abstract: We use the embedded-atom potential of Johnson to compute the length-distribution functions for a large number of fcc binary metallic alloys. From these distributions, we extract the mean lengths of the nearest-neighbor bonds, which compare well with recent EXAFS experiment in Ni x Au 1-x or with mean lattice parameter as determined by diffraction experiments read less

Abstract: This work proposes the separable potential method (SPM) as a unified treatment of a class of lattice models. The richness in structure of the unified approach is demonstrated through contact with some of the most important lattice models in the literature, the embedded-atom method (EAM) being one of such models. As a practical application of SPM some fairly extensive results for f.c.c. metallic copper are reported. Ce travail propose le «separable potential method (SPM)» pour le traitement unifie d'une classe des modeles lattice. La richesse en structure de cette approche est mise en evidence en comparant avec des modeles les plus importants trouves dans la litterature tel que l' «embedded atom method (EAM)». Des nombreuses resultats pour c.f.c. Cu sont presentes pour demontrer l'application de la SPM. read less

Abstract: Intermetallics such as the transition metal aluminides present theorists with a challenge since bonding is not well described by currently available pair or embedded atom potentials. We show that a new angularly dependent, many-body potential for the bond order has all the necessary ingredients for an adequate description. In particular, by linearizing the moment-recursion coefficient relations, a cluster expansion is derived which is applicable to any lattice and chemical ordering and which allows a derivation of the earlier ring ansatz. It can account for both the negative Cauchy pressure of cubic metals and the oscillatory behaviour across the transition metal aluminide series of the three-body cluster interaction Φ3. read less

Abstract: Finnis-Sinclair (F-S) type many-body potentials have been constructed for eight hexagonal metals: Co, Zr, Ti, Ru, Hf, Zn, Mg and Be. The potentials are parameterized using cubic splines and fitted to the cohesive energy, unrelaxed vacancy formation energy, five independent second-order elastic constants and two eauilibrium conditions. Hence, each of the constructed potentials represents a stable hexagonal close-packed lattice with a particular non-ideal c/a ratio. In the F-S scheme the many-body part is represented by a sauare root function and this form implies that C12 – C66>0. However, C 12 –C66 is negative for Zn, Be and Ru at Iow temperatures. For this reason a modified many-body function has been employed for these metals. To ensure the applicability of the potentials in modelling of extended lattice defects, the mechanical stability of the corresponding hexagonal close-packed lattice with respect to large homogeneous deformations has been tested. For all the metals considered, the h.c.p. l... read less

Abstract: The requirements for fitting bcc metals within the EAM format are discussed and, for comparative purposes, the EAM format is cast in a normalized form. A general embedding function is defined and an analytic first- and second-neighbor model is presented. The parameters in the model are determined from the cohesive energy, the equilibrium lattice constant, the three elastic constants, and the unrelaxed vacancy formation energy. Increasing the elastic constants, increasing the elastic anisotropy ratio, and decreasing the unrelaxed vacancy formation energy favor stability of a close-packed lattice over bcc. A stable bcc lattice relative to close packing is found for nine bcc metals, but this scheme cannot generate a model for Cr because the elastic constants of Cr require a negative curvature of the embedding function. read less

Abstract: A procedure based on the embedded atom method (EAM) is presented for developing atomistic models for use in computer simulation calculations, with an emphasis on simple but general schemes for matching experimental data with fitting parameters. Both the electron density function and the two-body potential are taken as exponentially decreasing functions and the model is derived for any choice of cutoff distance. The model has been applied successfully to seven fcc and three hcp metals, but the extension to bcc metals was unsuccessful because of difficulty in matching the shear anisotropy ratio. 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: We have introduced two quantum mechanics/molecular mechanics approaches for materials modeling. One is based on quantum mechanical coupling and the other on mechanical coupling. The formalism of both approaches is described in detail. The validations of the methods are demonstrated in terms of atomic and electronic structure. Finally, the applications of the methods are surveyed, including applications in vacancy clusters, dislocations, nanoindentations, and fractures. read less

Abstract: Nanowires and ultra-thin films have wide applications in the quickly developed nanotechnology and nanoscience. However, their Young’s modulus varies with the size, which is seemingly contradictory to the conventional continuum elasticity. Investigating and understanding the underlying mechanism of the size-dependent elastic properties in nanomaterials is of both academic and practical significance. In this work, both theoretical modeling and virtual experiments have been made on this issue. A nanoelement, from the traction free bulk lattice, undergoes an initial relaxation, during which its morphology changes and energy reduces, which is an emphasis in this developed methodology and is a distinction from almost other existing models. With different definitions of surfaces and edges, two models for a nanomaterial – a nanowire or an ultra-thin film – are derived based on the same thermodynamics framework. Comparing with most of others’ treatment, Model I has an advantage to mathematically treat a surface phase to be two-dimensional and an edge phase to be one-dimensional. Under external loadings, the initial relaxed state is taken as the reference. Experimentally, relaxation and tension/compression tests in different loading directions have been conducted on SiC, Si and Cu crystalline nanowires with different cross-sectional sizes and ultra-thin films with different thicknesses by Molecular Dynamics (MD) simulations. This systematic study successfully illustrates the intrinsic mechanism of the size-dependent Young’s modulus in nanomaterials and the proposed methodology facilitate characterizing mechanical properties of nanomaterials to some extent when continuum concepts, such as, surface energy and surface elastic constants, are used. read less

Abstract: Atomic scale computer simulations were used to investigate the surface stress induced deformation in nanoporous metals. A phase field model was used to generate digital nanoporous structures that are quantitatively similar to those created experimentally via dealloying. We analyze the important effects of surface relaxations on the macroscopic deformation in these samples as well as in small spherical clusters. read less

Abstract: An interatomic potential for aluminum was developed, which is based on empirical tight binding approximations. The model successfully reproduced the shear constants, structure energy differences, and phonon dispersion curves. This transferable potential was applied on static surface relaxations, and shows good agreements with experimental results on the oscillatory damped behavior of the multilayer relaxations and the expansion of the (111) surface. read less

Abstract: A scheme to produce density-of-states-(DOS)-dependent potentials for d-metals on the basis of the local density approximation calculations is suggested. As an example this scheme is applied to construct a DOS-dependent potential for tungsten. The second moment of the tungsten DOS is calculated. The authors show that the obtained potentials give a good agreement of cohesive properties with the experimental data. read less

Abstract: We give the closed-form solutions for the two-dimensional adhesive contact of a flat-ended wedge with an elastic half-space, including contact pressure distribution and load-contact width relationship. The approach is derived from contact mechanics in plane-strain elasticity and fracture mechanics concepts. The contact pressure has stress singularities both at the edges of contact due to molecular attractive forces and at the wedge corners, and is compared with those without adhesion. Under zero load, we find the contact strip has a finite width which is greater than that of the wedge end, and the central region of contact is under compression, similar to that of a flat punch problem, while the regions near the contact edges are under tension. Unlike the usual experiments with smooth and low modulus materials, we conduct molecular dynamics (MD) experiments via embedded-atom method (EAM), brownian dynamics algorithm and dynamical theory of crystal lattices. The results, including the “pull-off” force for contacting surfaces to peel apart, conform reasonably well with those derived from a continuum model. read less

Abstract: We report on the development of a novel computational method for molecular dynamics simulations which explicitly includes variable charge transfer between anions and cations. This method is found to be capable of describing the elastic properties, surface energies, and surface relaxation of crystalline metal-oxides accurately. We present results for a simulation of adhesive failure at a model metal/oxide heterophase interface between an aluminum (111) face and an α-alumina (0001) face. Our results indicate that this approach can provide physically realistic empirical potentials for future simulations on mixed metal/metal-oxide systems. read less

Abstract: We discuss the development of interaction potentials which explicitly allow for charge transfer in metallic oxides. The charge transfer is calculated self-consistently using a charge equilibration approach, which allows the amount of charge transferred to respond to the electrostatic environment. We model the metal-metal, metal-oxygen, and oxygen-oxygen interactions with Rydberg function pair potentials. By fitting the Rydberg potential parameters to the elastic and structural constants of the material, we arrive at an efficient model for the simulation of metallic oxides. We demonstrate the applicability of the model by describing some preliminary results on the rutile phase of titanium dioxide. read less

Abstract: A statistical thermodynamical theory based on a density-independent pair potential is first used in the hot crystal to relate the monovacancy formation energy Ev to properties of the liquid at the melting temperature Tm. Whereas the theory works quantitatively for the close-packed crystals Ar and Kr, there are larger deviations between theory and experiment for close-packed metals, and the reasons for this are discussed. For other solids, open structures stand out; b.c.c. metals, such as Na and K, require careful treatment of (a) ionic relaxation and (b) sp hybridization, and the theory is not appropriate for Ge and Si because of the change in the nature of the bonding through Tm. Two other correlations that are discussed are between (i)Ev and rigidity, G, and (ii)Ev and surface energy. Some brief comments are added on the energy of formation of self-interstitials.The second part of the paper is concerned with the consequences of the generalization of the density-independent pair potential theory to binary alloys. The new feature stressed is the size-difference, δ, between the components. A prediction concerning the effect of δ on the vacancy formation energy in a dilute mixture is shown to be borne out in a suitably chosen metallic binary alloy. The paper concludes with some comments on the relation of the present treatment to the embedded atom model predictions for vacancies in cold crystals, which complements the present work. read less

Abstract: This article presents the theoretical evaluation of the structural, mechanical, thermal and electrical properties of half-Heusler (ZrCo1-xNixBi = 0, 0.25, 0.75 and 1) alloys in the framework of density functional theory (DFT) that is implemented in WIEN2k code. Equilibrium lattice parameters are found agree with previous literature. Several calculated mechanical properties are revealed that all studied alloys are mechanically stable. According to the critical values for B/G, Ni-doped ZrCoBi alloys are ductile, whereas ZrCoBi and ZrNiBi are brittle. The band structure and density of states of the present compounds show that ZrCoBi has a semiconducting nature, while Ni-doped ZrCoBi has a half-metallic nature. The structural reforms, brought to ZrCoBi as the Ni-dopant concentration increases at the site of Co-atom, showed an increase in its metallicity, conductivity and ductility, and a decrease in its rigidity, stiffness, minimum thermal conductivity, melting and Debye temperatures. According to the results obtained, ( ZrCo1-xNixBi = 0, 0.25, 0.75 and 1) alloys could have potential thermal and electronic applications. read less

Abstract: As an efficient molecular simulation method, the smoothed molecular dynamics (SMD) method introduces background mesh and mapping process into molecular dynamics (MD) procedure to suppress high‐frequency modes, so that a much larger time step than that of MD can be adopted. SMD method can achieve a nice overall accuracy, but local atomic disorders cannot be described very well with original SMD method for smoothing out high‐frequency motions. An improved SMD method with two kinds of high‐order shape functions is proposed. A parameter indicating the smoothing degree in SMD method is also presented to assess the mapping process. The efficiency and the accuracy of the improved method are discussed in detail, and the proposed method is validated with several examples. The results demonstrate obvious improvement in accuracy, and more high‐frequency atomic motions can be captured. When the high‐order SMD method is coupled with the MD method, the non‐physical phonon reflection, which may occur at the interface in the coupling of MD and original SMD method, will decrease remarkably even if no transition scheme is employed. 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: A-Site doping with alkali ions, and/or metal substitution at the B and B′-sites, are among the key strategies in the innovative development of A2BB′X6 halide double perovskite semiconducting materials for application in energy and device technologies. To this end, we have investigated an intriguing series of five halide-based non-toxic systems, A2AgRhCl6 (A = Li, Na, K, Rb, and Cs), using density functional theory at the SCAN-rVV10 level. The lattice stability and bonding properties emanating from this study of A2AgRhCl6 matched well with those that have already been synthesized, characterized and discussed [viz. Cs2AgBiX6 (X = Cl, Br)]. Exploration of traditional and recently proposed tolerance factors has enabled us to identify A2AgRhCl6 (A = K, Rb and Cs) as stable double perovskites. The band structure and density of states calculations suggested that the electronic transition from the top of the valence band [Cl(3p)+Rh(4d)] to the bottom of the conduction band [(Cl(3p)+Rh(4d)] is inherently direct at the X-point of the first Brillouin zone. The (non-spin polarized) bandgap of these materials was found in the range 0.57–0.65 eV with SCAN-rVV10, which were substantially smaller than those computed with hybrid HSE06 and PBE0, and quasi-particle GW methods. This, together with the appreciable refractive index and high absorption coefficient in the region covering the range 1.0–4.5 eV, enabled us to demonstrate that A2AgRhCl6 (A = K, Rb, and Cs) are likely candidate materials for photoelectric applications. The results of our phonon calculations at the harmonic level suggested that the Cs2AgRhCl6 is the only system that is dynamically stable (no imaginary frequencies found around the high symmetry lines of the reciprocal lattice), although the elastic moduli properties suggested all five systems examined are mechanically stable. read less

Abstract: A first-principles calculation program is used for investigating the structural, mechanical, and electronic properties of the cubic NiTi shape-memory alloy (SMA) with the B2 phase under high pressure. Physical parameters including dimensionless ratio, elastic constants, Young’s modulus, bulk modulus, shear modulus, ductile-brittle transition, elastic anisotropy, and Poisson’s ratio are computed under different pressures. Results indicate that high pressure enhances the ability to resist volume deformation along with the ductility and metallic bonds, but the biggest resistances to elastic and shear deformation occur at P = 35   GPa for the B2-phase NiTi SMA. Meanwhile, the strong anisotropy produced by the high pressure will motivate the cross-slip process of screw dislocations, thereby improving the plasticity of the B2-phase NiTi SMA. Additionally, the results of the density of states (DOS) reveal that the B2-phase NiTi SMA is essentially characterized by the metallicity, and it is hard to induce the structural phase transition for the B2-phase NiTi SMA under high pressure, which provides valuable guidance for designing and applying the NiTi SMA under high pressure. read less

Abstract: In this work, structural, thermal, electro-magnetic and thermoelectric attributes of CoNb1−xTixSn (x  =  0, 0.75, 0.5, 0.25, 0) alloys have been investigated using density functional theory (DFT). The structural reforms, brought to CoNbSn, portray the increase in its rigidity when increasing the Ti content substituted at the Nb site. They also remodel the character of the alloy from semiconducting paramagnetic to half-metallic ferromagnetic nature. By investigating elastic properties that are interlinked with structural optimizations and enthalpy of formation, studied alloys displayed stable structure. Thermoelectric properties such as Seebeck coefficient (S), electrical conductivity (σ/τ) and power factor (S2 σ/τ), are calculated based on the Boltzmann transport theory. Results revealed that for x  =  0.75 in CoNb1−xTixSn, a temperature dependent switch from n-type to p-type is observed. According to the results obtained, CoNb1−xTixSn alloys could have potential thermoelectric applications. read less

Abstract: The mechanical, electronic and vibrational properties of Ir-based refractory superalloys (Ir 3 Hf and Ir 3 Nb) in the L1 2 structure were studied in the framework of ab initio calculations. The obtained equilibrium lattice constants and bulk modulus were reported and compared with the existing data. The elastic constants of alloys were determined using energy strain method. The results were utilised to evaluate mechanical stability of alloys in the crystal structure of L1 2 .  Both alloys were found to be mechanically stable based on the Pugh’s criteria. Subsequently, electronic band structures and partial and total densities of states have been obtained for Ir 3 Hf and Ir 3 Nb. The band structures of alloys demonstrated metallic behaviour whilst the conductivity was mostly governed by Ir 5d states. Moreover, phonon distribution curves of both alloys were obtained by employing the linear response technique within the density functional theory. Both alloys are found to be dynamically stable based on phonon modes evaluation. read less

Abstract: A calculation program based on the density functional theory (DFT) is applied to study the structural, mechanical, and electronic properties of TiV alloys with symmetric structure under high pressure. We calculate the dimensionless ratio, elastic constants, shear modulus, Young’s modulus, bulk modulus, ductile-brittle transition, material anisotropy, and Poisson’s ratio as functions of applied pressure. Results suggest that the critical pressure of structural phase transition is 42.05 GPa for the TiV alloy, and structural phase transition occurs when the applied pressure exceeds 42.05 GPa. High pressure can improve resistance to volume change, as well as the ductility and atomic bonding, but the strongest resistances to elastic and shear deformation occur at P = 5   GPa for TiV alloy. Furthermore, the results of the density of states (DOS) indicate that the TiV alloy presents metallicity. High pressure disrupts the structural stability of the TiV alloy with symmetry, thereby inducing structural phase transition. read less

Abstract: ABSTRACT Atomistic simulations are used to investigate the mechanical properties of silver nanowires (NWs) with kinked twin boundaries (TBs) under tensile loading. For comparison, a different ledge width of twinned NWs with both square and circular kink-steps are considered in this study. The embedded-atom-method potential is employed to describe the atomic interactions. To identify the defect evolution and incipient plastic deformation mechanism, the centrosymmetry parameter is implemented in our self-developed programme. Twinned NWs with both square and circular ledges are shown to have a reduced impact on yield stress as compared to their perfect TBs counterpart models in elastic deformation. In twinned NWs with rectangular ledges, a strain-hardening effect was observed in defective NWs. The incipient plastic deformation is influenced by the ledge width. While in twinned NWs with circular ledges, the ledges rather than the surface effect are the only dislocation source in their incipient plastic deformation. Our findings offer a view of imperfection in twinned NWs, and it is believed that the attention being paid to defective TBs will be helpful to further understanding of the mechanical properties of TB-strengthened NWs. read less

Abstract: The molecular dynamics simulation method is utilized to investigate the physical characteristics of nanoscale vanadium powder during 3D printing powder bed fusion laser sintering process. The radius of gyration, neck width, and root mean square of different powder size of nanoscale vanadium powder during 3D printing laser sintering process under different heating rate is analyzed and discussed. The neck width and internal lattice changes are also observed.It is observed that the temperature of solid state diffusion decrease with heating rate increasing, but the temperature of that increase with powder size decreasing. It is found that the coalescence temperature of nanoscale vanadium powder is range of between 1600K and 1950K, and the melting temperature of that is range of between 1850K and 1990K. read less

Abstract: ABSTRACT We studied the first Brillouin zone of the hexagonal close-packed (HCP) structure and the improved ones of the modified analytical embedded atom method (EAM) potentials were adopted to calculate the properties of the mono- and bi-vacancies and the phonon dispersions for HCP transition metals Ru, Sc, Ti, Y, and Zr. The agreements with the experimental data showed that the improved potentials for the HCP transition metals are available. read less

Abstract: An interatomic potential for the Ni–Mo binary alloy focusing on irradiation has been constructed with the modified analysis embedded atom method. The newly developed interatomic (Ni–Ni and Mo–Mo) potentials and the Ni–Mo cross-interactions are fitted to the ab initio results and experimental data, including defect energies, formation energies of three stable phases. The properties used for fitting are accurately reproduced by the present potentials for both pure elements and alloy systems. Those properties beyond the fitting ranges are also well predicted, demonstrating its excellent transferability. The advantages and certain weaknesses of the new potential are also discussed in detail compared with other existing potentials. The potential is expected to be especially suitable for irradiation simulations of Ni–Mo alloys. read less

Abstract: ABSTRACT The structural, elastic and electronic properties of Co7M6 (M = W, Mo, Nb) μ phases were investigated by first-principles calculations based on the density functional theory (DFT). The calculated cohesive energy indicates that Co7M6 (M = W, Mo, Nb) μ phases are thermodynamically stable. Besides, Co7W6 owns a higher structural stability than that of Co7Mo6 and Co7Nb6. The obtained elastic constant demonstrates that Co7M6 (M = W, Mo, Nb) are mechanically stable. With Voigt-Reuss-Hill (VRH) approximation, the elastic bulk modulus (B), shear modulus (G), Young's modulus (E) and Poisson's ratio (ν) were derived. The ductility and plasticity as well as the elastic anisotropy of the three phases were discussed in details. Finally, the density of states and charge density difference were also analysed to reveal the underlying mechanism of structural stability and the elastic properties. read less

Abstract: In this study, molecular dynamics (MD) are used to simulate the nanoimprinting behaviors of Ag thin films with impurities using a cuboid diamond punch. The results of the simulation show that the imprinting topography of Ag thin films depends on the slip behaviors of dislocations. In the punch indenting process, dislocations would be interfered by impurities defects. In the drafting process, the forming holes appear spring back and micro plastic recovery phenomenon, and the impurities affect the both behaviors. Although the impurities can result in a bigger springback, a better surface roughness can be obtained. read less

Abstract: We demonstrate that the embedded-atom method and related potentials predict many dimensionless properties of simple metals to depend predominantly on a single coefficient μ, which typically lies between 0.3 and 0.45. Among other relations presented in this work, we find that Ec∝Zμ, Ev/Ec=μ, and G/B∝μ hold within 25% accuracy and also find a linear dependence of the melting temperature on μ. The used variables are cohesive energy Ec, coordination number Z, vacancy energy Ev, and bulk modulus B, while G is the average of ordinary and tetragonal shear modulus. We provide analytical arguments for these findings, which are obeyed reasonably well by several metals. read less

Abstract: X-ray diffraction analysis, transmission electron microscopy, and thermodynamic calculation were used to investigate the effect of microstructural condition of austenite on the microstructural characteristics of the nanoscale bainite ferrite in a high carbon steel. As austenization temperature increases to 950 °C, there are a higher vacancy concentration and homogenized distribution level of the interstitial carbon atom in the austenite grains. The movement of more di-vacancies combination could encourage the generation of the γ → α embryo nucleus. The interstitial carbon atoms have a stronger inhibitory effect on the formation of the γ → α embryo nucleus and homogenized distribution of the interstitial carbon atoms are able to make the inhibitory effect exist everywhere in the austenite grains. In consequence, the bainite ferrite could only nucleate in a smaller area (several nanometers), and grow into slender laths in a smaller width and a larger length. read less

Abstract: We report ab initio study of the formation of chemically ordered athermal ω phase (trigonal symmetry, space group , non-ideal ω, ω′) in Zr3Al2Nb and (trigonal symmetry, space group P3m1, ideal ω) in Zr4AlNb alloy by coupled replacive–displacive transformation mode. The phase stability of body-centred cubic (bcc), non-ideal ω (ω′) and ideal ω structures of Zr3Al2Nb and Zr4AlNb was examined by first-principles calculations with projector augmented wave potentials and generalized gradient approximations. Position of Nb atom in the bcc lattice was decided by examining several atomic configurations, and it was also shown that the stability of each configuration is directly related to the number of Zr–Al bonds. Among these configurations, the most stable bcc Zr3Al2Nb and Zr4AlNb configurations showed instability for atomic displacements leading to ω-type structure in our ground-state energy calculations. In order to estimate strengths of Zr–Al, Nb–Al and Zr–Nb bonds, the heats of the formation for several virtual compounds were calculated and we find that the strength of Zr–Al bonds is higher than Nb–Al and Zr–Nb bonds. It was also confirmed that the formation of the ω phase in Zr3Al2Nb and Zr4AlNb is a combined displacive–replacive transformation. Moreover, our mechanical stability analysis predicts that both ω′-Zr3Al2Nb and B82-Zr2Al are mechanically stable phases with very poor ductility. In contrast, less pronounced directional bonding in ω-Zr4AlNb indicates this alloy to be ductile. read less

Abstract: First-principles calculations are used to investigate the structural, elastic and vibration properties of the binary intermetallic compounds AgRE (RE = Ho, Er, Tm) with a B2 (CsCl) structure using density functional theory. The calculated structural parameters, the bulk modulus and its derivative with respect to pressure are in good agreement with experimental and numerical data. Additionally, the negative energy of formation of AgRE (RE = Ho, Er, Tm) with a B2 structure is examined. The independent second order elastic constants and their related properties, which are essential for mechanical stability, are investigated under pressure (0–60 GPa). The phonon spectra and phonon density of states are also discussed. All calculated phonon modes for AgRE (RE = Ho, Er) are positive in the phonon dispersion diagrams. read less

Abstract: A systematical ab initio analysis on MP2O7 (M = Ti, Hf) is presented in this work. Density functional theory (DFT) computations were performed for the electronic, mechanical, and thermal properties of MP2O7. Heterogeneous bonding nature of MP2O7 was revealed by examining the structural and electronic properties, M–O bonds were weaker than P–O bonds. The elastic constants and polycrystalline mechanical properties of MP2O7 were reported. Based on the low shear-modulus-to-bulk-modulus ratios and positive Cauchy pressure, MP2O7 ceramics were predicted to be “quasi-ductile”. In addition, the minimum thermal conductivities were estimated to be 1.52 and 0.99 W·m−1·K−1 for TiP2O7 and HfP2O7, respectively. The ultra-low thermal conductivities were contributed to the lattice phonon scattering due to the heterogeneous bonding nature. Our theoretical results emphasize the importance of weak M–O bonds in the determination of mechanical and thermal properties of MP2O7. read less

Abstract: The formation of helium clusters in a copper crystal has been studied by means of ab initio calculations. Several He atoms have been placed either inside an n vacancy previously created or as interstitials inside the initially perfect bulk matrix. Based on density functional theory techniques, our results show that the first He atom inside the perfect crystal prefers a tetrahedral position instead of an octahedral as previously reported. When n vacancies are formed in the structure, He atoms start to aggregate forming small bubbles at these sites rather than at interstitial positions. The calculated formation and binding energies confirm the deep trapping and the stability of He atoms inside vacancies, as is well known for other metals. For a given number of He atoms inside an n vacancy, NHe, the minimum formation energy is found when NHe is equal to the number of vacancies n. Within each n vacancy, the formation energy increases (almost) linearly with the number of He atoms until NHe reaches the number of vacancies n. From this point onwards, the addition of new He atoms to the system implies a higher energy cost and consequently an abrupt decrease in the binding energy. read less

Abstract: Thermal stability is one of the main concerns for the synthesis of hollow nanoparticles. In this work, molecular dynamics simulation gave an insight into the atomic reconstruction and energy evolution during the collapse of hollow gold nanoballs, based on which a mechanism was proposed. The stability was found to depend on temperature, its wall thickness and aspect ratio to a great extent. The relationship among these three factors was revealed in geometric thermal phase diagrams (GTPDs). The GTPDs were studied theoretically, and the boundary between different stability regions can be fitted and calculated. Therefore, the GTPDs at different temperatures can be deduced and used as a guide for hollow structure synthesis. read less

Abstract: Nano-indentation is a sophisticated method to characterize mechanical properties of materials. This method samples a very small amount of material during each indentation. Therefore, this method is extremely useful to measure mechanical properties of nano-materials. The measurements using nanoindentation is very sensitive to the surface topology of the indenter and the indenting surfaces. The mechanisms involved in the entire process of nanoindentation require an atomic level understanding of the interplay between the indenter and the substrate. In this paper, we have used atomistic simulation methods with empirical potentials to investigate the effect of various types of pristine indenter on the defect nucleation and growth. Using molecular dynamics simulations, we have predicted the load-depth curve for conical, vickers, and sperical tip. The results are analyzed based on the coherency between the indenter tip and substrate surface for a fixed depth of 20 A. The depth of defect nucleation and growth is ... read less

Abstract: The surface relaxation and the formation of a single vacancy in very thin Cu (001) film formed by 2 ∼ 14 atomic layers have been studied by using MAEAM and MD simulation. For the surface relaxtion, the highest surface energy is in the l = 2 atomic layers. The multilayer relaxation mainly occurs between the first two atomic layers, and the maximum contractive displacement is obtained in the very thin Cu (001) film formed by l = 3 atomic layers. For the vacancy formed in l′ = 1 of the very thin Cu (001) film formed by l = 2 ∼ 14 layers, the most difficult site in the film formed by l = 3 atomic layers. read less

Abstract: We present an embedded atom method (EAM) potential for hexagonal beryllium, with a pair function in the form of a Morse potential and a Johnson embedding function with exponential electron density. The cohesive energy, elastic constants, lattice parameters and relaxed vacancy formation energy were used to fit the potential. The fitted-potential was validated by a comparison to first-principles and, wherever available, experimental results for the lattice energies of various crystal structures: vacancy cluster, interstitial formation and surface. Using a large cutoff distance of 5 Å, which includes interactions to approximately the eighth neighbor shell of beryllium, allows our potential to reproduce these quantities considerably better than previous EAM potentials. The accuracy obtained by our potential is similar to or in some cases even better than available modified EAM potentials, while being computationally less intensive. read less

Abstract: The interatomic interaction potential of tungsten and thorium crystals and those of hypothetical tungsten and thorium alloys within the embedded atom approach are considered. The corresponding Ansatz functions are fitted against full potential linear augmented plane wave data of real tungsten- and thorium- and hypothetical tungsten-thorium-crystals. The result is interatomic potentials, ready for use within classical molecular dynamics schemes. A cross check of the resulting force scheme derived by comparison of ab initio and classical molecular dynamics data is provided. Furthermore, we used the potentials to calculate the phonon dispersion relations, which then serve as an additional check. read less

Abstract: Ohmic RF-MEMS switches hold much promise for low power wireless communication, but long-term degradation currently plagues their reliable use. Failure in these devices occurs at the contact and is complicated by the fact that the same asperities that bear the mechanical load are also important to the flow of electrical current needed for signal processing. Materials selection holds the key to overcoming the barriers that prevent widespread use. Current efforts in materials selection have been based on the material's (or alloy's) ability to resist oxidation as well as its room-temperature properties, such as hardness and electrical conductivity. No ideal solution has yet been found via this route. This may be due, in part, to the fact that the in-use changes to the local environment of the asperity are not included in the selection criteria. For example, Joule heating would be expected to raise the local temperature of the asperity and impose a non-equilibrium thermal gradient in the same region expected t... read less

Abstract: A novel n-body potential for an Zr–Nb system was developed in the framework of the embedded-atom method. All the parameters of the constructed potential have been systematically evaluated by fitting to the ground state properties obtained from experimental measurements and first-principles calculations for pure elements and some alloys. It is shown that most of the static thermodynamics properties for Zr and Nb can be well reproduced by using the present potential. Some calculation results based on the present model are even closer to the experimental data than those based on previous potential models. The ground state properties of hypothetical Zr–Nb alloys were also calculated and found to be in agreement with first-principles calculations. Furthermore, the formation energies of random solid solutions of Zr–Nb with lattices of body centered cubic (bcc) and hexagonal close packed (hcp) type were calculated by fitting the energy–volume relations to Rose’s equation of state. These values were compared with those obtained by first-principles calculations based on special quasirandom structure models and the Miedema-ZSL-07 model (the improved Miedema model proposed by Zhang, Sheng and Liu in 2007). It is indicated that our n-body constructed potential for a Zr–Nb alloy provides an effective description for the interaction between the dissimilar ion interactions for hcp–bcc systems. read less

Abstract: Severe adhesive wear on a rough aluminum (Al) substrate is simulated by a hard Lennard-Jones asperity impacting an Al-asperity at high speeds using molecular dynamics (MD). Multiple simulations investigate the effects of variations in the inter-asperity bonding, the geometric overlap between two asperities, the relative impact velocity and the starting temperature. The effect of these experimental variables on degree of adhesive wear and the temperature profiles are discussed, and a design of experiments method is used to help interpret the results. The results indicate that increasing the inter-asperity bonding, the geometric overlap and the starting temperature of two asperities will substantially increase the wear rate, while raising the impact velocity slightly decreases the wear rate. It is observed that the deformation mechanism involves local melting and the formation of a liquid like layer in the contact area between two asperities, and the amorphous deformation of the Al-asperity. read less

Abstract: The structural, elastic, electronic, optical, and vibrational properties of cubic PdGa compound are investigated using the norm-conserving pseudopotentials within the local density approximation (LDA) in the framework of the density functional theory. The calculated lattice constant has been compared with the experimental value and has been found to be in good agreement with experimental data. The obtained electronic band structures show that PdGa compound has no band gap. The second-order elastic constants have been calculated, and the other related quantities such as the Young's modulus, shear modulus, Poisson's ratio, anisotropy factor, sound velocities, and Debye temperature have also been estimated. Our calculated results of elastic constants show that this compound is mechanically stable. Furthermore, the real and imaginary parts of the dielectric function and the optical constants such as the electron energy-loss function, the optical dielectric constant and the effective number of electrons per unit cell are calculated and presented in the study. The phonon dispersion curves are also derived using the direct method. read less

Abstract: We present an embedded-atom-method (EAM) model that accurately describes the vibrational dynamics in the alkali metals Li, Na, K, Rb and Cs. The bulk dispersion curves, frequency-moment Debye temperatures and temperature-dependent entropy Debye temperatures are all in excellent agreement with experimental results. The model is also well suited for studying surface vibrational dynamics in these materials, as illustrated by calculations for the Na(110) surface. read less

Abstract: The Cu coverage of trench filling enhanced by different low-index surfaces of tantalum in physical vapor deposition is studied by molecular dynamics simulation with the embedded atom method (EAM) as the interaction potential for the present alloy metal system. The deposition morphologies and bottom step coverage enhancement of trenches with three different aspect ratios are examined. It is found that the Cu adatom on Ta(110) with uniform and low surface diffusion barrier energy and that on Ta(111) with high surface energy lead to the improvement of the surface diffusion of Cu adatoms. The shadowing effect is inhibited on Ta(110) and Ta(111) such that the bottom step coverage of the trench is enhanced markedly at an early stage and the final coverage of trench filling is improved significantly. Also, the texture of deposition on the trench with Ta(110) has a uniform structure owing to the low surface energy, while that with Ta(111) has a nonuniform structure owing to the high surface energy on the sidewall. read less

Abstract: We report first-principles calculations of the structural, electronic, elastic, and vibrational properties of the semiconducting orthorhombic ZnSb compound. We study also the intrinsic point defects in order to eventually improve the thermoelectric properties of this already very promising thermoelectric material. Concerning the electronic properties, in addition to the band structure, we show that the Zn (Sb) crystallographically equivalent atoms are not exactly equivalent from the electronic point of view. Lattice dynamics, elastic, and thermodynamic properties are found to be in good agreement with the experiments and they confirm the nonequivalency of the zinc and antimony atoms from the vibrational point of view. The calculated elastic properties show a relatively weak anisotropy and the hardest direction is the $y$ direction. We observe the presence of low energy modes involving both Zn and Sb atoms at about 5--6 meV, similar to what has been found in Zn${}_{4}$Sb${}_{3}$, and we suggest that the interactions of these modes with acoustic phonons could explain the relatively low thermal conductivity of ZnSb. Zinc vacancies are the most stable defects, and this explains the intrinsic $p$-type conductivity of ZnSb. read less

Abstract: The Metropolis Monte Carlo method is used to demonstrate the relationship between the bulk phase diagram of alloys with limited miscibility and the equilibrium configurations of nanoparticles. Using the Au–Pt system as a case study, an embedded atom potential is parametrized so as to match the phase diagram exactly. The smooth temperature dependence of the short-range order parameter is shown correlated with an onion-like configuration intermediate between solid solution and phase separated states. read less

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

Abstract: A new interatomic potential is developed for the Al/Si system in the formulation of the recently developed angular-dependent embedded atom method (A-EAM). The A-EAM is formulated by combining the embedded atom method potential for Al with the Stillinger–Weber potential for Si. The parameters of the Al/Si cross-interactions are fitted to reproduce the structural energetics of Al/Si bulk alloys determined based on the results of density functional theory calculations and the experimentally observed mixing behavior of the AlSi liquid alloy at high temperatures. The ability to investigate the thermodynamic properties of the Al/Si system is demonstrated by computing the binary phase diagram of the Al–Si system as predicted by the A-EAM potential and comparing with that obtained using experiments. The ability to study the mechanical behavior of the Al/Si composite systems is demonstrated by investigating the micromechanisms related to dynamic failure of the Al/Si nanocomposites using MD simulations. read less

Abstract: The electronic state and potential data of U2 molecules are performed by first principle calculations with B3LYP hybrid exchange-correlation functional, the valence electrons of U atom are treated with the (5s4p3d4f)/[3s3p2d2f] contraction basis sets, and the cores are approximated with the relativistic effective core potential. The results show that the ground electronic state is X9Σ+g. The pair potential data are fitted with a Murrell-Sorbie analytical potential function. The U-U embedded atom method (EAM) interatomic potential is determined based on the generalized gradient approximation calculation within the framework of the density functional theory using Perdew-Burke-Ernzerhof exchange-correlation functional at the spin-polarized level. The physical properties, such as the cohesive energy, the lattice constant, the bulk modulus, the shear modulus, the sc/fcc relative energy, the hcp/fcc relative energy, the shear modulus and the monovacancy formation energy are used to evaluate the EAM potential parameters. The U-U pair potential determined by the first principle calculations is in agreement with that defined by the EAM potential parameters. The EAM calculated formation energy of the monovacancy in the fcc structure is also found to be in close agreement with DFT calculation. read less

Abstract: With modified analytical embedded-atom method and molecular dynamics simulation, this paper simulates the strain energy and the equilibrium core structure of a〈100〉 edge dislocation in BCC metal iron on atomistic scale. In addition, the trapping effect of dislocation on vacancy is investigated as well. The results show that the equilibrium dislocation core is quite narrow and has a C2v symmetry structure. Calculated strain energy Es of the dislocation is a linear function of ln(R/2b) while R ≥ 5.16 A (1 A = 0.1 nm), in excellent agreement with the elasticity theory prediction. Determined core radius and energy are 5.16 A and 0.62 eV/A, respectively. The closer the vacancy to the dislocation line is, the lower the vacancy formation energy is, this fact implies that the dislocation has a trend to trap the vacancy, especially for a separation distance of the vacancy from dislocation line being less than two lattice constants. read less

Abstract: The structural evolution of misfit dislocation networks at γ/γ′ phase interfaces in Ni-based single crystal superalloys under tensile loading and temperatures is simulated by molecular dynamics. From the simulation, we find that, with increasing load or temperature, the patterns of dislocation networks on the (100), (110) and (111) phase interfaces change from regular to irregular or disappear. Under the same load and temperature, the dislocation networks of the different phase interfaces show different degrees and patterns of damage. The density and stability of the dislocation networks decrease with increasing temperature. When the interfacial dislocation networks become more regular, the γ/γ′ interfaces become more stable. The simulated results are supported by related experimental findings. Moreover, based on MD simulations, the averaged stress–strain responses for different phase interfaces under loading are presented. The results indicate that the combined influences of temperature and load play an important role for the structure evolution of misfit dislocation networks at γ/γ′ phase interfaces of Ni-based superalloys. read less

Abstract: New parameters of nearest-neighbor EAM (1N-EAM), n-th neighbor EAM (NN-EAM), and the second-moment approximation to the tight-binding (TB-SMA) potentials are obtained by fitting experimental data at different temperatures. In comparison with the available many-body potentials, our results suggest that the 1N-EAM potential with the new parameters is the best description of atomic interactions in studying the thermal expansion of noble metals. For mechanical properties, it is suggested that the elastic constants should be calculated in the experimental zero-stress states for all three potentials. Furthermore, for NNEAM and TB-SMA potentials, the calculated results approach the experimental data as the range of the atomic interaction increases from the first-neighbor to the sixth-neighbor distance. read less

Abstract: The present work develops a physically reliable procedure for building the embedded-atom-method (EAM) interatomic potentials for the metals with fcc, bcc and hcp structures. This is mainly based on Chen–Möbius lattice inversion (Chen et al 1997 Phys. Rev. E 55 R5) and first-principles calculations. Following Baskes (Baskes et al 2007 Phys. Rev. B 75 094113), this new version of the EAM eliminates all of the prior arbitrary choices in the determination of the atomic electron density and pair potential functions. Parameterizing the universal form deduced from the calculations within the density-functional scheme for homogeneous electron gas as the embedding function, the new-type EAM potentials for Cu, Fe and Ti metals have successfully been constructed by considering interatomic interactions up to the fifth neighbor, the third neighbor and the seventh neighbor, respectively. The predictions of elastic constants, structural energy difference, vacancy formation energy and migration energy, activation energy of vacancy diffusion, latent heat of melting and relative volume change on melting all satisfactorily agree with the experimental results available or first-principles calculations. The predicted surface energies for low-index crystal faces and the melting point are in agreement with the experimental data to the same extent as those calculated by other EAM-type potentials such as the FBD-EAM, 2NN MEAM and MS-EAM. In addition, the order among the predicted low-index surface energies is also consistent with the experimental information. read less

Abstract: In this paper, we represented our numerical results on LIBS (laser-induced breakdown spectroscopy) process in Lead, excited by intense laser pulses. We used the EAM scheme and the proposed potential form for Lead in molecule dynamics simulation of the ablated process without taking into account the complex solid-liquid-gas phase transition process, which can greatly simplify phase variation of Lead and provide good insight into energies of dislodged ions and crater formation. In our work, we mainly focused on the crater morphology during the laser ablation process, and made comparison of erosion processes for different pulse durations. For the plasma emission, we used statistical method to study the energy distribution of the jetted Lead ions and analyzed the its relevance with the spectral line intensity of Lead, which contributed to the characteristic 406-nm peak of Lead. read less

Abstract: The micromechanisms related to ductile failure during dynamic loading of nanocrystalline Cu are investigated in a series of large-scale molecular-dynamics MD simulations. Void nucleation, growth, and coalescence are studied for a nanocrystalline Cu system with an average grain size of 6 nm under conditions of uniaxial tensile strain and triaxial tensile strain at a strain rate of 10 8 s �1 . The MD simulations of deformation of the nanocrystalline system under conditions of triaxial tensile stress show random nucleation of voids at grain boundaries and/or triple point junctions. The initial shape of the voids is nonspherical due to growth of the voids along the grain boundaries. Void growth is observed to occur by the creation of a shell of disordered atoms around the voids and not by nucleation of dislocations from the void surface. Void coalescence occurs by the shearing of the disordered regions in between the voids. The nucleation and growth of voids result in the relaxation of tensile stresses, after which growth of the voids is slower. The slower growth is accompanied by recrystallization of the surrounding disordered regions resulting in near-spherical shapes of the voids. read less

Abstract: The embedded atom method (EAM) is used to construct an interatomic potential for modelling interfaces in Cu–Nb nanocomposites. Implementation of the Ziegler–Biersack–Littmark (ZBL) model for short-range interatomic interactions enables studies of response to ion bombardment. Collision cascades are modelled in fcc Cu, bcc Nb, and in Cu–Nb layered composites in the experimentally-observed Kurdjumov–Sachs (KS) orientation relation. The interfaces in these composites reduce the number of vacancies and interstitials created per keV of the primary knock-on atom (PKA) by 50–70% compared to fcc Cu or bcc Nb. read less

Abstract: In this work we review some recent research on the surface diffusion-mediated decay of two-dimensional nanostructures. These results include both a continuous, vectorial model and a discrete kinetic Monte Carlo approach. Predictions from the standard linear continuous theory of surface-diffusion-driven interface decay are contrasted with simulational results both from kinetic and morphological points of view. In particular, we focused our attention on high-aspect-ratio nanostructures, where strong deviations from linear theory take place, including nonexponential amplitude decay and the emergence of several interesting nanostructures such as overhangs developing, nanoislands and nanovoids formation, loss of convexity, nanostructures-pinch off and nanostructures-break off, etc. read less

Abstract: Three-dimensional molecular dynamics simulations are conducted to investigate the effect of reciprocating nanomachining process on the subsurface damaged layers, surface integrity, cutting force, stress variation of subsurface, and changes of energy and defects in the workpiece. Results show that there is no obvious shear zone ahead the tool during nanomachining. Dislocation nucleations are near the free surface ahead the tool and the interface of the tool and the workpiece and propagate in the surface and downward in the workpiece. There are the generations of dislocation jog and dislocation loops ahead the tool during the reciprocating cutting. The values of the reciprocating cutting force for the (111) orientation and (100) orientation under offset distance of 0a0 are not zero but 11.756 and 13.0498 nN, respectively. When the offset distance of the tool is up to 10a0, the ratio of primary cutting force to reciprocating force is nearly 90%. The shape of the machined groove in the (111) orientation remai... read less

Abstract: In the context of the investigation of particle formation, a potential model by means of the embedded atom method is developed for the hexagonal close packed metal zinc. This type of model includes many-body interactions caused by delocalised electrons in metals. The effective core charge as function of the distance is calculated here by an integral over the electron distribution function rather than fitting it to experimental data. In addition, the dimer potential is included in the parameterisation because we focus on the formation of nanoparticles from the vapour phase. With this potential model, the growth of zinc clusters consisting of 125 to 1000 atoms is investigated, which takes place at elevated temperatures in a liquid-like cluster state. The growing clusters are embedded in an argon carrier gas atmosphere which regulates the cluster temperature. The average thermal expansion of the clusters and the different lattice constants are analysed. For the determination of the cluster structure, the common-neighbour analysis method is extended to hexagonal close packed surface structures. During growth, small clusters with less than approximately 60 atoms develop transient icosahedral structure before transforming into hexagonal close-packed structure. The surface of the clusters exhibits a transformation from planes with high surface energy to the most stable ones. Besides ambiguous surface structures the final clusters are almost completely in an hexagonal close packed structure. read less

Abstract: A multiscale model is developed to study to the femtosecond laser single pulse and pulse train processing of the metal films. In our model, molecular dynamics simulation combined with the improved two-temperature model is employed in the ablation area and the improved two-temperature model is applied in heat-affected zone. This paper extends the improved two-temperature model to describe higher laser fluences processing by introducing the phase change. The phase change mechanisms of the non-equilibrium thermal melting and vaporization are both analyzed, which has a strong impact on the lattice temperature evaluation. The model can simulate phase change process of gold with higher accuracy. It is demonstrated that the pulse train could improve the fabrication accuracy, repeatability, and controllability. read less

Abstract: The structure and binding energy of copper clusters of the size range 70 to 150 were studied by using the embedded-atom method. The stability of the structure of the clusters was studied by calculating the average binding energy per atom, first difference energy and second difference energy of copper cluster. Most of the copper clusters of the size n = 70–150 adopt an icosahedral structure. The results show that the trends are in agreement with theoretic prediction for copper clusters. The most stable structures for copper clusters are found at n = 77, 90, 95, 131, 139. read less

Abstract: Both the formation energies and the intra‐ and inter‐layer diffuse activation energies of a vacancy in the first six lattice planes of Mg (001) surface have been calculated by combining the modified analytical embedded‐atom method (MAEAM) with molecular dynamics (MD). The results show that the effect of the surface on the formation and migration of the vacancy is only down to the third‐layer. It is easer for a single vacancy to form and to migrate in the first layer. Furthermore, the vacancy in the second layer is favorable to migrate to the first layer. This is in agreement with the experimental results that the first layer has the highest concentration of the vacancy. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) read less

Abstract: Based on the atomic local structure, a nanocrystalline material is separated into two parts: inner grains and grain boundaries. Molecular dynamics simulations have been performed to study the difference in their vibrational properties in nanocrystalline nickels. The results obtained reveal that a similar phonon frequency softening as in the perfect lattice mainly focuses on grains in nanocrystalline materials, and for the grain boundary part there is no obvious change in the vibrational density of states in the temperature range 300–900 K, especially in the lower frequency range. Comparing with conventional crystals, the higher specific heat and vibrational entropy, and lower vibrational free energy with decreasing mean grain size, result from the high proportion of grain boundaries in nanocrystals. In addition, the composite model provides a good description of the contributions of grain and grain boundary phases to thermodynamic quantities. Supposing the nanocrystalline material can be treated as a composite composed of grain and grain boundary network, the vibrational thermodynamic quantities of the system can be well determined from the proportion of grain boundaries and the corresponding thermodynamic quantities of grain and grain boundary parts. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) read less

Abstract: Molecular statics method incorporating minimum energy concept was employed to study the one-dimensional copper nanospring with faced-center-cubic crystal structure. Various geometric sizes (wire diameter, radius, pitch), numbers of turns and crystal orientations of nanosprings were systematically modeled to investigate the size dependence of elastic properties. It was observed that as the wire diameter increases and the radius and number of turns decrease, the nanospring stiffness would increase irrespective of the crystal orientations. Moreover, the elastic constants of nanosprings would become larger while the pitches become smaller for almost all the crystal orientations. Also the simulation results were compared to the predictions based on continuum theory in order to clarify whether the classical theory could apply to nanosprings. read less

Abstract: The melting process of Ag nanowires was studied by molecular dynamics (MD) simulations at the atomic level. It is indicated that the Ag nanowires with Ni coating can be superheated depending on their radius and size. Also, in this paper the mechanism of superheating was analyzed and ascribed to the epitaxial Ag/Ni interface suppressing the nucleation and growth of melt. For the analysis, a thermodynamic model was constructed to describe the superheating mechanism of the Ni-coated Ag nanowires by considering the Ag/Ni interface free energy. We showed that the nucleation and growth of the Ag melt phase are both suppressed by the low energy Ag/Ni interfaces in Ni-coated Ag wires and the suppression of melt growth is crucial and plays a major role in the process of melting. The thermodynamic analysis gave a quantitative relation of superheating with the Ag wire radius and the contact angle of melting. The superheating decreased with Ag wire radius and also depended on the Ag/Ni interfacial condition. The results of the thermodynamic model were consistent with those of the MD simulations. read less

Abstract: Through ab initio density functional theory based calculations, we find that the anomalously large shear modulus and the intrinsic brittleness of face-centred cubic (fcc) iridium (Ir) are primarily a consequence of its relatively strong bonds. Comparative analysis of the bond order, which dictates the bond strength, the localization of the valence electrons and the elastic constants of Ir and a selection of fcc metals allows us to rationalize the peculiarities of Ir in terms of its strong and directional bonds. Furthermore, the similarities between the failures of Al and Ir are suggested to reflect the resemblance existing between the angular features of their bonds. read less

Abstract: Ab initio spin polarized calculations were carried out to study the elastic, thermal physical properties and electronic structures of Fex Ga1–x alloys. To evaluate the elastic properties, dilute supercell models were constructed for Fe–6.25 at% Ga and Fe–12.5 at% Ga alloys. In addition, possible atomic models were explored for Fe–18.75 at% Ga, in which one model that contains a pair of Ga atoms forming a cluster was suggested to represent the quench state of the alloy. The experimentally observed softness of the tetragonal shear modulus 1/2(C11 – C12) in Fe–Ga alloys was reproduced in the calculations. The ductility of Fe–Ga alloys was analyzed in terms of the ratio G /B, where G and B are the shear and bulk modulus, respectively, and the Cauchy pressure 1/2(C12 – C44). The results show that the ductility of Fe–Ga was enhanced with increasing Ga concentration. The relation between linear thermal expansion coefficients α (T) and Ga content was also examined. It was demonstrated that both bulk modulus B and the Grüneisen parameter γ play a major role in determining the trend in thermal expansion coefficients. The electronic structures of Fe–Ga alloys were investigated, and the characteristics of electronic density of states were analyzed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) read less

Abstract: This study analyzes the plastic deformation on the atomic scale of Cu nanowires (NWs) with and orientations during uniaxial tension and compression, using a molecular dynamic simulation. The maximum local stress (MLS) method is employed to evaluate mechanical behavior during deformation. Following yielding, the flow stress strongly depends on the variation in the degree of orientation caused by twinning. Both the tension of the NW and the compression of the NW cause twin deformation and consequent geometrical softening. In contrast, the compression of the NW and the tension of the NW form twin bands and cause geometrical hardening. These behaviors result in the stress–strain curves that reveal the pseudo-skew-symmetry characteristic. With respect to the difference between the critical resolved shear stress (τc) associated with the distinct orientations, τc depends strongly on the surface critical resolved stress (τsc). Under tension, τsc depends on the degree of lattice distortion. A larger lattice distortion (pre-tensile stress) corresponds to higher τsc. However, under compression, a geometrical factor can be used to describe the difference in τsc between the different orientations. A larger geometrical factor corresponds to a larger τsc. read less

Abstract: Both molecular statics and molecular dynamics methods were employed to study the mechanical properties of copper nanowires. The size effect on both elastic and plastic properties of square cross-sectional nanowire was examined and compared systematically using two molecular approaches. It was found consistently from both molecular methods that the elastic and plastic properties of nanowires depend on the lateral size of nanowires. As the lateral size of nanowires decreases, the values of Young’s modulus decrease and dislocation nucleation stresses increase. However, it was shown that the dislocation nucleation stress would be significantly influenced by the axial periodic length of the nanowire model using the molecular statics method while molecular dynamics simulations at two distinct temperatures (0.01 and 300 K) did not show the same dependence. It was concluded that molecular statics as an energy minimization numerical scheme is quite insensitive to the instability of atomic structure especially without thermal fluctuation and might not be a suitable tool for studying the behaviour of nanomaterials beyond the elastic limit. read less

Abstract: The surface tension of liquid Co–Mo alloys is calculated by the Monte Carlo (MC) method using embedded atom method (EAM) potentials. For liquid Co–10% Mo alloy, the calculated surface tension at the liquidus temperature is 1.51 N/m, which is 20% smaller than the experimental result. The temperature coefficient of the surface tension is 4.72 × 10−4 N/m · K. The simulated surface tension of liquid Co–37.6% Mo alloy at the liquidus temperature is 1.95 N/m, which is only 2% lower than its measured value. The dependence of surface tension on the degree of undercooling of liquid Co–16% Mo and Co–30% Mo alloys has also been obtained. Comparison between the simulated and experimental results shows that better agreement is obtained with higher Mo content in the liquid alloys. read less

Abstract: We demonstrate that the wide range of magnitudes observed for the slopes of the melting curves at zero pressure for some transition metals with incomplete d-shells are consistent with their elastic moduli data. The near-zero slopes observed for W and Mo are not anomalous as has been suggested recently. We propose an empirical non-linear relationship between the slope and the compressibility. By examining a model for the vacancy formation energy in these metals, we show that the slopes can also be related to the slopes of the melting curves. read less

Abstract: Molecular dynamics simulation study based on the EAM potential is carried out to investigate the effect of pressure on the rapid solidification of Aluminum. The radial distribution function is used to characterize the structure of the Al solidified under different pressures. It is indicated that a high pressure leads to strong crystallization tendency during cooling. read less

Abstract: Nanoindentation tests performed in an atomic force microscope have been utilized to directly measure the mechanical properties of single crystal metal thin films fabricated by the vacuum vapour deposition technique. Nanoindentation tests were conducted at various indentation depths to study the effect of indentation depths on the mechanical properties of thin films. The results were interpreted by using the Oliver-Pharr method with which direct observation and measurement of the contact area are not required. The elastic modulus of the single crystal copper film at various indentation depths was determined as 67.0 > 6.9 GPa on average, which is in reasonable agreement with the results reported by others. The indentation hardness constantly increases with decreasing indentation depth, indicating a strong size effect. In addition to the experimental work, a three-dimensional nanoindentation model of molecular dynamics (MD) simulations with embedded atom method (EAM) potential is proposed to elucidate the mechanics and mechanisms of nanoindentation of thin films from the atomistic point of view. MD simulations results show that due to the size effect no distinct dislocations were observed in the plastic deformation processes of the single crystal copper thin films, which is significantly different from the plastic deformation mechanism in bulk materials. read less

Abstract: The melting and freezing processes of CuN (N = 180, 256, 360, 408, 500, 628 and 736) nanoclusters are simulated by using micro-canonical molecular dynamics simulation technique. The potential energies and the heat capacities as a function of temperature are obtained. The results reveal that the melting and freezing points increase almost linearly with the atom number in the cluster increasing. All copper nanoclusters have negative heat capacity around the melting and freezing points, and hysteresis effect in the melting/freezing transition is derived in CuN nanoclusters for the first time. read less

Abstract: A number of experimental studies of condensed matter assemblies with different types of chemical bonding will provide the focus of this work. Condensed compounds X(CH3)4, with X = C, Si or Ge, are the first of such assemblies; two phase boundaries in the pressure--temperature plane being studied: melting and a solid phase boundary heralding orientational disordering of molecules still however on a lattice. Secondly, directionally bonded d-electron transition metals such as Ni, Pd and Nb will be treated. Here, melting is the main focus, but the precursor transition is now the separation of a high-temperature ductile solid from a lower temperature mechanically brittle phase. A dislocation-mediated model of these transitions is discussed, leading into the third area of covalently bonded solids graphite and silicon. Here topological defect models again provide the focus; both dislocations and rotation-dislocations now being invoked. Some qualitative suggestions are made to interpret the melting curve of graphite subjected to high pressure. read less

Abstract: The kinetic details of the nucleation in a supercooled Ni6Cu4 melt are investigated using molecular dynamics simulation method. It is indicated that the nucleus is a random mixture of a large number of fcc structures and a small number of hcp structures. The growth rate of the nucleus increases linearly with undercooling (ΔT). The results show the microstructure evolution in the supercooled liquid of Ni6Cu4 clearly. read less

Abstract: We propose an extended Finnis–Sinclair (FS) potential by extending the repulsive term into a sextic polynomial for enhancing the repulsive interaction and adding a quartic term to describe the electronic density function. It turns out that for bcc metals the proposed potential not only overcomes the ‘soft’ behaviour of the original FS potential, but also performs better than the modified FS one by Ackland et al, and that for fcc metals the proposed potential is able to reproduce the lattice constants, cohesive energies, elastic constant, vacancy formation energies, equations of state, pressure–volume relationships, melting points and melting heats. Moreover, for some fcc–bcc systems, e.g. the Ag–refractory metal systems, the lattice constants, cohesive energies and elastic constants of some alloys are reproduced by the proposed potential and are quite compatible with those directly determined by ab initio calculations. read less

Abstract: We fit a new gold embedded atom method (EAM) potential using an improved force matching methodology which included fitting to high-temperature solid lattice constants and liquid densities. The new potential shows a good overall improvement in agreement to the experimental lattice constants, elastic constants, stacking fault energy, radial distribution function, and fcc/hcp/bcc lattice energy differences over previous potentials by Foiles, Baskes, and Daw (FBD) [Phys. Rev. B 33, 7983 (1986)] Johnson [Phys. Rev. B 37, 3924 (1988)], and the glue model potential by Ercolessi et al. [Philos. Mag. A 50, 213 (1988)]. Surface energy was improved slightly as compared to potentials by FBD and Johnson but as a result vacancy formation energy is slightly inferior as compared to the same potentials. The results obtained here for gold suggest for other metal species that further overall improvements in potentials may still be possible within the EAM framework with an improved fitting methodology. On the other hand, we also explore the limitations of the EAM framework by attempting a brute force fit to all properties exactly which was found to be unsuccessful. The main conflict in such a brute force fit was between the surface energy and the liquid lattice constant where both could not be fitted identically. By intentionally using a very large number of spline sections for the pair potential, electron-density function, and embedding energy function, we eliminated a lack of functional freedom as a possible cause of this conflict and hence can conclude that it must result from a fundamental limitation in the EAM framework. read less

Abstract: The key problem for applying the Monte Carlo (MC) method to segregation lies in the selection of an appropriate energy model for the simulations. Zhang et al. proposed a modified analytic embedded atomic method (MAEAM), which has been applied to a variety of fundamental problems in metals and alloys. We used this MAEAM and MC method for the simulations of segregation of AuCu and Au3Cu alloys. For the (100) surface, the calculations show that Au is enriched in the 1st layer (70 at% for AuCu, 99 at% for Au3Cu), while Cu is enriched in the 2nd layer (50–62 at% for AuCu, 35–48 at% for Au3Cu). The composition profiles are generally oscillating. For the (111) surface, we also found Au to be enriched in the 1st layer. However, Cu is not enriched in the 2nd layer, it reaches the bulk composition from 2nd to 4th layer. Au increases from 92 to 97 at% and 99 to 100 at% for Au3Cu(111) and (100), respectively, when the temperature varies from 1000 to 200 K, which is basically in agreement with the measurements for Au3Cu(100) by Taglauer et al. However, the Au concentration does not change for the AuCu alloy in the same temperature range. We also calculate the segregation energy, the simulation results agree qualitatively with the experimental data available. Our results demonstrate that the MAEAM model provides an effective means for simulating the segregation of alloys. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) read less

Abstract: Recently a method termed constrained fluid lambda-integration was proposed for calculating the free energy difference between bulk solid and liquid reference states via the construction of a reversible thermodynamic integration path; coupling the two states in question. The present work shows how the application of the constrained fluid lambda-integration concept to solid/liquid slab simulation cells makes possible a generally applicable computer simulation methodology for calculating the free energy of any surface and/or surface defect structure, including surfaces requiring variations in surface atom or density number, such as the (1 x 5) Au(100) or (1 x 2) missing row Au(110) reconstructed surfaces or excess adatom/vacancy/step populated surfaces. We evaluate the methodology by calculating the free energy of various disordered high temperature Au(110) embedded atom method surfaces constrained to differing excess surface atom numbers [including those corresponding to the (1 x 2) missing row reconstructed surface] and obtained the interesting result that at 1000 K (as distinct from lower temperatures) the free energy difference between these surfaces is reduced to zero; a result which is consistent with an expected order-disorder phase transition for the Au(110) surface at such high temperatures. read less

Abstract: This paper provides the results on experimentally measured and numerically predicted surface tensions of undercooled liquid cobalt. The experiments were performed by using the oscillation drop technique combined with electromagnetic levitation. The simulations are carried out with the Monte Carlo (MC) method, where the surface tension is predicted through calculations of the work of cohesion, and the interatomic interaction is described with an embedded-atom method. The maximum undercooling of the liquid cobalt is reached at 231 K (0.13Tm) in the experiment and 268 K (0.17Tm) in the simulation. The surface tension and its relationship with temperature obtained in the experiment and simulation are σexp = 1.93 − 0.000 33 (T − T m) N m−1 and σcal = 2.26 − 0.000 32 (T − T m) N m−1 respectively. The temperature dependence of the surface tension calculated from the MC simulation is in reasonable agreement with that measured in the experiment. read less

Abstract: Analytic modified embedded atom method (AMEAM) type many-body potentials have been constructed for ten hcp metals: Be, Co, Hf, Mg, Re, Ru, Sc, Ti, Y and Zr. The potentials are parametrized using analytic functions and fitted to the cohesive energy, unrelaxed vacancy formation energy, five independent second-order elastic constants and two equilibrium conditions. Hence, each of the constructed potentials represents a stable hexagonal close-packed lattice with a particular non-ideal c/a ratio. In order to treat the metals with negative Cauchy pressure, a modified term has been added to the total energy. For all the metals considered, the hcp lattice is shown to be energetically most stable when compared with the fcc and bcc structure and the hcp lattice with ideal c/a. The activation energy for vacancy diffusion in these metals has been calculated. They agree well with experimental data available and those calculated by other authors for both monovacancy and divacancy mechanisms and the most possible diffusion paths are predicted. Stacking fault and surface energy have also been calculated and their values are lower than typical experimental data. Finally, the self-interstitial atom (SIA) formation energy and volume have been evaluated for eight possible sites. This calculation suggests that the basal split or crowdion is the most stable configuration for metals with a rather large deviation from the ideal c/a value and the non-basal dumbbell (C or S) is the most stable configuration for metals with c/a near ideal. The relationship between SIA formation energy and melting temperature roughly obeys a linear relation for most metals except Ru and Re. read less

Abstract: Based on an embedded-atom method, an n-body potential is developed for the Ni-Nb system and the potential is then applied in molecular-dynamics simulation to study the detailed process of interfacial reaction in an Ni-Nb sandwich model. It turns out that the solid-state amorphization is initiated by interface-crossing atomic migration and governed by diffusion-limited reaction in the Ni-Nb system characterized by a negative heat of formation. The simulation results are confirmed by thermal annealing experiments conducted at medium temperatures. read less

Abstract: Pair potentials afford a quantitative starting point for studying the vacancy formation energy E v in hot close-packed crystals such as Ar. A summary will be given of the relation of E v to the melting temperature T m, via liquid structure, and a brief comment made on the role of three-body-forces. This leads into a discussion of ‘criteria’ characterizing the solid—liquid phase transition, one of these being the assertion that a close-packed crystal melts when the internal energy required to create a localized hole (the unrelaxed vacancy in the hot crystal) is equal to the change in internal energy at T m required to expand the liquid by one atomic volume. When N-body forces become important and are treated by so-called glue models, exemplified in the work of R. A. Johnson (1988, Physical Review B, 37, 3924), on a model of Cu metal, the important role of the shear modulus in determining both E v (now at T = 0) and the divacancy binding energy is stressed. Finally, E v in, for example, Al is argued to be closely connected with surface energy, through the similarity of conduction-electron density profiles around the vacant site and through the planar surface. This leads to a brief account of unpublished work on mechanical properties in the liquid at T m, and in particular shear viscosity, which is related to surface tension via the velocity of sound and the thickness of the liquid vapour interface. read less

Abstract: Using the analytic modified embedded atom method (MEAM), the embedding, potential and modifying functions for the Mg and rare earth (RE) metals (RE = Sc, Y, Pr, Nd, Gd, Tb, Dy, Ho and Er) are presented. The thermodynamic properties, such as the dilute-limit heats of solution, enthalpies of formation of disordered solid solutions and intermetallic compounds, for the binary Mg-RE alloys are calculated. The obtained results are in good agreement with the available experimental data and with the results calculated using Miedema theory. The calculations of the enthalpies of formation of Mg3 RE, Mg2 RE, MgRE, MgRE2 and MgRE3 with various ordered structures (DO3 , DO19 , L12 , C15, MoPt2 , B2, W1 and L10 ) indicate that the trends in the structural stability can be interpreted directly in terms of the formation energy. Moreover, the lattice constants and volume contractions of alloys with various compositions are determined based on the relation between the formation energy and the interatomic distance. The correlation between the enthalpy of formation and volume contraction for intermetallic compounds is discussed. read less

Abstract: The embedded-atom model (EAM) is applied to the study of vacancy formation in bulk aluminium and lithium. A systematic study is undertaken into the sensitivity of the EAM potentials and embedding energy functionals as a function of the unrelaxed vacancy formation energy which is normally obtained via ab initio density functional calculations. The effect of this `empirical' input parameter on the vacancy relaxation energy, formation volume and structural relaxation is also investigated using super-cell sizes not normally accessible in orbital-based ab initio relaxation studies. We find that for aluminium, for which at most a fifth-nearest-neighbour model is required, the vacancy relaxation energy and formation volume are not sensitive functions of the unrelaxed vacancy formation energy. For lithium, for which at least a ninth-nearest-neighbour model is needed, the situation is somewhat different: both the vacancy relaxation energy and the formation volume are found to be a noticeably related to the unrelaxed vacancy formation energy. For both solids, the structural relaxation was found to be largely insensitive to the unrelaxed vacancy formation energy, agreeing well with previous ab initio calculations. In particular for aluminium, the EAM result agrees extremely well with recent orbital-free density functional calculations which use super-cell sizes approaching those used here. Finally, we find that for lithium, the embedding energy functional has negligible curvature for a wide range of local electronic densities, justifying the use of a simpler pair potential description for lithium in mildly inhomogeneous systems. read less

Abstract: The universal form of embedding function suggested by Banerjea and Smith together with a pair-potential of the Morse form are used to obtain embedded atom method (EAM) potentials for fcc metals: Cu, Ag, Au, Ni, Pd, and Pt. The potential parameters are determined by fitting to the Cauchy pressure ( C _12 − C _44)/2, shear constant G _V = ( C _11 − C _12 + 3 C _44)/5, and C _44, the cohesive energy and the vacancy formation energy. The obtained parameters are utilized to calculate the unrelaxed divacancy binding energy and the unrelaxed surface energies of three low-index planes. The calculated quantities are in reasonable agreement with the experimental values except perhaps the divacancy energy in a few cases. In a further application, lattice dynamics of these metals are discussed using the present EAM potentials. On comparison with experimental phonons, the agreement is good for Cu, Ag, and Ni, while in the other three metals, Au, Pd, and Pt, the agreement is not so good. The phonon spectra are in reasonable agreement with the earlier calculations. The frequency spectrum and the mean square displacement of an atom in Cu are in agreement with the experiment and other calculated results. read less

Abstract: At low coverages, lead adatoms form a single layer on the gold (110) surface. However, if a critical coverage is exceeded, a second adlayer forms. At zero temperature, various properties such as the energy and atomic spacing of atoms in the first adlayer are discontinuous at . The effects of temperature on the discontinuities and location of are reported. Results for clean Au(110) and Pb(110) surfaces are also reported. read less

Abstract: The temperature dependence of the elastic constants of Ni is calculated using molecular dynamics (MD) simulations in conjunction with the embedded atom method (EAM). The Parrinello - Rahman version of molecular dynamics is employed along with the fluctuation formulae in the and EhN ensembles at various temperatures from 0 K to somewhat below the melting point (experimental value 1725 K). The calculated results for the elastic constants, compressibility, linear coefficient of thermal expansion, specific heat and the melting temperature compare reasonably well to experiment. read less

Abstract: The elastic constants of fcc transition metals are studied as functions of the exponential parameters of a many-body potential based on the second moment approximation of the d-electron density of states. It is shown that this model gives a reasonable agreement with the experimental values not only for all three independent elastic constants but also for the vacancy formation energy for Cu, Ag, Au and Ni. Less satisfactory results are obtained for Ir, Rh, Pt and Pd. read less

Abstract: Calculations on point-defect clusters in Zn are performed using a static simulation technique in conjunction with a many-body potential. The results are compared with the available experimental observations, theoretical estimates and previous computations in hcp metals with c/a ratios below the ideal value. Both monovacancy and divacancy defects demonstrate an anisotropic migration with almost the same activation energy, corresponding to that observed in recovery stage IV. Small vacancy clusters are found to favour three-dimensional agglomerates. Stability conditions for vacancy clusters to nucleate or break up (recovery stage V) are discussed. Small clusters of interstitials are shown to grow on the basal plane in forms based on the octahedral point defect, although the most stable single interstitial is the non-basal crowdion. Large single- and double-layer interstitial clusters are assigned to embryos of the experimentally observed dislocation loops. read less

Abstract: Based on Born's criteria we studied phase stability and theoretical strength of fcc crystals of copper and nickel under [100] uniaxial loading. The calculation was carried out using a simple and completely analytical embedded atom method (EAM) potential proposed by the present authors. For Cu, the calculated value of its theoretical strength (0.33 ? 1011 dyn?cm-2) agrees well with the experimental value (0.30 ? 1011 dyn?cm-2), while the calculated strain (9.76%) is somewhat larger than the experimental one (2.8%). For Ni, its theoretical strength and strain predicted using the EAM potential are found smaller than those predicted using a pair potential. It is worthy to note that unlike previous calculations, in which pair potentials were used and three unstressed fcc, bcc, and fct structures included (for Ni only fcc state is found stable, while for Cu both fcc and bcc states are predicted stable), in present calculations using EAM potential the [100] primary loading path passes through only two zeroes (a stable unstressed fcc structure and an unstable stress-free bcc structure) either for Cu or for Ni. read less

Abstract: Based on the embedded-atom method, molecular dynamics simulations have been performed to study the structural features of Al and Cu in the liquid and solid states during rapid solidification. The calculated pair correlation functions above the melting points of Al and Cu are found to be in good agreement with experiment, especially for Cu. The results show that the EAM can correctly and efficiently predict the glass transition and crystallization during rapid solidification from liquid metals, and can also describe microstructures of liquids, supercooled liquids, glasses and crystalline phases. In addition, structural analysis using bond orientational order and pair analysis techniques have been made in detail, and the effect of cooling rate on microstructures during rapid solidification has been analysed. read less

Abstract: A realistic theoretical study to predict the phonon dispersion and binding energy of h.c.p. metals is developed. The total interaction among the constituents is divided into two parts, ion-ion part and electron-ion-electron part. The former part is expressed by the modified form of the generalized exponential potential (MGEP) developed by the present group. To highlight the characteristic features of the MGEP other similar potentials are compared. The latter part is represented by the modified Krebs scheme, involving the dielectric screening due to Vashista and Singhwi. These interactions are blended to develop a model for predicting the binding energy and the phonon dispersion of the scantily studied h.c.p. metals like thallium. read less

Abstract: A detailed comparison of the separable potential method (SPM), recently proposed by one of us, with the embedded-atom method (EAM) allows a characterization of the embedding function F through a nonlinear differential equation. The complete solutions of the nonlinear differential equation are given, and as a demonstration of the applicability of the method, one of the solutions is employed to calculate various physical quantities for some f. c. c. metals. The applicability of the unified approach discussed here, goes beyond f. c. c. metals. We illustrate this important point by comparing some of our results obtained for b. c. c. metallic vanadium with similar results obtained recently by Adam and Foiles in 1990. Une comparaison detaillee de la “separable potential method” (SPM) recement propose par l'un de nous, avec la “embedded-atom method” (EAM) permis une caracterisation de la fonction d'incrustration F au moyen d'une equation differentielle non-lineaire. Etant donnes les solutions de l'equations differentielle non-lineaires, et pour demontrer le mode d'application de la methode, une des solutions sert a calculer les differents quantites physiques pour quelques metaux c. f. c. Le mode d'application de la methode unie, dont il est ici question, est au dela des metaux c. f. c. Nous illustrons cette point important avec des comparaisons de nos resultats pour le b. c. c. vanadium metallique avec les resultats similaire recement obtenus par Adam et Foiles. read less

Abstract: Defect energies in two- and three-dimensional classical crystals correlate with the shear modulus mu ; in turn this relates the melting temperature Tm intimately to mu . Thus, a model is first proposed for the shear modulus for two-dimensional Wigner crystals. The melting temperature is then determined from the Kosterlitz-Thouless melting criterion or from an anharmonic instability inherent in the model. The relative positrons of these transitions depend on the model parameters used. The calculation is generalized to include (a) zero-point motion which is dominant in the quantum limit and (b) the effect of a magnetic field. For the high field case, this modelling allows Tm to be plotted versus the Landau-level filling factor nu . The predictions of the model are thereby brought into contact with the experiments of Andrei et al. (1988), and Glattli et al. (1990) which have been interpreted as evidence for a magnetically induced Wigner solid (MIWS) in the electron assembly in a GaAs/AlGaAs heterojunction in strong magnetic fields. The model exhibits of the features observed experimentally. read less

Abstract: The class of Complex Metallic Alloys (CMAs) is interesting for its wide range of physical properties. There are materials that exhibit high hardness at low density or good corrosion resistance, which is important for technological applications. Other compounds are superconductors, have strong anisotropic transport coefficients or exhibit a novel magnetic memory effect. The theoretical investigation of CMAs is often very challenging because of their inherent complexity and large unit cells with up to several thousand atoms. Molecular dynamics simulations with classical interaction potentials are well suited for this task – they can handle hundreds of thousands of atoms in reasonable time. Such simulations can provide insight into static and dynamic processes at finite temperatures on an atomistic level. The accuracy of these simulations depends strongly on the quality of the employed interaction potentials. To generate physically relevant potentials the force-matching method can be applied. A computer code called potfit has been developed at the Institute for Theoretical and Applied Physics (ITAP) especially for this task. It uses a large database of quantum-mechanically calculated reference data, forces on individual atoms and cohesive energies, to generate effective potentials. The parameters of the potential are optimized in such a way that the reference data are reproduced as accurately as possible. The potfit program has been greatly enhanced as part of this thesis. The optimization of analytic potentials, new interaction models as well as a new optimization algorithm were implemented. Potentials for two different complex metallic alloy systems have been generated and used to study their properties with molecular dynamics simulations. The first system is an approximant to the decagonal Al-Pd-Mn quasicrystal. A potential which can reproduce the cohesive energy with high accuracy was generated. With the help of this potential a refinement of the experimentally poorly determined structure model could be performed. The second class of potentials was fitted for intermetallic clathrate systems. They have interesting thermoelectric properties which originate from their special structure. Silicon- and germanium-based clathrate potentials were derived and the influence of the complex structure on the thermal conductivity has been studied. Komplexe Intermetallische Verbindungen (CMAs) sind aufgrund ihrer vielfaltigen physikalischen Eigenschaften sehr interessant fur technologische Anwendungen. Dabei ist z.B. hohe Harte bei geringer Dichte und Korrosionsbestandigkeit wichtig. Neben Supraleitern gibt es Materialien mit anisotropen Transporteigenschaften oder einem neuartigen magnetischen Memory Effekt. Theoretische Untersuchungen von CMAs stellen durch ihre inharente Komplexitat und die riesigen Einheitszellen mit mehreren tausend Atomen oft eine grose Herausforderung dar. Molekulardynamiksimulationen mit effektiven Potenzialen konnen dazu eingesetzt werden; sie ermoglichen die Berechnung von hunderttausenden von Atome in annehmbarer Zeit. Damit kann ein Einblick in sowohl statische als auch dynamische Prozesse auf atomarer Ebene gewonnen werden. Die Ergebenisse solcher Simulationen hangen jedoch sehr stark von der Qualitat der eingesetzten Wechselwirkung ab. Um physikalisch gerechtfertigte Potenziale zu erzeugen, kann die Force-Matching-Methode angewandt werden. Dazu wurde am Institut fur Theoretische und Angewandte Physik (ITAP) ein Programm mit dem Namen potfit entwickelt. Es verwendet eine grose Datenbank von quantenmechanisch berechneten Referenzgrosen wie z.B. Krafte auf die einzelnen Atome und die Kohasionsenergie, um effektive Potenziale zu generieren. Die freien Parameter des Potenzials werden optimiert, um die Referenzdaten so gut wie moglich zu reproduzieren. Fur diese Arbeit wurde potfit deutlich erweitert. Es konnen nun analytisch definierte Potenziale optimiert werden, neue Wechselwirkungen wurden implementiert und ein neuer Optimierungsalgorithmus wurde hinzugefugt. Damit wurden effektive Potenziale fur zwei verschiedene CMA Systeme gefittet und deren Eigenschaften mit Molekulardynamik untersucht. Fur die Approximanten eines decagonalen Al-Pd-Mn Quasikristalls, den Xi-Phasen, wurde ein Potenzial fur die Strukturbestimmung erzeugt. Es kann die Kohasionsenergien verschiedener Strukturen mit groser Genauigkeit wiedergeben. Ein aus experimentellen Daten ungenau bestimmtes Strukturmodell konnte damit erheblich verbessert werden. Auserdem wurden Potenziale fur Intermetallische Klathrate erzeugt. Diese Systeme besitzen interessante thermoelektrische Eigenschaften aufgrund ihrer speziellen Kafigstruktur. Effektive Wechselwirkungen fur silizium- und germaniumbasierte Klathrate wurden erzeugt. Damit wurde der Einfluss der komplexen Struktur auf die thermische Leitfahigkeit des Gitters untersucht. read less

Abstract: In this paper, we propose a computational method for coarse graining the atomistic description at finite temperature using formal asymptotics. The method is based on the ansatz that there exists a separation of scales between the time scale of the atomic fluctuations and that of the thermodynamic processes, such as thermal expansion. We use the WKB method to propose an averaging scheme for treating the thermal degrees of freedom and deriving an effective Hamiltonian for the atomistic system. This energy functional is incorporated into the quasicontinuum framework to achieve a seamless coarse graining on the spatial scale. Numerical validation is performed by computing the thermal equilibrium properties of selected materials. The scope of the method based on the use of perturbation theory is discussed, and its capability is illustrated by way of simulating dislocation nucleation under a nanoindenter. read less

Abstract: In a number of areas of application, the behavior of systems depends sensitively on properties that pertain to the atomistic scale, i. e., the angstrom and femtosecond scales. However, generally the behaviors of interest are macroscopic and are characterized by slow evolution on the scale of meters and years. This broad disparity of length and time scales places extraordinary challenges in computational material science. The overarching objective of this dissertation is to address the problem of multiple space and time scales in atomistic systems undergoing slow macroscopic evolution while retaining full atomistic detail. Our approach may be summarized as follows: (1) The issue of accounting for finite temperature in coarse grained systems has not been solved entirely. For finite temperature systems at equilibrium, constructing an effective free energy in terms of a reduced set of atomic degrees of freedom is still an open area of research. In particular, the thermal vibrations of the missing degrees of freedom need to be accounted for. This is specially important if the aim of the simulation is to determine the dynamic properties of a system, or to allow the transmission of dynamic information between regions of different spatial discretization. To this end, we introduce a framework to simulate (spatially) coarse dynamic systems using the Quasicontinuum method (QC). The equations of motion are strictly derived from dissipative Lagrangian mechanics, which provides a classical Langevin implementation where the characteristic time is governed by the vibrations of the finest length scale in the computational cell. In order to assess the framework's ability to transmit information across scales, we study the phonon impoverish spectra in coarse regions and the resulting underestimation of thermal equilibrium properties. (2) Atomistic simulations have been employed for the past thirty years to determine structural and thermodynamic (equilibrium) properties of solids and their defects over a wide range of temperatures and pressures. The traditional Monte Carlo (MC) and Molecular Dynamics (MD) methods, while ideally suited to these calculations, require appreciable computational resources in order to calculate the long-time averages from which properties are obtained. In order to permit a reasonably quick, but accurate determination of the equilibrium properties of interest, we present an extension of the “maximum entropy” method to build effective alloy potentials while avoiding the treatment of all the system's atomic degrees of freedom. We assess the validity of the model by testing its ability to reproduce experimental measurements. (3) Based upon these effective potentials, we present a numerical framework capable of following the time evolution of atomistic systems over time windows currently beyond the scope of traditional atomistic methods such as Molecular Dynamics (MD) or Monte Carlo (MC). This is accomplished while retaining the underlying atomistic description of the material. We formulate a discrete variational setting in which the simulation of time-dependent phenomena is reduced to a sequence of incremental problems, each characterized by a variational principle. In this fashion we are able to study the interplay between deformation and diffusion using time steps or strain rates that are orders of magnitude larger or smaller than their MD|MC counterparts. (4) We formulate a new class of “Replica Time Integrators” (RTIs) that allows for the two-way transmission of thermal phonons across mesh interfaces. This two-way transmission is accomplished by representing the state of the coarse region by a collection of identical copies or “replicas” of itself. Each replica runs at its own slow time step and is out-of-phase with respect to the others by one fast time step. Then, each replica is capable of absorbing from the fine region the elementary signal that is in phase with the replica. Conversely, each replica is capable of supporting --and transmitting to the fine region-- an elementary signal of a certain phase. Since fine and coarse regions evolve asynchronously in time, RTIs permit both spatial and temporal coarse graining of the system of interest. Using a combination of phase-error analysis and numerical testing we find that RTIs are convergent, and allow step waves and thermal phonons to cross mesh interfaces in both directions losslessly. read less

Abstract: First-principles calculations of the electronic structure of silver infinite nanotubes, chains, and nanowires are reported. For the purpose of this study, the focus is on two specific types of silver nanostructure. We have calculated the electronic structures for the silver (2,3) and the silver (3,4) extended nanostructures. For the (2,3) structure, we modeled the single-walled tube, and for the (3,4) structure, we modeled the hollow single-walled tube, as well as the tube with a linear chain inserted along the tube axis. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 read less

Abstract: The approach of molecular dynamics simulation is one of the most promising methods for investigating the mechanical behavior of nanostructures, such as nanorods and nanowires, which are used to design nano-devices. The mechanical behavior of a copper nanorod subjected to axial compressive force is simulated using the embedded atom method (EAM). The results obtained show that the mechanical behavior in nanoscale is very different from that in macrosclae. This is due to size and surface effects. It has been found that when the external axial force is below certain critical value, the nanorod contracts in length. However, when the force reaches the critical value, the rod exhibits transverse bending, which is known as buckling. After reaching the critical value, the nanorod is able to withstand further increase in external loading and remains stable, which is so called post-buckling. When the external loading is too big, the nanorod collapses. read less

Abstract: A comprehensive study has been carried out in order to reveal physical properties of antiperovskite compound GeNCa 3 in the cubic structure. This study presents extensive properties including mechanical, electronic, vibrational and thermodynamical by means of generalised approximation approach within the density functional theory. The equilibrium lattice constant and bulk modulus of the compound are obtained via energy-volume data. The mechanical stability evaluation is conducted based on Born’s criteria using elastic constants. Subsequently, electronic band structures and partial and total densities of states are computed for antiperovskite GeNCa 3 . The electronic band structure of the compound reveals a metallic character with highest contribution to the conductivity Ge 4p and Ca 3d states. Moreover, phonon distribution curve is obtained by employing the linear response technique. The results indicate a dynamically stable compound. Finally, thermodynamic properties such as entropy and specific heat value and the results are presented. 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 structural, mechanical, electronic and phonon properties of antiperovskite SnNCa 3 compound in the cubic phase were systematically investigated by means of the density functional theory. The computed lattice constants and bulk modulus are well in accordance with the literature. The mechanical stability of the compound was examined via obtained elastic constants. The results indicated that SnNCa 3 antiperovskite compound is mechanically stable and brittle based on the Pugh`s criteria. The electronic band structure of the compound suggest that the material is metallic; the largest contribution to the conductivity are due to electrons of Sn-5p, N-2p and Ca-3d orbitals. In addition, phonon distribution curves and their corresponding density of states were obtained for the first time using the linear response approach by means of the density functional perturbation theory. The phonon properties investigation exhibited that SnNCa 3 antiperovskite compound is dynamically stable. read less

Abstract: In order to meet the continuously increasing requirements in nearly all fields of technology, an ongoing development and optimization of new and existing materials, components and manufacturing facilities is necessary. The rapidly growing demand on the application side implies a constant acceleration of the complete development process. In the past, development and optimization were often based on experiments. Indeed, the efforts for this approach are mostly extensive, time consuming and expensive, which significantly restricts the development speed. The development of numerical methods and physical models as well as steadily increasing computer capacities allow for the employment of numerical simulations during materials development and optimization. Thus the experimental efforts can be considerably reduced. Moreover, the application of computational methods allows for the investigations of physical phenomena, which are "inaccessible" from the experimental point-of-view, such as trapping behaviour of hydrogen or carbon at different lattice defects (vacancies, dislocations, grain boundaries, etc.) within an Fe-based matrix, see e.g. (Desai et al., 2010; Hristova et al., 2011; Lee, 2006; Lee & Jang, 2007; Nazarov et.al., 2010). In steel production for example, the goal is pursued to set up a so-called ’digital plant’, in which it is possible to calculate the behavior of material and components up to the application level, see Figure 1. Such a digital production line provides deep insight into the materials response and the involved physical effects at each step of the process chain. Furthermore material parameters can be calculated, which will be used as input data to perform calculations of subsequently following process steps. In fact, if the production process chain can be completely reproduced, a backwards approach will be possible, which allows for the transfer from application requirements to the materials design (computer aided material design). A fully theoretical, sufficiently accurate reproduction of all steps of materials processing is as far as we know still not possible. To achieve reliable simulation results in manageable computational times, (semi-)empirical models are widely used at nearly all production 6 read less

Abstract: We review the studies on the thermophysical properties of undercooled metals and alloys by molecular simulations in recent years. The simulation methods of melting temperature, enthalpy, specific heat, surface tension, diffusion coefficient and viscosity are introduced and the simulated results are summarized. By comparing the experimental results and various theoretical models, the temperature and the composition dependences of the thermophysical properties in undercooled regime are discussed. read less

Abstract: The formation energy of the mono-vacancy and both the formation energy and binding energy of the di-and tri-vacancy in BCC alkali metals and transition metals have been calculated by using the modified analytical embedded-atom method (MAEAM). The formation energy of each type of configuration of the vacancies in the alkali metals is much lower than that in the transition metals. From minimum of the formation energy or maximum of the binding energy, the favorable configuration of the di-vacancy and tri-vacancy respectively is the first-nearest-neighbor (FN) or second-nearest-neighbor (SN) di-vacancy and the [112] tri-vacancy constructed by two first-and one second-nearest-neighbor vacancies. It is indicated that there is a concentration tendency for vacancies in BCC metals. read less