Embedded Atom Model Research Articles

Reliability of two embedded atom models for the description of Ag@Au nanoalloys

The validation of embedded atom models (EAMs) for describing nanoalloys requires the verification of both a faithful description of the individual phases and a convincing scheme for the mixed interactions. In this work, we present a systematic benchmarking of two widely adopted EAM parameterizations, i.e., by Foiles [Phys. Rev. B 33, 7983 (1986)] and by Zhou [Phys. Rev. B 69, 144113 (2004)] with density functional theory calculations for the description of processes at Ag@Au nanoalloy surfaces and nanoclusters.

Interactions of plasticity and phase transformation under shock in iron bicrystals

The interaction of plasticity and martensitic transformation in iron bicrystals under shock has been investigated via nonequilibrium molecular-dynamic simulations with our modified analytic embedded-atom model potential. Our results show that grain boundaries (GBs) can change the kinetics of α → ɛ martensitic transformations. The condition that GBs trigger strain induced transformation (SIT) is revealed, that is, the local structure of GBs can be converted to a hexagonal close packing (HCP) lattice with a lower potential barrier than the BCC → HCP transformation (stress assist transformation, SAT) because the driving energy of transition can be partly provided by the energy of GBs. It can explain why the threshold of the phase transition can be greatly reduced in samples containing some types of GBs. The threshold of SIT is lower than that of SAT, which agrees well with experimental results. Then, the nucleation kinetics of phase transition induced by dislocations are described. The propagation of dislocation provides the driving force for the nucleation of the phase transition. The dislocations can be directly emitted from the GBs under shock, which provides a new view that the phase transition is not always preceded by dislocations near GBs. In addition, dislocations can be induced by pre-existing dislocations under shock, which makes it understandable that GBs can emit dislocations. Our findings have an important significance for experimental studies and macroscopic and mesoscopic scale simulations regarding iron martensitic transformation.

Reproducibility of a heterophase structure emergence effect when changing the ignition temperature of SHS in a layered nanosized nonstoichiometric Ti-Al system

The simulation results of SHS process in a package of alternating nanoscale layers of the crystal lattices of Ti and Al atoms for two systems with various numbers of atoms and nonstoichiometric ratios are given. For simulation the LAMMPS package supporting parallel computations and the interatomic interaction potential in the “embedded atom” model (EAM) have been used. For two systems with various numbers of atoms and nonstoichiometric ratios the sets of temperature and density profiles along the layers of the structure at successive instants of time (up to 16 ns) have been obtained. For a system with 453974 atoms and a nonstoichiometric ratio of 1.23 the heterostructure emergence with an alternation of intermetallic phases when changing the SHS “ignition” temperature has been reproduced.

Embedded atom method potentials for La-Al-Ni ternary alloy

Based on a series of the first-principles and experimental data, a new interatomic potential for the La-Al-Ni ternary system has been developed in the framework of an embedded-atom model using a force matching technique. With the optimized potential, the ground states of various elemental phases of La, Al, Ni, and La-Al-Ni intermetallic compounds are accurately reproduced. Also, the liquid dynamic behavior and glass forming ability of the La-Al-Ni system are validated, and the simulation results show great agreement with those calculated by ab initio molecular dynamic and related experimental data, indicating the reasonability and availability of our developed embedded-atom-method potential.

Applicability of molecular dynamics method to the prediction of the melting point of refractory metals and compounds

Various methods for measuring the melting point using embedded atom model (EAM) and from first principles modeling are considered. The features of each of them are shown. The results of the work of these methods are compared with the experimental values on the example of systems consisting of pure metals such as Fe, Pd, Ti, Pt Zr, Mo, Ta, W. Good agreement of the estimated melting points with experimental values is shown.

Modeling Liquid Antimony by Means of Molecular Dynamics

The potential of the embedded-atom model (EAM) for liquid antimony is calculated, and the molecular dynamics models are constructed for antimony at temperatures of up to 2023 K and under conditions of shock compression up to a pressure of 131 GPa. It is established that the EAM potential describes the behavior of the shoulder of pair correlation function (PCF). Good agreement with the experimental data is obtained for the structure and density of the liquid, the speed of sound at the binodal, and discrepancies are obtained with the experimental data for the energy. It is found that the self-diffusion coefficient is overstated near the melting point, but the discrepancy disappears upon heating; the calculated shock adiabat agrees with the experimental results; and the structure of liquid antimony models at pressures up to 8 GPa is not consistent with the diffraction data regarding the shape of the first PCF peak. It is concluded that the structural features of the anomalous metal (antimony) are determined by an existence of the interval to the right of the first PCF peak, at which the curvature of the interparticle potential is negative.

Singularity at the Point of Transition from Equilibrium to Metastable States of a Metallic Melt

Equilibrium and metastable states of a liquid are studied near the liquid-crystal phase transition point in a metallic melt using a time-dependent four-point correlation function. This function was proposed earlier for revealing the collective atomic motion in equilibrium liquids. The characteristic changes in the behavior of liquid when it passes to the region of metastability are found. At the quantitative level, such changes manifest themselves in the specific features of the four-point correlation function in the region of crossover from the equilibrium to metastable state arising on isochoric cooling. As specific examples, the embedded atom models for copper and nickel are considered using the method of molecular dynamics.

Size dependent stability and surface energy of amorphous FePt nanoalloy

The effects of particle size (2–6 nm) and temperature (300–2700 K) on the stability and local structural evolutions of amorphous equiatomic FePt bulk/nanoalloys have been investigated by combining Embedded Atom Model (EAM) with classical molecular dynamics (MD) simulation method. The three dimensional (3D) atomic configuration of amorphous FePt NPs by means of Voronoi analysis reveals that, deformed bcc (d-bcc) and icosahedron (d-ico) type structures are most probable local atomic configurations for 2–6 nm sized Fe50Pt50 NPs both in liquid (1700 K) and glassy (300 K) states. It was shown that nano-scale phase separation takes place around 300 K for 2 nm sized FePt NPs that leads to formation of spherical core-shell segregated structure having Pt-Fe-rich core and Fe-rich surface. Compared to core region, more ordered and high dense configuration of atoms at the surface gives rise to decrease in surface entropy of NPs and hence bringing about surface energy anomaly with positive temperature coefficient above the glass transition temperatures. Below glass transition temperatures of the FePt nanoalloy particles, the thermodynamically stable amorphous phase (TSA) would appear. Glass transition temperature of amorphous Fe50Pt50 NPs increases with increasing particle size and eventually approaches to the glass temperature of bulk system for NPs diameters ≥ ∼16 nm. Results of current predictions are in good qualitative and semi-quantitative agreement with other theoretical and experimental findings reported in the literature.

Structure, Thermodynamic Properties, and Shock Compression of Bismuth–Lead Melts and Correlation Effect during Diffusion. Molecular Dynamics

The models of Bi–Pb solutions were constructed by molecular dynamics at temperatures of up to 30 000 K and pressures of up to 282 GPa. The embedded atom model (EAM) potentials for pure components and the Lennard-Jones potential for pairs 12 are used. The density isobar at normal pressure and the solution formation heat isotherm at 700 K were in good agreement with experiment. The thermodynamic (density, energy, heat capacity, compression modulus, sound velocity, and Gruneisen coefficients), structural, and diffusion properties of the eutectic Bi55Pb45 melt at normal pressure and temperatures of up to 2900 K and under conditions of shock compression were calculated. At elevated pressure, the negative deviations from ideality change to positive ones. A molecular-dynamic calculation of the mutual diffusion coefficients at 550–928 K was performed on the melt models. The interdiffusion coefficients found by the molecular dynamics method are smaller than those measured experimentally by the capillary method by a factor of 1.25–1.64. The correlation effect during diffusion in melts similar to the cataphoretic effect in ionic solutions is confirmed. In Bi–Pb solutions with negative deviations from ideality, the correlation factor is substantially smaller than unity. In the equation for the interdiffusion coefficient, the thermodynamic factor and the correlation factor partially cancel each other.

Atomistic studies of shock-induced plasticity and phase transition in iron-based single crystal with edge dislocation

In engineering applications, various types of defects inevitably exist in materials which would critically affect their properties. By introducing an edge dislocation into the model of single-crystal iron, the plasticity and α→ε phase transition in iron under shock loading have been investigated by means of non-equilibrium molecular dynamics (NEMD) simulations with a modified analytic embedded-atom model potential. Plastic slip is clearly observed in our work, which is induced by the initial edge dislocation under shock loading. The activated slip systems are considered as {110}<111> at the nucleation stage of dislocation generation, and then a self-adaptive transformation process occurs, which results in transforming the activated slip systems from {110}<111> into {112}<111>. The α→ε phase transition only occurs after the activation of dislocation slip, which indicates there is a strong connection between plasticity and the phase transition. The process of this plasticity-controlled phase transition is illustrated. By calculating the resolved shear stress along the shuffle plane of the shocked crystal iron at different moments, the mechanism of the plasticity-controlled phase transition is deeply discussed. Our findings could contribute to the better understanding of the behaviors of iron or iron-based alloy under high pressure conditions.

Improved embedded-atom model potentials of Pb at high pressure: application to investigations of plasticity and phase transition under extreme conditions

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

Temperature and Strain Rate Dependent Mechanical Properties of a Square Nickel Plate with Different Shaped Central Cracks: A Molecular Dynamics Study

In our present study, under uniaxial tension, atomistic simulations were conducted to explore the crack propagation mechanism of Square Nickel Plate (SNP) for two distinct shaped cracks (Rectangular and Circular) at center separately. Here, for modeling the inter-atomic potential between atoms, Embedded Atom Model (EAM) was used. In case of both types, the crack size was varied keeping a constant strain rate of 2×109s-1and temperature of 300 k for investigation of the effects of crack geometry and size on the behavior of crack propagation. Along with the size and geometry of crack, the effects of different strain rates (1×109, 2×109and 4×109s-1) and temperatures (300 K, 600 K and 900 k) were also studied. From the simulations, the declination nature of peak stress can be deduced for both of the geometries by increasing the crack size. It can also be concluded that when crack area was same, the peak stresses were higher in SNP with Circular crack than with the SNP with Rectangular one. Besides, increasing and decreasing nature of peak stress were found for two genres with the increment of strain rate and temperature separately.

The study of microstructure and propagation of the combustion wave of SHS in nanodimensional multilayer systems of Ni-Al with using molecular-dynamic simulation

Computational experiments (CEs) on the molecular-dynamic simulation of the propagation of the combustion wave of the SH-synthesis process in a stack of alternating layers of nanoscale crystal lattices of Ni and Al atoms have been carried out. In the calculations are used two versions of the interatomic interaction potential in the “embedded atom model” (EAM) and the LAMMPS package taking into account parallel computings. As the results of computational experiments for two varieties of the EAM potential, a family of temperature profiles along the layers of the structure at successive instants of time (up to 40 ns) and the corresponding set of microsections (snapshots) of the layered structure are given. For large values of the initial heating temperature and the stoichiometric ratio of Ni and Al atoms in the SHS sample, the computational experiments were confirmed the heterogeneous SHS-reaction mechanism, referred to in scientific publications as “mosaic-dissolution-precipitation”. In addition, when under such conditions a combustion wave of SHS passes through, the effect of heat localization is observed with the establishment of a higher temperature in the final region of the sample compared with the initial region, as a result of which on the “plateau” of the temperature profile corresponding to 16 ns a smooth “dip” is observed. Then, at subsequent times, there is a continuation of combustion (more precisely, after combustion) in the inverse direction with the equalization of the temperature plateau corresponding to a higher temperature value of the final region of the SHS-sample.

Nucleation instability in supercooled Cu-Zr-Al glass-forming liquids.

Few general models representing certain classes of real glass-forming systems play a special role in computer simulations of supercooled liquid and glasses. Recently, it was shown that one of the most widely used model glassformers-the Kob-Andersen binary mixture-crystalizes in quite lengthy molecular dynamics simulations, and moreover, it is in fact a very poor glassformer at large system sizes. Thus, our understanding of crystallization stability of model glassformers is far from complete due to the fact that relatively small system sizes and short time scales have been considered so far. Here we address this issue for two embedded atom models intensively used last years in numerical studies of Cu-Zr-(Al) bulk metallic glasses. Exploring the structural evolution of Cu64.5Zr35.5 and Cu46Zr46Al8 alloys at continuous cooling and isothermal annealing, we observe that both systems nucleate in sufficiently lengthy simulations, although critical nucleation time for the latter is an order of magnitude higher than that for the former. We show that Cu64.5Zr35.5 is actually unstable to crystallization for large system sizes (N > 20 000). Both systems crystallize with the formation of tetrahedrally close packed Laves phases of different types. We argue that nucleation instability of the simulated Cu64.5Zr35.5 alloy is due to the fact that its composition is very close to that for the stable Cu2Zr compound with a C15 Laves phase structure.

Polytetrahedral structure and glass-forming ability of simulated Ni-Zr alloys.

Binary Cu-Zr system is a representative bulk glassformer demonstrating high glass-forming ability (GFA). From the first glance, the Ni-Zr system is the most natural object to expect the same behavior because nickel and copper are neighbors in the periodic table and have similar physicochemical properties. However, it is known that the Ni-Zr system has worse GFA than the Cu-Zr one. To understand the underlying physics, we investigate the Ni α Zr1-α system in whole concentration range α ∈ [0, 1]. Doing molecular dynamic simulations with a reliable embedded atom model potential, we show that the simulated Ni-Zr system also has relatively low GFA, which is comparable to that for an additive binary Lennard-Jones mixture without any chemical interaction. Icosahedral local ordering in Ni-Zr alloys is known to be less pronounced than that in the Cu-Zr ones; we see that as well. However, the icosahedron is not the only structural motif responsible for GFA. We find that the local structure of glassy Ni α Zr1-α alloys at 0.3 < α < 0.65 can be described in terms of Z11-Z16 Kasper polyhedra with high density of topological defects including icosahedra as a part of this family. Concentration of topologically perfect Kasper polyhedra appears to be several times smaller than that in Cu-Zr. This is the reason for relatively poor GFA of the Ni-Zr system.

Accelerating high-throughput searches for new alloys with active learning of interatomic potentials

We propose an approach to materials prediction that uses a machine-learning interatomic potential to approximate quantum-mechanical energies and an active learning algorithm for the automatic selection of an optimal training dataset. Our approach significantly reduces the amount of density functional theory (DFT) calculations needed, resorting to DFT only to produce the training data, while structural optimization is performed using the interatomic potentials. Our approach is not limited to one (or a small number of) lattice types (as is the case for cluster expansion, for example) and can predict structures with lattice types not present in the training dataset. We demonstrate the effectiveness of our algorithm by predicting the convex hull for the following three systems: Cu-Pd, Co-Nb-V, and Al-Ni-Ti. Our method is three to four orders of magnitude faster than conventional high-throughput DFT calculations and explores a wider range of materials space. In all three systems, we found unreported stable structures compared to the AFLOW database. Because our method is much cheaper and explores much more of materials space than high-throughput methods or cluster expansion, and because our interatomic potentials have a systematically improvable accuracy compared to empirical potentials such as embedded atom model, it will have a significant impact in the discovery of new alloy phases, particularly those with three or more components.

Molecular dynamics simulation of alloying during sintering of Li and Pb metallic nanoparticles

The sintering-alloying process of Li and Pb nanoparticles at three typical temperatures, 200 K, 250 K and 400 K, are studied by molecular dynamics simulation with the modified analytic embedded-atom model (MAEAM) potential. The three stages, containing the disordering of the contact region, the nucleation and growth of a new phase at the mixed region, are distinctly observed in the three sintering processes. It is verified that the alloying products are B2-LiPb. However, the amount of B2-LiPb is the fewest at 250 K for the reason that an alloying process like liquid-solid sintering occurs as a result of the collapse of Li nanoparticle although the temperature of 250 K is lower than the melting points of Li and Pb nanoparticles. With time, the thickness of the mixed region thickens gradually as growth rate decreases. Besides, the growth rate increases with temperature rising. Moreover, the values of the shrinkage ratio and the average nearest coordination number are increased with time among the three sintering temperatures. As a result, the sinterability and the alloying degree are the highest at 400 K and the lowest at 200 K. In addition, the sphere polycrystalline structure forms at 400 K, while the egg-like configurations are found at the other two temperatures within the simulation time. Furthermore, surface segregation and its effect on sintering alloying process are discussed.

Thermal properties of fcc titanium and aluminum thin films

The molecular dynamics method with many-body potential of interatomic interaction constructed in the embedded atom model is used to study the thermal characteristics of aluminum and metastable fcc titanium nanofilms with (001) and (110) surface orientation. It is shown that in Al films the linear coefficients of thermal expansion are positive in all the directions, except for the area close to the melting temperature. For fcc titanium the linear coefficients of thermal expansion in the film plane are negative in a wide temperature range. With increasing temperature, in Ti films the local vibrational densities of states of the surface atomic layers, polarized along the x,y,z axes, shift to the low-energy part of the spectrum only in those directions where a decrease in the lattice parameters is observed. The changes in the interface areas of Al films occurring with increase in temperature, lead to the growth of anharmonicity and a “softening” of the local vibration spectra near the critical temperature. It is shown that the negative coefficient of linear expansion is an indicator of the simulated system being in metastable state.

Synthesis of Sm–Al metallic glasses designed by molecular dynamics simulations

Solidification of the Sm-rich Al–Sm alloys has been studied using a newly developed Al–Sm embedded atom model potential by molecular dynamics (MD) simulations. We use descriptors for glass forming ability that have been previously validated for Al-rich Al–Sm alloys, and we predict that among the Sm-rich alloys the best glass formers have compositions between 65 and 75 at.% of Sm. To test this prediction, Al35Sm65 and Al25Sm75 alloys were synthesized by melt spinning. These alloys have been found to be good glass formers, in agreement with predictions from MD simulations. We also synthesized Al10Sm90, which according to MD results should have a low glass forming ability. Indeed, after the melt spinning, the Al10Sm90 was found to be crystalline as verified by multiple experimental characterization techniques. This is the first time that Sm-rich Al–Sm metallic glasses with enhanced glass forming ability have been experimentally synthesized. Our results also demonstrate the capability of specific structural descriptors to predict glass forming ability of Al-based alloys.

Grain-Boundary Resistance in Copper Interconnects: From an Atomistic Model to a Neural Network

Orientation effects on the resistivity of copper grain boundaries are studied systematically with two different atomistic tight binding methods. A methodology is developed to model the resistivity of grain boundaries using the Embedded Atom Model, tight binding methods and non-equilibrum Green's functions (NEGF). The methodology is validated against first principles calculations for small, ultra-thin body grain boundaries (<5nm) with 6.4% deviation in the resistivity. A statistical ensemble of 600 large, random structures with grains is studied. For structures with three grains, it is found that the distribution of resistivities is close to normal. Finally, a compact model for grain boundary resistivity is constructed based on a neural network.