Embedded Atom Model Potential Research Articles

Structure and thermodynamics of liquid ruthenium and ruthenium-based alloys from ab initio and classical molecular dynamics with embedded atom model potentials

The combination of classical and ab initio molecular dynamics simulations for computing structural and thermodynamic properties of metallic liquids is illustrated on the example of ruthenium and ruthenium-based alloys. The classical simulations used embedded atom model (EAM) potentials parametrized with the force matching method. The ab initio reference data were obtained using two electronic structure codes implementing the density functional theory plane wave/pseudopotential method. Several methodological aspects in the determination of structural and thermodynamic properties in the liquid phase are examined, first for pure ruthenium. The efficiency of this combined method is finally illustrated on the structure and the pressure of ternary alloys of platinum group metals of interest in the treatment of nuclear wastes.

Response embedded atom model potential of Pb at finite temperature: application on the dislocation mobility

Dislocation is a major carrier of plastic deformation for metal materials and are crucial. Understanding the mechanism of dislocation motion is beneficial for understanding the plastic deformation of materials under dynamic loading. In this work, a new response EAM (REAM) potential is developed for the applications under high pressure and finite temperature conditions. We use the REAM potential to investigate the behaviors of edge and screw dislocations in Pb by molecular dynamics (MD) simulations, and compare it with two commonly used EAM potentials. Specially, we examine the influence of the stacking fault energy and the temperature-dependent elastic constants on the dislocation motions. Our results show that the temperature-dependent elastic constants do not considerably affect the dislocation motion at the linear region of low stress, while the stacking fault energy plays a significant role. In the nonlinear region, the stacking fault energy and elastic constant together influence the dislocation motion. In subsonic and low transonic regimes, the dislocation width oscillates with time, but eventually fluctuates around equilibrium width.

The interplay between size, shape, and surface segregation in high-entropy nanoalloys.

The respective influences of particle shape and size on the energetic stability of five-component multimetallic nanoparticles have been computationally investigated for AlCuFeCrNi and AuCuPdNiCo mixtures at equiconcentration. Using available embedded-atom model potentials, exchange Monte Carlo simulations possibly assisted with systematic quenching, we explore tools to approach ideal phase equilibrium in such high-entropy nanoalloys. In particular, we show how deviations to ideal solid solution behaviors can be characterized using percolation analyses, and how the contribution of alloying fluctuations at finite temperature can be inferred to evaluate the entropy of mixing in such nonideal cases. An approximation to the entropy of mixing based on pair correlations only is also found to capture the behavior of the thermodynamical mixing entropy quite well, and can be used as an order parameter of mixing. While the AlCuFeCrNi mixture appears to mix reasonably well in all cases considered, cobalt and nickel segregate significantly in AuCuPdNiCo nanoparticles, deviating strongly from ideal random mixtures. A simple Gaussian regression model applied to a coarse distribution of concentrations is found to correctly predict conditions for optimising the mixing thermodynamical properties of the miscible AlCuFeCrNi nanoparticle.

Atomistic simulation of α-Fe(100)-lead-bismuth eutectic (LBE) solid-liquid interface

Behaviors of body centered cubic (bcc) Fe(100)-liquid lead-bismuth eutectic (LBE) solid-liquid interface have been studied by classical molecular dynamics simulation (MD) with embedded-atom model potential under anaerobic conditions. Both static and dynamic simulation results indicate that Fe atoms prefer to bind with Bi atoms rather than Pb atoms. Trajectories of Bi and Pb perpendicular to the interface show obvious thermal vibrations of both liquid components at the time scale of about 0.9 ns for atoms Bi, and about 0.3 ns for atoms Pb, respectively. Temperature effect on penetration of liquid atoms into bulk Fe and dissolution of Fe into liquid in the solid-liquid interface system show that interactions between the bcc Fe(100) and liquid LBE mount perceptibly in the temperature range of 823 to 873 K. This result indicates that the critical temperature of the interaction between the bcc Fe(100) and liquid LBE is between 823 K and 873 K, which is consistent with the designed operating temperature for LBE coolant.

Nucleation of Twinning Dislocation Loop in Pt: A Computational Approach

Twinning plays a critical part in the plastic deformation of the materials and the strengthening mechanisms and is hence considered as one of the most prevalent deformation mechanisms in metals. Because to the high stacking fault energy of the fcc metals like Al, Pd, and Pt, the extended dislocations are believed to be energetically favored over isolated partials, thereby rendering deformation twinning unfeasible. Nevertheless, some recent experimental researches have confirmed a potential deformation twinning pathway in nanocrystalline platinum. This alternate-shear mechanism has a much lower energy barrier than the usual layer-by-layer twinning. We utilize computations involving atomistic calculations and continuum modeling in this study to examine the genesis of deformation twins in Pt. Atomistic simulations provides the generalized planer fault energy using an EAM (embedded-atom-model) potential. Moreover, a potential energy-based method, namely; a nudged-elastic band (chain of states) has been used to compute the activation energy barrier for the nucleation of the twinning dislocation loop in the alternate-shear model. The critical stress needed for the nucleation of the twinning dislocation loop in platinum is estimated using some of the parameters acquired from atomistic calculations and using them as fitting parameters in the continuum model. The minimum-energy path between the two end states can be identified using this methodology. Through the unusual alternate-shear approach, the results provide a rudimentary but essential dislocationbased perspective of the occurrence of deformation twins in fcc metals.

Computer Modeling of Sodium in the Embedded Atom Model

The pair contribution to the potential of sodium in the embedded atom model (EAM) is refined. Two potentials (EAM-2 and EAM-3) that differ in the shape of the embedding potentials are calculated from data on the shock compression and isothermal compression of sodium at 298 K. The main thermodynamic, structural, and diffusion properties of the models are calculated in two variants. The EAM-2 potential poorly describes the properties of sodium at 298 K, while the EAM-3 potential inaccurately characterizes the properties under conditions of shock compression. This means the fixed EAM potential fails to describe the temperature dependence of the properties of the metal. At 900 K, the pressure in the models with the EAM-3 potential is close to the data obtained ab initio. There is no prepeak of the pair correlation function of sodium, but the anomalous behavior of the pressure of the sodium bcc lattice at ∼19–25 GPa is confirmed. The melting lines in the sodium models are calculated in two variants with maxima at around 30 GPa. The problem of the predictive power of the embedded atom model is discussed.

Analysis of Shock Compression Data for Porous Samples

The literature data on shock compression of compact and porous samples of a number of metals (copper, tin, lead, bismuth, iron, and nickel) and the use of these data in calculations of interparticle potentials in the embedded atom model (EAM) were considered. The Hugoniot shock adiabats of compact metals provide information for calculations of EAM potentials, but as the porosity of the initial samples increases, they become unsuitable for this purpose because of inconsistency with the data obtained on compact samples. Possible reasons for this inconsistency were considered.

Pressure-Temperature Phase Diagram of Lithium, Predicted by Embedded Atom Model Potentials.

In order to study the performance of interatomic potentials and their reliability at higher pressures, the phase diagrams of two different embedded-atom-type potential models (EAMs) and a modified embedded-atom model (MEAM) of lithium are compared. The calculations were performed by using the nested sampling technique in the pressure range 0.01-20 GPa, in order to determine the liquid-vapor critical point, the melting curve, and the different stable solid phases of the compared models. The low-pressure stable structure below the melting line is found to be the body-centered-cubic (bcc) structure in all cases, but the higher pressure phases and the ground-state structures show a great variation, being face-centered cubic (fcc), hexagonal close-packed (hcp), a range of different close-packed stacking variants, and highly symmetric open structures are observed as well. A notable behavior of the EAM of Nichol and Ackland (Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93, 184101) is observed, that the model displays a maximum temperature in the melting line, similarly to experimental results.

MXE: A package for simulating long-term diffusive mass transport phenomena in nanoscale systems

We present a package to simulate long-term diffusive mass transport in systems with atomic scale resolution. The implemented framework is based on a non-equilibrium statistical thermo-chemo-mechanical formulation of atomic systems where effective transport rates are computed using a kinematic diffusion law. Our implementation is built as an add-on to the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) code, it is compatible with other LAMMPS’ functionalities, and shows a good parallel scalability and efficiency. In applications involving diffusive mass transport, this framework is able to simulate problems of technological interest for exceedingly large time scales using an atomistic description, which are not reachable with the state-of-the-art molecular dynamics techniques. Several examples, involving complex diffusive behavior in materials, are investigated with the framework. We found good qualitative and quantitative comparison with known theories and models, with Monte Carlo methods, as well as with experimental results. Thus, our implementation can be used as a tool to understand diffusive behavior in materials where experimental characterization is difficult to perform. Program summaryProgram Title: MXE packageProgram Files doi:http://dx.doi.org/10.17632/s2mhjb8hyk.1Licensing provisions: GNU GPLv3Programming language: c++Supplementary material: The user manual and examples are provided along with the source code.External routines: MPI, LAMMPS, 12 December 2018 (http://lammps.sandia.gov/)Nature of problem: The simulation of diffusive mass transport in atomistic systems, involving vacancies, interstitials and solute atoms, is often challenging due to the exceedingly large time scale involved in these slow processes. Thus, molecular dynamics, a preferred technique to simulate atomistic systems, is not capable of accessing these long-term diffusive timescale, hindering its application to this kind of phenomena.Solution method: We present an implementation for simulating diffusive mass transport in atomic system. The methodology is based on two main pillars: (i) A non-equilibrium thermodynamic formulation of atomic system (Venturini et al., 2014); and (ii) Fokker–Planck master equation that encompasses the time evolution of the atomic molar fraction field in atomic systems (Ponga and Sun, 2018). The proposed implementation is built as a user package of the popular Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). The implementation is flexible and robust, shows good parallel scalability and efficiency, and is compatible with all features available in LAMMPS.Additional comments including restrictions and unusual features: Unique features of the implementation involve the simulation of vacancy, interstitial, solutes and substitutional alloys for exceedingly large time scales.The current implementation in this package is limited to the Embedded Atom Model potentials.

Polytetrahedral short-range order and crystallization stability in supercooled Cu64.5Zr35.5 metallic liquid

Development of reliable interatomic potentials is crucial for theoretical studies of relationship between chemical composition, structure and observable properties in glass-forming metallic alloys. Due to ambiguity of potential parametrization procedure, some crucial properties of the system, such as crystallization stability or symmetry of the ground state crystal phase, may not be correctly reproduced in computer simulations. Here we address this issue for ${\rm Cu_{64.5}Zr_{35.5}}$ alloy described by two modifications of embedded atom model potential as well as by \textit{ab initio} molecular dynamics. We observe that, at low supercooling, both models provide very similar liquid structure, which agrees with that obtained by \textit{ab initio} simulations. Hoverer, deeply supercooled liquids demonstrate essentially different local structure and so different crystallization stability. The system, which demonstrate more pronounced icosahedral sort-range order, is more stable to crystallization that is in agreement with Frank hypothesis.

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.

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.

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.

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.

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.

Effect of grain boundaries on shock-induced phase transformation in iron bicrystals

Non-equilibrium molecular-dynamic simulations with a modified analytic embedded-atom model potential have been performed to investigate the effect of three kinds of grain boundaries (GBs) on the martensitic transformation in iron bicrystals with three different GBs under shock loadings. Our results show that the phase transition was influenced by the GBs. All three GBs provide a nucleation site for the α → ε transformation in samples shock-loaded with up = 0.5 km/s, and in particular, the elastic wave can induce the phase transformation at Σ3 ⟨110⟩ twist GB, which indicates that the phase transformation can occur at Σ3 ⟨110⟩ twist GB with a much lower pressure. The effect of GBs on the stress assisted transformation (SAT) mechanisms is discussed. All variants nucleating at the vicinity of these GBs meet the maximum strain work (MSW) criterion. Moreover, all of the variants with the MSW nucleate at Σ5 ⟨001⟩ twist GB and Σ3 ⟨110⟩ tilt GB, but only part of them nucleate at Σ3 ⟨110⟩ twist GB. This is because the coincident planes between both sides of the GB would affect the slip process, which is the second stage of the martensitic transformation and influences the selection of variant. We also find that the martensitic transformation at the front end of the bicrystals would give rise to stress attenuation in samples shock-loaded with up = 0.6 km/s, which makes the GBs seem to be unfavorable to the martensitic transformation. Our findings have the potential to affect the interface engineering and material design under high pressure conditions.

Advanced approach to the local structure reconstruction and theory validation on the example of the W L3-edge extended x-ray absorption fine structure of tungsten

Atomistic simulations of the experimental W L3-edge extended x-ray absorption fine structure (EXAFS) of bcc tungsten at T = 300 K were performed using classical molecular dynamics (MD) and reverse Monte Carlo (RMC) methods. The MD-EXAFS method based on the results of MD simulations allowed us to access the structural information, encoded in EXAFS, beyond the first coordination shell and to validate the accuracy of two interaction potential models—the embedded atom model potential and the second nearest-neighbor modified embedded atom method potential. The RMC-EXAFS method was used for more elaborate analysis of the EXAFS data giving access to thermal disorder effects. The results of both methods suggest that the correlation in atomic motion in bcc tungsten becomes negligible above 8 Å. This fact allowed us to use the EXAFS data to determine not only mean-square relative displacements of atomic W–W pair motion but also mean-square displacements of individual tungsten atoms, which are usually accessible from diffraction data only.

Molecular dynamics simulation of the thermodynamic properties of mercury at pressures below 2.5 GPa and temperatures below 10000 K

Models of mercury were constructed by molecular dynamics using the interparticle potential of the embedded atom model (EAM) at temperatures below 10 000 K and pressures below 2.5 GPa. The thermodynamic properties of the models were presented on the isobars of 0.5, 1.0, 1.5, 2.0, and 2.5 GPa. The compressibility factors Z = pV/(RT) were calculated; the coordinates of the inversion points of the Joule–Thomson coefficient below 5600 K were found from the positions of minima on the Z(p, T) isobars. At densities above 8–9 g/cm3, the results of simulation agreed well with experiment; at lower densities there were discrepancies associated with a loss of metal properties by real mercury. The behavior of the models was analyzed in the region of the van der Waals loop. The calculated critical temperature of mercury was found to be significantly overestimated relative to the experiment. Modeling the “meta-mercury” with the EAM potential with excluded embedded potential contribution gave better agreement with the equation of state of mercury at lower densities. The states with Z = 1 can be observed below 1.0 GPa. The calculated temperature of the inversion of the Joule–Thomson coefficient increased monotonically to 5600 K as the pressure increased to 2.5 GPa.

Network Connectivity in Icosahedra Medium-Range Order of Metallic Mg&lt;sub&gt;70&lt;/sub&gt;Zn&lt;sub&gt;30&lt;/sub&gt; Alloy

The molecular dynamics simulations with embedded atom model (EAM) potential had performed to investigate the icosahedral network connectivity in Mg70Zn30 alloy. The microstructure was detected with a new precise method of largest standard cluster analysis. It was validated that the EAM potential is succeed in reflecting the objective physical nature of Mg-Zn alloy systems. Results shows that large amount of nanoclusters consist of ICOIs, which shows large connectivity variations, formed in the system with decreasing temperature. And the ICOIs connect over extended range act as backbone for a networked structure.