Embedded-atom Potential Research Articles

Obtaining Composite Nanoparticles via Electron Beam Irradiation and Modeling the Processes of their Formation by Means of Molecular Dynamics

The partial pressure of vapors of silicon and such metals as silver, copper, and tantalum is estimated and Ag–Si and Ta–Si systems are modeled thermodynamically. Nanoparticles of two types (core–shell and Janus-like) are obtained experimentally via high-temperature gas-phase synthesis using a direct-action electron accelerator. Characteristic features of the formation of composite nanoparticle structures are revealed by means of molecular dynamics, based on modified embedded-atom potentials.

Vitrification and nanocrystallization of pure liquid Ni studied using molecular-dynamics simulation.

Structural variation, vitrification, and crystallization processes in liquid nickel are simulated on continuous cooling and isothermal holding using a classical molecular-dynamics computer simulation procedure with an embedded-atom method potential at constant pressure. Structural changes are monitored with direct structure observation in the simulation cells, as well as by pair distribution and radial distribution functions created using the atomic coordinates. A cluster analysis is also performed. The crystallization kinetics is analyzed under isothermal conditions by monitoring density and energy variation as a function of time. As a result, a time-temperature-transformation diagram can be constructed over a wide temperature range.

Modified embedded-atom method interatomic potential for the Mg–Zn–Ca ternary system

Mg–Zn–Ca alloys are representative Mg alloys with high formability at room temperature. Their high formability is thought to be related to slip, twinning, and recrystallization of the alloys, but the detailed mechanisms have not yet been clarified. To enable atomistic simulations for investigating those behaviors, an interatomic potential for the Mg–Zn–Ca ternary system was developed. The development was based on the second nearest-neighbor modified embedded-atom method formalism, combining previously developed Mg–Zn and Mg–Ca potentials with the newly developed Zn–Ca binary potential. The Zn–Ca and Mg–Zn–Ca potentials reproduce structural, elastic, and thermodynamic properties of compounds and solution phases of relevant alloy systems in reasonable agreement with experimental data, first-principles and CALPHAD calculations. The applicability of the developed potentials is demonstrated through calculations of the effects of Zn and Ca solutes on the generalized stacking fault energy for various slip systems, segregation energy on twin boundaries, and volumetric misfit strain.

Modified embedded-atom method potential for cadmium

The second nearest-neighbor modified embedded atom method (2NN MEAM) is a semi-empirical simulation technique designed to calculate materials properties of metallic and covalent solids. It is a strong candidate for calculation of site occupation, defect association, and diffusion mechanisms in intermetallic compounds, which would provide a tool for interpreting experimental results and for predicting new, interesting measurements to perform. Potentials for 39 elements can be found in the literature, but cadmium, an important element for perturbed angular correlation spectroscopy (PAC), is absent. The purpose of this work was to develop a 2NN-MEAM potential for cadmium. Empirical parameters were determined through optimization of the potential’s ability to reproduce known physical properties of pure cadmium including cohesive energy, lattice parameters, elastic constants, structural enthalpy differences, surface enthalpy, vacancy formation energy, thermal expansion, and specific heat. Prospects for using this new potential with 2NN MEAM potentials for other elements to help interpret previous PAC studies of cadmium diffusion in intermetallic compounds will be discussed.

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.

Embedded atom potential for Sm–Co compounds obtained by force-matching

Abstract We present the results of ab initio simulations of the series of intermetallic Sm–Co compounds. We show that the “open core” approach allows calculating intrinsic properties of Sm–Co compounds with sufficient accuracy for all structures of the series. The calculated magnetic moments, both total and on the cobalt atom, as well as the elastic constants agree with the available experimental data. Using the data, we recover embedded atom potential for Sm–Co alloys, which is suitable for studying the interface of coherent SmCo5 and Sm2Co17 phases.

A systematic study of grain boundary segregation and grain boundary formation energy using a new copper–nickel embedded-atom potential

In this atomistic study on the copper–nickel system, a new embedded-atom alloy potential between copper and nickel is fitted to experimental data on the mixing enthalpy, taking available potentials for the pure components from literature. The resulting phase boundaries of the new potential are in very good agreement with a recent CALPHAD prediction. Using this new potential, a high angle symmetrical tilt Σ5 and a coherent Σ3 twin grain boundary (GB) are chosen for a systematic investigation of equilibrium GB segregation in the semi-grandcanonical ensemble at temperatures from 400 K to 800 K. Applying thermodynamically accurate integration techniques, the GB formation energies are calculated exactly and as an absolute value for every temperature and composition, which also enables the evaluation of GB excess entropies. The thorough thermodynamic model of GBs developed by Frolov and Mishin is excellently confirmed by the simulations quantitatively, if the impact of both segregation and GB tension on the change in GB formation energy is accounted for. In the case of the Σ3 coherent GB, it turns out that the change in GB formation energy at low temperatures is for the most part attributed to the GB tension, while segregation only has a small influence. This demonstrated effect of GB tensions should also be taken into account in the interpretation of experiments.

Study of structural stability of copper crystal with voids from molecular dynamics simulations

We perform molecular dynamics simulations to understand the melting mechanism in Cu crystals (modeled using Embedded Atom Potential) with spherical voids of radii 0.5, 0.6 and 1.0 nm. Along with these systems defect-free Cu and a Cu crystal with layered void is also studied. We analyse crystallinity in these systems using Q6 bond-orientational order parameter, Lindemann index, radial distribution functions and coordination numbers. At 1 atm, all crystals melt between 1700 and 1800 K and melting temperature decreases with increase in void fraction. It is also found that at 900 K voids disintegrate and become a part of the crystal and between 900 and 1700 K crystals amorphize losing long-range order.

Mobility of tungsten clusters on tungsten surfaces

We report on an accelerated molecular dynamics investigation of surface transport mechanisms on W surfaces. These mechanisms likely influence the formation of nano-fuzz in fusion plasma-facing conditions. The simulations use an embedded-atom potential to investigate the diffusion mechanisms of W clusters (Wn, n = 2–9) on W(1 1 0) and W(1 0 0) surfaces at 1000 K, reaching timescales beyond 10 microseconds. A number of fast moving adatom species, Wn (n = 2–6), were observed. In contrast, slightly larger clusters Wn (n = 7–9), especially W8, exhibit significantly lower diffusivities. The diffusion mechanisms of Wn clusters on the W(1 1 0) surface are complex, with the most common elementary steps involving hops of monomers and dimers. Moreover, we observe tetramer hopping in the diffusion processes of Wn (n = 4–7), trimer hopping in Wn (n = 5, 6, 8), and pentamer hopping in W5 on the W(1 1 0) surface. Wn (n ≤ 3) and metastable Wn (n > 3) clusters on the W(1 0 0) surface diffuse predominantly through monomer exchange with surface atoms. However, W clusters containing square tetramers are essentially immobile on the W(1 0 0) surface at 1000 K.

Embedded-atom method interatomic potential for boron nanostructures

Parameters of embedded-atom method interatomic potential for boron are presented in this paper. The potential parameters were determined by means of ab initio data for boron cluster B20, triangular boron sheet, and body-centered cubic structure. The potential has been tested against basic properties of various boron structures. They are face-centered cubic, diamond-like, body-centered tetragonal, icosahedron B12 and icosahedral chain structures. One can conclude that the proposed potential provides a reasonable representation of the interatomic interaction in boron nanostructures, and it is intended for use in large-scale molecular dynamics simulations of boron nanomaterials.

Computer simulation of thermodynamic factors in Ni-Al and Cu-Ag liquid alloys

In this paper, thermodynamic factors of two liquid alloys, Ni-Al and Cu-Ag, are studied by means of molecular dynamics simulation (with the most reliable embedded-atom method potentials) over both wide composition and temperature ranges. For the first time, a comparison is possible between Ni-Al (with a strong ordering tendency) and Cu-Ag (with a strong phase separation tendency). The calculations were performed based on the evaluation of pair correlation functions and mean square thermal fluctuations in composition (the thermodynamic limit of one of the Bhatia-Thornton partial structure factors, SCC). A novel but straightforward computational approach is developed and successfully applied to these calculations. In order to ensure reliability of the employed model description of Ni-Al liquid alloys, the calculated partial and total structure factors are verified by data from neutron scattering experimental measurements (performed as part of this work as well as also being available in the literature). Good agreement is observed for the structure factor SCC for all values of q including the thermodynamic limit q→0. On the other hand, for the total and Faber-Ziman partial structure factors, good agreement is observed only for q values outside the limit q→0. In addition, a similarity between the shapes of the thermodynamic factor composition dependence and the shape of the liquidus lines in both Ni-Al and Cu-Ag liquid alloys is noted. We highlight the significance of the presented computational and experimental study for developing and testing various models and frameworks.

Modified embedded-atom method potential of niobium for studies on mechanical properties

Niobium (Nb) as a refractory metal has appealing mechanical properties due to high ductility at room temperature. Density functional theory (DFT) calculations confirmed its intrinsic ductility by showing its perfect lattice generates an elastic shear deformation preceding its cleavage fracture under 〈100〉 ideal tensile deformation. Based on DFT calculations of simple atomic configurations, we applied an evolution strategy method to build a modified embedded-atom method (MEAM) spline-interpolation potential that can reproduce the intrinsic ductility of Nb during ideal tensile deformation. Further examinations show that simulations based on this MEAM potential can exhibit other essential characteristics of deformation behavior, such as ideal shear strengths, generalized stacking fault energies, twin boundary energies, phonon spectrum, etc., that are consistent with DFT calculations. This potential also outputs the structures, energetic stability and glide mechanism of 12〈111〉 screw dislocations, such as dislocation core structures, core energies, migration barriers and dislocation core trajectories, consistent with DFT calculations. Therefore, this MEAM potential is suitable to study the mechanical behaviors of Nb, especially those under extreme loading conditions and considering non-Schmid effects.

Modeling solution hardening in BCC refractory complex concentrated alloys: NbTiZr, Nb1.5TiZr0.5 and Nb0.5TiZr1.5

Large scale, atomistic simulations of the core structure and mobility of ½[111] screw, edge and mixed dislocations in ternary multicomponent alloys (e.g. High Entropy alloys), NbTiZr, Nb1.5TiZr0.5 and Nb0.5TiZr1.5, are presented. The core structure of ½[111] screw dislocations continuously varies from compact to 3-fold with decreasing Nb content. The screw dislocation core structures in NbTiZr and Nb1.5TiZr0.5 are calculated using Embedded Atom Potentials (Johnson-Zhou) and compared with first-principles calculations of the screw dislocation in a quasi-random structure. In both simulations the dislocation core spreads on different (110) glide planes as the composition varies along the dislocation line in stoichiometric NbTiZr. The Nb-rich composition Nb1.5TiZr0.5 shows a compact core with very little core structure variation along the dislocation line in both First Principles and atomistic simulations. The screw dislocation deposits interstitial and vacancy dipole debris as it moves under stress. Average solute-dislocation core interaction energies in NbTiZr, Nb1.5TiZr0.5 and Nb0.5TiZr1.5 are derived from the average interatomic potential derived for each of the three systems. The interaction energies are used to determine the critical stress for the motion of ½[111] screw dislocations in the three systems as a function of temperature using the Suzuki model of kink migration controlled mobility developed for concentrated BCC random alloys. This analysis shows that the relatively high barrier for kink migration caused by fluctuations in solute concentration along the screw dislocation line and the dipole dragging stress associated with the screw dislocation motion results in a shallow fall-off of critical stress with temperature in these alloys as compared to simple BCC metals. Finally, the screw dislocation to edge and mixed dislocation critical stress ratio in NbTiZr are shown to be low, ∼2 at 5 K, in contrast to simple BCC metals, where it could be as high as 100–1000.

An optimized approach for computing coincidence-site-lattice grain boundary energy

In this study, the grain boundary energy of symmetric and asymmetric iron bicrystals is calculated for Σ3, Σ9 boundaries with 110 tilt axis and Σ5 with <100> tilt axis. The calculations are carried out using molecular dynamics simulations with the embedded-atom method potential. A modified method for creating grain boundary atomic structure is proposed that has sufficient accuracy and its computation cost will be considerably lower than the previously used methods. The effect of three parameters namely rigid body movement, overlapping distance, and reduction side is investigated and compared to previous studies and the optimal parameters are introduced which leads to a better performance in bicrystal modeling.

A computational study of the mixed-mode crack behavior by molecular dynamics method and the multi-parameter crack field description of classical fracture mechanics

The overarching objective of the paper is to analysis the mixed-mode crack propagation direction angles by molecular dynamics method and to investigate the validity of continuum-based linear elastic fracture mechanics crack growth criteria. For this purpose, an embedded atom potential (EAM) available in LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) molecular dynamics (MD) software is utilized to accurately pinpoint mixed-mode crack growth. The study is focused on the application of the different approaches for the determination of the initial crack propagation angle. Copper and aluminum plates with the central crack under complex mechanical stresses (Mode I and Mode II loading) are studied by extensive MD simulations. Williams’ expansion for the crack tip fields containing the higher-order terms is used. The crack propagation direction angles for combinations of Mode I and Mode II loadings are obtained by 1) the multi-parameter fracture mechanics approach based on three fracture mechanics criteria: maximum tangential stress (MTS), maximum tangential strain and strain energy density (SED); 2) atomistic modeling for the mixed-mode loading of the plane medium with the central crack. The temperature effects during fracture processes in MD simulations are considered and the temperature field distributions for mixed-mode crack propagation are obtained.

Molecular dynamics simulation on mechanical behaviors of Ni&lt;sub&gt;x&lt;/sub&gt;Al&lt;sub&gt;100−x&lt;/sub&gt; nanowires under uniaxial compressive stress

This article investigates the nanoscale mechanical properties and deformation mechanism of Nix–Al100−x metallic glasses nanowires (NWs) subjected to uniaxial compressive stress. Molecular dynamics (MD) simulation is carried out using the program package LAMMPS with Embedded-Atom potential. Simulation is performed and focused on the effects of different slenderness ratio, quenching rate, alloy ratio, compression rate, temperature, defects and fracture process of Nix–Al100−x metallic glasses NWs on the mechanical behaviors of these materials. Simulation results show that three possible deformation mechanisms, namely compressive deformation, buckling of structural instability, and lateral extrudes, may occur under different conditions. When the quenching rate is slow, the formation of amorphous phase after quenching is low, but both the corresponding ultimate stress and the Young’s modulus become high. Moreover, under the same quenching rate, the ultimate stress increases with the decrease of the slenderness ratio. For different alloy ratio, it is found that B2 phase of this alloy system exhibits the highest magnitude of both ultimate stress and Young’s modulus. In addition, the concentration effects of point defects on mechanical behaviors of materials are also evaluated and discussed.

A Systematic Study of Grain Boundary Segregation and Grain Boundary Formation Energy Using a New Copper-Nickel Embedded-Atom Potential

Abstract In this atomistic study on the copper–nickel system, a new embedded-atom alloy potential between copper and nickel is fitted to experimental data on the mixing enthalpy, taking available potentials for the pure components from literature. The resulting phase boundaries of the new potential are in very good agreement with a recent CALPHAD prediction. Using this new potential, a high angle symmetrical tilt Σ 5 and a coherent Σ 3 twin grain boundary (GB) are chosen for a systematic investigation of equilibrium GB segregation in the semi-grandcanonical ensemble at temperatures from 400 K to 800 K. Applying thermodynamically accurate integration techniques, the GB formation energies are calculated exactly and as an absolute value for every temperature and composition, which also enables the evaluation of GB excess entropies. The thorough thermodynamic model of GBs developed by Frolov and Mishin is excellently confirmed by the simulations quantitatively, if the impact of both segregation and GB tension on the change in GB formation energy is accounted for. In the case of the Σ 3 coherent GB, it turns out that the change in GB formation energy at low temperatures is for the most part attributed to the GB tension, while segregation only has a small influence. This demonstrated effect of GB tensions should also be taken into account in the interpretation of experiments.

Dislocation Interaction and V-Shaped Growth of the Distorted Structure During Nanoindentation of Cu20Ni20Al20Co20Fe20 (high-entropy alloy)-Coated Copper: A Molecular Dynamics Simulation-Based Study

In this paper, the deformation behavior of Cu20Ni20Al20Co20Fe20 high-entropy alloy-coated single-crystal Cu substrate which undergoes nanoindentation has been investigated under molecular dynamic simulation with embedded-atom method potential. The dynamic structural evolutions under nanoindentation are presented using centrosymmetry parameter analysis, common neighbor analysis and radial distribution function plots. In the initial level of nanoindentation, the interface deformation is greatly confronted by the confined V-shaped growth of the distorted structure. But the sudden discrete dislocation burst account for avalanche break-down in the interface layer, which further get influenced by the evolution of multiple dislocation nodes that is significantly governed by core spreading, extended misfit dislocation generation and relative rotation. In the meanwhile, the subsequent generation of dislocation locks, complicated multiple dislocation loops, dislocation junctions and limited cross-slip in wide stacking faults (SFs) hasten the work hardening and in turn slows down the deformation progress. On the other hand, the intermediate appearance of narrow SFs and slip bands significantly reduces the work hardening rate that increases the optimum fracture strain value of the specimen. Moreover, the overall increase in dislocation density and dislocation length leads to a significant growth in dislocation sources which leads to forest hardening in the later stage.

Atomic Scale Study on the Tension-Compression Asymmetry of Single Crystalline and Nanocrystalline NiTi Shape Memory Alloy

The tension-compression asymmetry of single crystalline and nanocrystalline NiTi shape memory alloys (SMAs) is investigated at atomic scale utilizing molecular dynamics (MD) simulation. This study is based on the second nearest neighbor modified embedded-atom method interatomic potential for Ni-Ti binary systems. First of all, multiple variants of martensite are simulated via temperature-induced martensitic transformation in single crystal and nanocrystalline. The characteristics of forward and reverse martensitic transformation is derived by means of the atomic structural evolution. Secondly, it is observed that the tension-compression asymmetry for single crystalline NiTi is attributed to types of stress induced martensitic variants and martensitic reorientation, which respectively leads to stress asymmetry and strain asymmetry. However, the phenomenological tension-compression asymmetry in nanocrystalline is mainly credited to stress-induced martensitic variants, no obvious martensitic reorientation occurs under tension and compression loads. The corresponding atomic-scale structural evolution also explains why the length of tensile stress plateau in nanocrystalline is shorter than that of the single crystal. Finally, the nanocrystalline tension-compression asymmetric behavior is simulated under a broad range of temperature to obtain the critical stress-temperature relation. The simulation results are compared with the corresponding results obtained by experiment reported in the literature. The tension-compression asymmetry mechanism of single crystalline and nanocrystalline NiTi shape memory alloy is derived based on numerical results at atomic scale.

Embedded-atom potential for Ni-Al alloy

We construct a new embedded-atom potential for Ni-Al system. The lattice constants, cohesive energies, elastic constants, vacancy formation energies, stacking fault energies and equations of state of Ni and Al are included in the fitting process of potentials for pure Ni and pure Al. The cross-interaction potential is fitted to the lattice constants, cohesive energies, elastic constants of Ni3Al and NiAl and the energies of (100) APB and (111) SISF of Ni3Al. The potential accurately reproduces the various physical properties of Ni, Al, Ni3Al and NiAl phases. The results of the new embedded-atom potential and other embedded-atom potentials are compared and discussed in detail.