Atom Model Potential Research Articles

Role of grain size on the martensitic transformation and ultra-fast superelasticity in shape memory alloys

We use multi-million-atom molecular dynamics (MD) simulations with an embedded atom model potential parameterized for NiAl to study temperature- and stress-induced martensitic phase transformations in nanocrystalline shape memory alloys. Nucleation of the martensite phase occurs in the grain interiors and grows outward up to the point where further transformation is hindered by the constraints imposed by neighboring grains. Decreasing grain size inhibits the transformation process and the temperature-induced transformation is completely suppressed for samples with average grain sizes of 7.5nm and less. Interestingly, mechanical loads can induce the martensitic transformation in samples with ultra-fine grains and, quite surprisingly, the sample with 7.5nm grain size exhibits improved, ultra-fast, superelasticity as compared with its coarser grain counterparts. The simulations provide a picture of the processes that govern the performance and fundamental limits of nanocrystalline shape memory alloys with atomistic resolution.

Molecular-dynamic modeling of thermodynamic properties of the lunar Fe-S core

The physical properties (density, modules of compression, heat capacity, and velocity of the P-waves) of the lunar Fe-S core are modeled with molecular dynamics and the Embedded Atom Model potential at 0–18 at % S content, temperatures up to 2500 K, and pressures up to 5 GPa. The thermodynamic calculations of the velocity of P-waves in the liquid lunar core are consistent with the interpreted results of seismic processing of the Apollo observations. According to calculations, the density of the liquid core may vary from 7.4 (pure liquid iron) to 6.75 (10 at % S at 1950 ± 50 K) g/cm3. This approach provides more reliable restrictions for the inner structure of the Moon.

Negative thermal expansion and anomalies of heat capacity of LuB50 at low temperatures.

Heat capacity and thermal expansion of LuB50 boride were experimentally studied in the 2-300 K temperature range. The data reveal an anomalous contribution to the heat capacity at low temperatures. The value of this contribution is proportional to the first degree of temperature. It was identified that this anomaly in heat capacity is caused by the effect of disorder in the LuB50 crystalline structure and it can be described in the soft atomic potential model (SAP). The parameters of the approximation were determined. The temperature dependence of LuB50 heat capacity in the whole temperature range was approximated by the sum of SAP contribution, Debye and two Einstein components. The parameters of SAP contribution for LuB50 were compared to the corresponding values for LuB66, which was studied earlier. Negative thermal expansion at low temperatures was experimentally observed for LuB50. The analysis of the experimental temperature dependence for the Gruneisen parameter of LuB50 suggested that the low-frequency oscillations, described in SAP mode, are responsible for the negative thermal expansion. Thus, the glasslike character of the behavior of LuB50 thermal characteristics at low temperatures was confirmed.

Atomic collective excitations in liquid lead

The atomic dynamics of liquid lead at the temperature T = 600 K has been simulated on the basis of the embedded atom model potential (the “embedded” atom model making it possible to effectively take into account the many-particle interactions) in order to study the mechanisms of formation of the atomic collective excitations. Spectra of the dynamic structure factor S(k, ω) and the spectral densities of the time correlation functions of the longitudinal \(\tilde C_L\) (k, ω) and transverse \(\tilde C_T\) (k, ω) currents have been calculated for the wavenumber region 0.11 A−1 ≤ k ≤ 2.01 A−1. It has been established that the dynamics of density fluctuations is characterized by two dispersion “acoustic-like” branches of the longitudinal and transverse polarization.

Vapor-liquid equilibrium simulation of binary and ternary mixtures of CH4, C2H4 and iso-C4H10

The vapor-liquid equilibrium (VLE) properties for the binary and ternary mixtures of CH4, C2H4 and iso-C4H10 are of great importance in the recovery of ethylene from mixture containing CH4 and C2H4 with iso-C4H10 as solvent. Hence, Gibbs ensemble Monte Carlo (GEMC) simulations were used to estimate vapor-liquid equilibrium for the binary and ternary mixtures of CH4, C2H4 and iso-C4H10 with the united atom potential NERD model. The selected simulation conditions are based on the experiment in the literature. The results of this work were shown to be in satisfactory agreement with available experimental data and predictions of Peng-Robinson equation of state. The structure of simulated liquid phase is also characterized by radial distribution function (RDF), which contributes to further understanding of the VLE curve of these systems. RDF is not sensitive to the pressure and temperature range. With the increase of pressure or the decrease of temperature, the molecules tend to gather together.

Calculation of the surface tension of pure tin from atomistic simulations of liquid–vapour systems

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

Role of atomic variability and mechanical constraints on the martensitic phase transformation of a model disordered shape memory alloy via molecular dynamics

We use molecular dynamics (MD) with an embedded atom model potential parameterized for NiAl to study martensitic phase transformations in a disordered shape memory alloy. We focus on the role of intrinsic atomic-level variability and mechanical constraints on the martensite and austenite transformation temperatures and on the martensite microstructure for specimens with varying size. We find that periodic system size has a weak effect on transformation temperature all the way to the nanometer scale, with the entropy-stabilized austenite phase slightly penalized with decreasing size. Atomic-level variability in these random alloys leads to significant sample-to-sample variability in transformation temperature. The uncertainty in the austenite transformation temperature increases with decreasing size, reaching ∼10% of the mean value for samples 10nm on the side. Interestingly, the variability of the high-temperature martensite transition shows little size dependence. We find that a critical size of ∼40nm is required to develop multidomain martensite microstructures, and mechanical constraints reduce this critical size to ∼7nm, while significantly affecting the transformation temperatures. These results contribute to the understanding of martensitic transformation in nanocrystalline samples and of the fundamental limits of miniaturization of these alloys.

Precipitates of Cr at Σ3 <110> {112} GB in α-Fe

ABSTRACTPrecipitates of Cr at Σ3 <110> {112} GB in α-Fe have been studied using molecular dynamics with a two-band embedded atomic model potential. The accumulation and segregation of Cr atoms and the evolution of the GB depend on local Cr concentration and temperature. At the early stage, with the existence of vacancies, the strong attraction of Cr with the GB core provides a pathway for Cr atoms to quickly accumulate within the GB core. With the increase of Cr concentration, the size of Cr dilute precipitates increases dramatically. And the strong segregation of Cr at the GB is observed, when Cr concentration reach 20%. Also, the size of Cr precipitates increases with increasing the temperatures from 300 K to 1000 K. The accumulation and segregation of Cr atoms at the GB lead to significant deformation of the GB structure and the formation of GB steps, causing the displacement and broadening of the GB.

Molecular dynamics simulations of shock-induced plasticity in tantalum

We present Non-Equilibrium Molecular Dynamics (NEMD) simulations of shock wave compression along the [001] direction in monocrystalline Tantalum, including pre-existing defects which act as dislocation sources. We use a new Embedded Atom Model (EAM) potential and study the nucleation and evolution of dislocations as a function of shock pressure and loading rise time. We find that the flow stress and dislocation density behind the shock front depend on strain rate. We find excellent agreement with recent experimental results on strength and recovered microstructure, which goes from dislocations to a mixture of dislocations and twins, to twinning dominated response, as the shock pressure increases.

Structure and dynamics in the crystallization of Ni500 nanocluster

Molecular dynamics simulations are performed on nano clusters with 500 nickle atoms (Ni500) to determine the structure and dynamics in the isothermal crystallization using embed atom model potentials. Through the study of nano Ni cluster dynamic behavior, we find that multi-step crystallization process of Ni500 is at temperatures far below the melting temperature (Tm), whereas one-step crystallization process is at the temperatures just below Tm. Multi metastable states are observed before Ni500 becomes fully crystallized in multi-step crystallization process. We find that atom rearrangements take not only string-like cooperative motion manner but also other collective motion manner.

Molecular Dynamics Simulation of Liquid-Vapor Coexistence Curves of Metals

A Molecular dynamics implementation of Gibbs ensemble method is applied to determine liquid-vapor coexistence curves of metals using embedded atom model potentials. As an application of the code we developed, the liquid-vapor coexistence curves of Aluminum and Copper are simulated using Cai and Ye potential. The critical constants obtained were found to be slightly lower than the range of experimental results. The results show that the potentials parameters obtained by fitting to low temperature solid properties are not adequate to accurately determine the liquid-vapor phase diagram.

Computer simulation of the properties of liquid metals: Gallium, lead, and bismuth

The embedded atom model (EAM) potentials of liquid gallium, lead, and bismuth calculated by the author using the Schommers algorithm were refined and written in a unified analytic form more convenient for applications. Pair contributions to EAM potentials are described by piecewise continuous functions. The form of EAM potentials admits the transition to a high-density state characteristic of shock compression. Series of models of these liquid metals were constructed by the molecular dynamics method at temperatures up to 1500 (Zn), 3000 (Ga, Pb), and 1800 K (Bi). For all the metals, close agreement with experiment was obtained over the whole temperature range for density, structure, bulk compression modulus, and self-diffusion coefficient. The standard deviations of model pair correlation functions (PCF) from the diffraction PCFs of gallium and lead were on the order of 0.01. As distinct from alkali metals, the calculated energy of gallium and lead models was close to actual energy over the whole temperature range, and excess electronic heat conductivity was almost unobservable. With bismuth, agreement with experiment for energy and structural characteristics was noticeably worse, which shows that the embedded atom model is less applicable to bismuth.

Computer simulation of liquid zinc

An embedded atom model potential for zinc has been developed, which makes it possible to calculate liquid zinc properties both under normal pressure and in strongly compressed states using the molecular dynamics method. In order to calculate the potential, the data on density, energy, and compressibility of liquid zinc and the data on shock compression of zinc were used. Pair contribution to the potential and the embedding potential are represented by analytical functions. Liquid zinc properties are calculated at temperatures up to 1500 K. The values of energy, bulk compression modulus, and self-diffusion coefficient, as well as pair correlation functions at T < 1000 K, agree well with the experiment. The electron contribution to the thermal capacity at those temperatures is not high. Zinc models are constructed for densities up to 15.86 g/cm3 and pressures up to 773 GPa. Zinc models melt in the case of shock compression at compression rates of V/V0 < 0.7 and temperatures above 1900 K. A significant contribution of electron excitation energy to the zinc energy is observed at temperatures above 20000 K. The estimated average surplus thermal capacity of electrons at 30000–50000 K is ∼12 J/mole K. Discrepancies between the molecular dynamic calculation and the Gruneisen model at low temperatures are relatively low; however, they rise as temperature increases. A series of zinc nanocluster models with magic sizes of 55 and 147 atoms is constructed. The clusters have an amorphous structure with slightly lower energy than that of icosahedral or cuboctahedral configuration, after cooling from 600 to 10 K. The surface energy of zinc at T = 0 calculated based on the dependence of energy of clusters on size is 1.3 J/m2.

Atomistic simulation of fcc—bcc phase transition in single crystal Al under uniform compression

By molecular dynamics simulations employing an embedded atom model potential, we investigate the fcc-to-bcc phase transition in single crystal Al, caused by uniform compression. Results show that the fcc structure is unstable when the pressure is over 250 GPa, in reasonable agreement with the calculated value through the density functional theory. The morphology evolution of the structural transition and the corresponding transition mechanism are analysed in detail. The bcc (011) planes are transited from the fcc (111̄) plane and the (11̄1) plane. We suggest that the transition mechanism consists mainly of compression, shear, slid and rotation of the lattice. In addition, our radial distribution function analysis explicitly indicates the phase transition of Al from fcc phase to bcc structure.

Critical assessment of UO2 classical potentials for thermal conductivity calculations

This article reviews the thermal transport properties as predicted by 26 classical interatomic potentials for uranium dioxide, an important nuclear fuel material, determined using a lattice dynamics-based method. The calculations reveal structural instabilities for multiple potentials, as well as the presence of lower energy structures even for potentials in which the fluorite structure is stable. Both rigid atom and shell model potentials are considered, and general trends in their representation of the thermal conductivity are identified. Reviewed classical potentials predict thermal conductivity in the range of ~5–22 W/mK, compared to the experimental value of 8.9 W/mK. The quality of the anharmonicity correction that is based on the correlation between thermal expansion and thermal conductivity is investigated, and it found to generally improve thermal conductivities results.

Calculation of migration rates of vacancies and divacancies in α-Iron using transition path sampling biased with a Lyapunov indicator

ABSTRACTPredicting the microstructural evolution of radiation damage in materials requires handling the physics of infrequent-events, in which several time scales are involved. The reactions rates characterizing these events are the main ingredient for simulating the kinetics of materials under irradiation over large time scales and high irradiation doses. We propose here an efficient, finite temperature method to compute reaction rate constants of thermally activated processes. The method consists of two steps. Firstly, rare reactive trajectories in phase-space are sampled using a transition path sampling (TPS) algorithm supplemented with a local Lyapunov bias favoring diverging trajectories. This enables the system to visit transition regions separating stable configurations more often, and thus enhances the probability of observing transitions between stable states during relatively short simulations. Secondly, reaction constants are estimated from the unbiased fraction of reactive trajectories, yielded by an appropriate statistical data analysis tool, the multistate Bennett acceptance ratio (MBAR) package. We apply our method to the calculation of reaction rates for vacancy and di-vacancy migration in α-Iron crystal, using an Embedded Atom Model potential, for temperatures ranging from 300 K to 800 K.

Molecular dynamics simulation of shock compression of metals: Iron and iron-sulfur solutions

The embedded atom model potential suggested earlier was improved to correctly describe iron at high pressures and temperatures. Correction was introduced using the shock compression data. The properties of body- and face-centered cubic (BCC and FCC) lattices and liquid iron at compression degrees up to 50% of the normal volume and temperatures up to 10000 K were calculated. At degrees of compression 0.7–0.6 and 0 K, the FCC lattice is thermodynamically stable. The temperature of fusion increases to ≈9700 K at compression to 50% of initial volume (pressure 585 GPa). The pressure of pure iron at 5000 K and density 12.5 g/cm3 is ≈250 GPa and is substantially lower than in the center of the Earth according to the geophysical data (360 GPa). An embedded atom model potential for a 10 at % solution of sulfur in iron which allows the properties of the melt in the center of the Earth to be described correctly is suggested; the viscosity of the melt under these conditions is not high (0.0156 Pa s); these results are close to those obtained in ab initio calculations. The possibility of partial Earth core crystallization is shown.

Molecular dynamics study of the melting curve of NiTi alloy under pressure

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

Structures of AgPd nanoclusters adsorbed on MgO(100): A computational study

The structures of AgPd clusters supported on MgO(100) are investigated via a combination of global optimization searches within an atom–atom potential model and density-functional calculations. Although pure Ag and Pd clusters favor fcc structures in (100) epitaxy with the substrate, it is found that AgPd mixing creates a pocket of stability for polyicosahedral structures in the AgPd/MgO(001) nanoalloy phase diagram. Polyicosahedra are very stable for size around 40 atoms, where they compete with fcc(100) and decahedral structures. For clusters with up to 200 atoms and 50%–50% composition, these last two structural motifs are the most stable ones at the atom–atom potential level. For sizes of 400 atoms, fcc structures in (111) epitaxy with the substrate become close in energy to fcc(100) and Dh clusters, indicating a likely transition to (111) epitaxy for larger sizes.

Molecular dynamics simulation of mechanical properties of Ni–Al nanowires

We employed molecular dynamics simulations to study mechanical properties of Ni–Al nanowires by calculating the stress–strain response of the wires under various loading conditions. For this purpose, nanowires were subjected to uniaxial strain at different strain rates and temperatures using embedded atom model potential. The behaviour of the wires at lower and higher strain rates was investigated, and the yield and rupture strain values and also Young’s Modulus were obtained which are essential factors for the ductility of the wires. This work indicates that how the stress–strain response of the nanowires are affected by varying strain rates and temperatures.