Modified Embedded Atom Method Research Articles

Atomistic studies of transformation pathways and energetics in plutonium

One of the most challenging problems in understanding the structural phase transformations in Pu is to determine the energetically favoured, continuous atomic pathways from one crystal symmetry to another. This problem involves enumerating candidate pathways and studying their energetics to garner insight into instabilities and energy barriers. The purpose of this work is to investigate the energetics of two transformation pathways for the δ → α′ transformation in Pu that were recently proposed on the basis of symmetry. These pathways require the presence of either an intermediate hexagonal closed-packed (hcp) structure or a simple hexagonal (sh) structure. A subgroup of the parent fcc and the intermediate hexagonal structure, which has trigonal symmetry, facilitates the transformation to the intermediate hcp or sh structure. Phonons then break the translational symmetry from the intermediate hcp or sh structure to the final monoclinic symmetry of the α′ structure. We perform simulations using the modified embedded atom method (MEAM) for Pu to investigate these candidate pathways. Our main conclusion is that the path via hcp is energetically favoured and the volume change for both pathways essentially occurs in the second step of the transformation, i.e. from the intermediate sh or hcp to the monoclinic structure. Our work also highlights the deficiency of the current state-of-the-art MEAM potential in capturing the anisotropy associated with the lower symmetry monoclinic structure.

Atomistic modeling of III–V nitrides: modified embedded-atom method interatomic potentials for GaN,InN and Ga1−xInxN

Modified embedded-atom method (MEAM) interatomic potentials for the Ga–N and In–Nbinary and Ga–In–N ternary systems have been developed based on the previously developedpotentials for Ga, In and N. The potentials can describe various physical properties(structural, elastic and defect properties) of both zinc-blende and wurtzite-type GaN andInN as well as those of constituent elements, in good agreement with experimental data orhigh-level calculations. The potential can also describe the structural behavior ofGa1−xInxN ternary nitrides reasonably well. The applicability of the potentials toatomistic investigations of atomic/nanoscale structural evolution inGa1−xInxN multi-component nitrides during the deposition of constituent element atoms is discussed.

Simulating radiation damage in δ-plutonium

Radiation events in δ-Pu (fcc) have been simulated in an attempt to understand the fundamental mechanisms that contribute to the Pu ageing process. The Pu interactions are modelled using a potential based on the modified embedded atom method (MEAM). The energetics of point defects have been investigated using static calculations together with molecular dynamics (MD) to simulate radiation events. All MD simulations were carried out with Pu initially in the face-centred-cubic (fcc) structure, although this is not the lowest energy configuration for the pure metal.The point defect study suggests that the mono-vacancy has the lowest formation energy (0.46eV), with interstitial defects favouring the 〈100〉 – split orientation over occupation of the native fcc octahedral site. Displacement threshold energy calculations at room temperature give a minimum value of between 5 and 6eV, increasing to 8–14eV along the major crystallographic directions.Low energy collision cascades, initiated with energies in the range of 0.4–1keV, show that the cascades form in a similar manner to other fcc metals with a vacancy rich zone at the cascade core, surrounded by isolated interstitial defects. Higher energy cascades show similar features but with occasional channelling of energetic atoms and sub-cascade branching which significantly reduces defect production. A common trait observed across all the cascades was the relatively slow annealing period, compared to cascades in other fcc metals, with simulations at energies above 5keV requiring many 10’s of picoseconds before the ballistic phase was completed.

Understanding the Plasticity of Amorphous Alloys Via the Interpretation of Structural Evolution Inside a Shear Band

)Abstract The effect of the initial packing structure on the plasticity of amorphous alloys was investigated bytracing the structural evolution of the amorphous solid inside a shear band. According to the moleculardynamics simulations, the structural evolution of the amorphous solids inside the shear band was more abruptin the alloy with a higher initial packing density. Such a difference in the structural evolution within the shearband observed from the amorphous alloys with different initial packing density is believed to cause differentdegrees of shear localization, providing an answer to the fundamental question of why amorphous alloys showdifferent plasticity. We clarify the structural origin of the plasticity of bulk amorphous alloys by exploring themicrostructural aspects in view of the structural disordering, disorder-induced softening, and shear localizationusing molecular dynamics simulations based on the recently developed MEAM (modified embedded atommethod) potential.Key wordsamorphous alloys, free volume, structural disorder, molecular dynamics.

Modified embedded-atom method interatomic potentials for the Fe–Ti–C and Fe–Ti–N ternary systems

Modified embedded-atom method (MEAM) interatomic potentials for the Fe–Ti–C and Fe–Ti–N ternary systems have been developed based on the previously developed MEAM potentials for sub-unary and binary systems. An attempt was made to find a way to determine ternary potential parameters using the corresponding binary parameters. The calculated coherent interface properties, interfacial energy, work of separation and misfit strain energy between body-centered cubic Fe and NaCl-type TiC or TiN were reasonable when compared with relevant first-principles calculations under the same condition. The applicability of the present potentials for atomistic simulations to investigate nucleation kinetics of TiC or TiN precipitates and their effects on mechanical properties in steels is also demonstrated.

MEAM study of carbon atom interaction with Ni nano particle

Carbon, Ni, and C–Ni alloy modified embedded atom method (MEAM) potentials were developed to study the initial process of carbon nanotube growth on Ni catalyst particles. The MEAM potentials were used to study the atomistic interaction between a carbon atom and a fcc Ni nano particle, both on the particle surfaces and inside the Ni nano particles. The result shows that surface carbon atom is more stable than those in the bulk and sub-surface interstitial positions. Carbon atoms are expected to diffuse from the bulk to the surface, and the single walled and double-walled carbon nanotubes would be more favorable to form on Ni nano particle catalyst. The carbon and Ni nano particle interaction calculation shows that the corner and the edge of the particle are the energetically more favorable sites for the carbon adatom. The carbon nanotube may grow from the corner and edge of the particle.

Atomistic simulation on size-dependent yield strength and defects evolution of metal nanowires

A simple derivation based on continuum mechanics is given, which shows the surface stress is critical for yield strength at ultra-small scales. Molecular dynamics (MD) simulations with modified embedded atom method (MEAM) are employed to investigate the mechanical behaviors of single-crystalline metal nanowires under tensile loading. The calculated yield strengths increasing with the decrease of the cross-sectional area of the nanowires are in accordance with the theoretical prediction. Reorientation induced by stacking faults is observed at the nanowire edge. In addition, the mechanism of yielding is discussed in details based on the snapshots of defects evolution. The nanowires in different crystallographic orientations behave differently in stretching deformation. This study on the plastic properties of metal nanowires will be helpful to further understanding of the mechanical properties of nanomaterials.

Missing row reconstruction on three low index surfaces of ten FCC metals

AbstractWith modified embedded‐atom method (MEAM), the surface energies of the perfect and (1×2) missing row structure on (100), (110) and (111) surfaces, and (1×3), (1×4) and (1×5) missing row structure on (110) surface have been calculated for ten FCC metals Pb, Au, Pt, Ir, Ag, Pd, Rh, Cu, Ni and Al. According to the energy minimization, we know that the (1×2) missing row reconstruction cannot be formed spontaneously on (100) and (111) surfaces of all ten FCC metals. However, not only (1×2) but also (1×3) missing row reconstruction can be formed on (110) surface of Au, Pt and Al spontaneously. These are consistent with the experimental results except Al for which the experiment has not been reported. The results are also analyzed in terms of the surface configurations and valence electron structures. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A modified embedded atom method interatomic potential for the Ti–N system

A modified embedded atom method (MEAM) interatomic potential for the Ti–N system has been developed using previous MEAM potentials of Ti and N. The materials parameters, such as the cohesive energy, equilibrium atomic volume, and bulk modulus, were used to determine the MEAM parameters. The MEAM potential was used to calculate the surface energy of TiN. The lattice parameters, the bulk modulus, and the enthalpy of formation of Ti2N were also calculated. It was found that the potential gives a good agreement with experimental data or first-principles calculations. The surface energy and the formation energy of one nitrogen vacancy of Ti2N were predicted using the potential.

Construction of modified embedded atom method potentials for the study of the bulk phase behaviour in binary Pt–Rh, Pt–Pd, Pd–Rh and ternary Pt–Pd–Rh alloys

Abstract A first attempt is made to simulate the solid part of the phase diagram of the ternary Pt–Pd–Rh system. To this end, Monte Carlo (MC) simulations are combined with the Modified Embedded Atom Method (MEAM) and optimised parameters entirely based on Density Functional Theory (DFT) data. This MEAM potential is first validated by calculating the heat of mixing or the demixing phase boundary for the binary subsystems Pt–Rh, Pt–Pd and Pd–Rh. For the disordered alloy systems Pt–Rh and Pt–Pd, the MC/MEAM simulation results show a slightly exothermic heat of mixing, thereby contradicting any demixing behaviour, in agreement with other theoretical results. For the Pd–Rh system the experimentally observed demixing region is very well reproduced by the MC/MEAM simulations. The extrapolation of the MEAM potentials to ternary systems is next validated by comparing DFT calculations for the energy of formation of ordered Pt–Pd–Rh compounds with the corresponding MEAM energies. Finally, the validated potential is used for the calculation of the ternary phase diagram at 600 K.

Molecular dynamics calculation of thermophysical properties for a highly reactive liquid

In order to further understand the physical characteristics of liquid silicon, the thermophysical properties are required over a broad temperature range. However, its high reactivity brings about great difficulties in the experimental measurement. Here we report the thermophysical properties by molecular dynamics calculation, including density, specific heat, diffusion coefficient, and surface tension. The calculation is performed with a system consisting of 64,000 atoms, and employing the Stillinger-Weber (SW) potential model and the modified embedded atom method (MEAM) potential model. The results show that the density increases as a quadratic function of undercooling, and the value calculated by SW potential model is only 2-4 % smaller than the reported experimental data. The specific heat is obtained to be 30.95 J mol;{-1}K;{-1} by SW potential model and 32.50 J mol;{-1}K;{-1} by MEAM potential model, both of which are constants in the corresponding ranges of temperature. The self-diffusion coefficient is exponentially dependent on the temperature and consistent with the Arrhenius equation. The surface tension increases linearly with the rise of undercooling and agrees well with the reported experimental results. This work provides reasonable data in much wider temperature range, especially for the undercooled metastable state.

Modified embedded-atom method interatomic potentials for pure Mn and the Fe–Mn system

Modified embedded-atom method (MEAM) interatomic potentials for pure Mn and the Fe–Mn binary system have been developed using a previously developed MEAM potential for Fe. The potentials can describe various fundamental physical properties of pure Mn (cohesive energy, structural energy differences, lattice parameters, elastic constants, vacancy formation energy, surface energy, etc.) and alloy behaviors (enthalpy of mixing in face-centered cubic and liquid phases, composition dependency of lattice parameters in various solid solutions) in reasonable agreement with experimental information or other empirical approaches. The applicability of the potential to atomistic investigations on a wide range of mechanical or deformation properties of the Fe–Mn alloys is demonstrated.

Effects of Crystallographic Structure and Cr on the Rate of Void Nucleation in BCC Fe: An Atomistic Simulation Study

Atomistic Monte Carlo simulations based on modified embedded-atom method (MEAM) interatomic potentials have been carried out to clarify the differences in swelling rates between bcc and fcc Fe and between pure bcc Fe and bcc Fe−Cr alloys. Assuming that the transient regimes prior to the onset of steady-state swelling correspond to the void nucleation stage, the effect of crystallographic structure (bcc vs. fcc) or Cr alloying on the void nucleation rate under a given amount of supersaturated vacancies was examined. It was found that the void nucleation rate is much higher in fcc Fe than in bcc Fe. Randomly distributed Cr atoms slightly increase the void nucleation rate in bcc Fe, but microstructural evolutions such as the precipitation of Cr-rich phase have more decisive effects, serving as a vacancy sink. The reasons for the individual results are rationalized in terms of the binding energy of vacancy clusters and the size difference between Fe and Cr atoms.

Surface segregation in ternary Pt–Pd–Rh alloys studied with Monte Carlo simulations and the modified embedded atom method

Recently, new parameters for the modified embedded atom method (MEAM) were derived for the ternary Pt–Pd–Rh system. In this work, this validated potential is used in conjunction with Monte Carlo (MC) simulations to study segregation to the (1 1 1) surface for the entire phase diagram of this ternary system. At 1400 K, these simulations reveal two distinct regions. In the major part of the phase diagram, Pd is the segregating component. However, close to the binary Pt–Rh axis, a region is observed in which Pt and Pd co-segregate to the surface. This co-segregation occurs only at higher temperatures as it is the result of two competing exothermic segregation reactions.

Modified embedded-atom method interatomic potentials for the Ti–C and Ti–N binary systems

Modified embedded-atom method (MEAM) interatomic potentials for the Ti–C and Ti–N binary systems have been developed using previously developed MEAM potentials of Ti, C and N. The potential parameters were determined by fitting to experimental data on the enthalpy of formation, lattice parameter, elastic constants, thermal linear expansion of NaCl-type TiC and TiN, and dilute heat of solution of carbon and nitrogen atoms in hexagonal close-packed Ti. The potentials can describe fundamental physical properties (structural, elastic, thermal and surface properties) of the alloys well, in good agreement with experimental information or first-principles calculations. The applicability of the potentials to atomistic investigations of interactions between TiC or TiN precipitates and matrix, dislocations or other defects, and their effects on deformation and mechanical behaviors of metallic alloys is discussed.

A modified embedded-atom method interatomic potential for the Cu–Zr system

A modified embedded-atom method (MEAM) interatomic potential for the Cu–Zr system has been developed based on the previously developed MEAM potentials for pure Cu and Zr. The potential describes fundamental physical properties and alloy behavior of the Cu–Zr binary system reasonably well. The applicability of the potential to atomistic investigations of mechanical and deformation behavior for the Cu–Zr binary and Cu–Zr-based multicomponent amorphous alloys is also demonstrated by showing that fully relaxed and realistic amorphous structures can be generated by molecular dynamics simulations.

Simulations of vacancy cluster behavior in δ-Pu

Using rates for vacancy diffusion in plutonium (Pu) found with parallel-replica dynamics, we develop a kinetic Monte Carlo (KMC) model of void growth and mobility. We compare and contrast the behavior of voids in Pu as predicted using vacancy mobilities from two different modified embedded atom method (MEAM) descriptions of Pu. We find that void behavior depends sensitively on the values used for vacancy mobility. In particular, we find that voids are very mobile in one model of Pu, but are essentially immobile in another, leading to very different void structures over time. This second model also predicts lifetimes for voids that are extremely long, and seemingly unphysical, suggesting that the first model is more representative of real Pu.

A modified embedded-atom method interatomic potential for Germanium

A semi-empirical interatomic potential for germanium has been developed based on the modified embedded-atom method (MEAM) formalism. The new potential describes various fundamental physical properties of germanium: elastic, structural, point defect, surface, thermal properties (except melting point), etc., in better agreement with experimental data or first principles calculations than any other empirical potential ever developed. When compared to the previously developed MEAM Ge potential [M.I. Baskes, J.S. Nelson, A.F. Wright, Phys. Rev. B 40 (1989) 6085], certain improvements are made in descriptions of surface relaxations, point defects, thermal expansion and amorphous structure. The potential has the same formalism as already developed MEAM potentials for bcc, fcc and hcp elements, and can be easily extended to describe various metal–silicon multi-component systems.

Molecular dynamics simulations of deformation in nanocrystalline Al–Pb alloys

A modified embedded-atom method (MEAM) potential was developed and used for molecular dynamics (MD) straining simulations of Al–Pb alloys with a grain size of 10 nm and Pb content up to 3 at.%. Monte Carlo (MC) simulations done at 300 K indicated that all the Pb is segregated to the grain boundaries in these alloys. As the Pb content increases, partial dislocation nucleation at grain boundaries is suppressed, and the plastic strain is accommodated by mechanisms other than dislocation slip. The increasing Pb content was accompanied by a reduction in the yield and peak stress values.

Surface segregation in CuPt alloys by means of an improved modified embedded atom method

We present a procedure to investigate surface structures in CuPt alloys by combining the modified embedded atom method (MEAM) and the cluster expansion method (CEM). While the MEAM provides structural information for the description of extended anisotropic defects, the CEM improves the ability to correctly reproduce the relevant ground state structures in agreement with ab initio data. The procedure is validated with the reproduction of surface energies of pure Cu and Pt, the prediction of ${T}_{C}$ for order-disorder transitions, the surface and segregation energies in ordered CuPt alloys, and Monte Carlo (MC) simulations of temperature-dependent surface segregation profiles. A complete MEAM-CEM/MC study of the surface segregation in ${\mathrm{Cu}}_{3}\mathrm{Pt}$, CuPt, and $\mathrm{Cu}{\mathrm{Pt}}_{3}$ alloys is presented, engaging only 11 composition- and volume-independent alloy-specific parameters. Results are critically compared with experimental data from literature and with an independent set of ab initio periodic density functional theory calculations.