We have used molecular dynamics and Monte Carlo methods to simulate the structure and phase stability of a Pd crystal in thermodynamic equilibrium with molecular hydrogen gas at temperature T and pressure PH2 g. The pressurecomposition-temperature (PCT) curves were deduced under the extreme conditions of an open system (Pd crystal in equilibrium with a large-volume H2 gas reservoir) and a closed system (Pd crystal in equilibrium with H2 gas reservoir of infinitesimal volume). The PCT curves from the open simulations reproduce the experimental observations, including the ubiquitous hysteresis. The PCT curves from the closed-system simulations are continuous curves. Below a tricritical point, the Pd-H system decomposes into two coherent phases. We find excellent agreement between the present simulation results and the predictions of the Schwarz-Khachaturyan theory for the decomposition of a Pd-H alloy into two coherent hydride phases.

The continuing search for broadly applicable, predictive, and unique potential functions led to the invention of the multi-state modified embedded atom method (MS-MEAM) (Baskes et al 2007 Phys. Rev. B 75 094113). MS-MEAM replaced almost all of the prior arbitrary choices of the MEAM electron densities, embedding energy, pair potential, and angular screening functions by using first-principles computations of energy/volume relationships for multiple reference crystal structures and transformation paths connecting those reference structures. This strategy reasonably captured diverse interactions between atoms with variable coordinations in a face-centered-cubic (fcc)-stable copper system. However, a straightforward application of the original MS-MEAM framework to model technologically useful hexagonal-close-packed (hcp) metals proved elusive. This work describes the development of an hcp-stable/fcc-metastable MS-MEAM to model titanium by introducing a new angular function within the background electron density description. This critical insight enables the titanium MS-MEAM potential to reproduce first principles computations of reference structures and transformation paths extremely well. Importantly, it predicts lattice and elastic constants, defect energetics, and dynamics of non-ideal hcp and liquid titanium in good agreement with first principles computations and corresponding experiments, and often better than the three well-known literature models used as a benchmark. The titanium MS-MEAM has been made available in the Knowledgebase of Interatomic Models (https://openkim.org/) (Tadmor et al 2011 JOM 63 17).

We explore whether the continuum scaling behavior of the fracture energy of metals extends down to the atomistic level. We use an embedded atom method (EAM) model of Ni, thus bypassing the need to model strain-gradient plasticity at the continuum level. The calculations are performed with a number of different 3D periodic size cells using standard molecular dynamics (MD) techniques. A void nucleus of a single vacancy is placed in each cell and the cell is then expanded through repeated NVT MD increments. For each displacement, we then determine which cell size has the lowest energy. The optimal cell size and energy bear a power-law relation to the opening displacement that is consistent with continuum estimates based on strain-gradient plasticity (Fokoua et al., 2014, "Optimal Scaling in Solids Undergoing Ductile Fracture by Void Sheet Formation," Arch. Ration. Mech. Anal. (in press); Fokoua et al., 2014, "Optimal Scaling Laws for Ductile Fracture Derived From Strain-Gradient Microplasticity," J. Mech. Phys. Solids, 62, pp. 295-311). The persistence of power-law scaling of the fracture energy down to the atomistic level is remarkable.

In many metallic alloys, complex microstructures form as a consequence of local atomic ordering that depends on the processing path. This research uses an atomistic approach to study microstructural morphology and evolution in order to investigate how temperature and alloy concentration affect ordering. A semi-empirical Modified Embedded Atom Method (MEAM) is used in conjunction with molecular dynamics (MD) and Monte Carlo (MC) simulations to investigate the properties and equilibrium configurations of the high temperature body-centered-cubic (bcc) uranium-zirconium (U-Zr) alloys. Atomic simulations conducted with the MEAM potential show the thermodynamic driving force to the lamellar structure for the melt-casted U-rich alloys and the finely acicular microstructure of the water quenched U-rich alloys. In addition, when the U-rich U-Zr alloy is equilibrated at a lower temperature, the lamellar/acicular microstructures begin to spheroidize, as observed in experiments. In the intermediate Zr concentration region, the ordering seen is able to facilitate the structure to the partially ordered delta-UZr2 phase seen experimentally. Lastly, the Zr-rich region is able to successfully show the thermodynamic driving force to the acicular, Widmanstatten, and martensitic needles type microstructures observed experimentally. These simulations are able to successfully replicate some of the fundamental thermo-physical and microstructural characteristics following fabrication and irradiation of the U-Zr metallic fuels. Published by Elsevier Ltd on behalf of Acta Materialia Inc.

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. (C) 2009 Elsevier B.V. All rights reserved.

Using parallel-replica dynamics and temperature accelerated dynamics, we extract the rates for mono- and di-vacancy diffusion in delta-plutonium (Pu) using two parameterizations of the modified embedded atom method (MEAM). We find that mono-vacancy diffusion is faster in "pure" Pu than in delta-stabilized Pu. Also, at higher temperatures, the rate of double jumps is nearly the same as single jumps in pure Pu. Since these double jumps contribute four times as much as single jumps to the diffusion constant, models incorporating mono-vacancy diffusion must account for this mechanism to predict mass transport in Pu. While di-vacancies are energetically only slightly preferred compared to mono-vacancies, they are C significantly more mobile. Surprisingly, this enhanced mobility is due to the prefactor; the migration barrier is essentially identical. The di-vacancy dissociates at a rate similar to the mono-vacancy hop rate. (c) 2006 Elsevier B.V. All rights reserved.

We employ molecular dynamics and Monte Carlo (MC) methods to simulate the rearrangement of Ga atoms from a randomly distributed Pu-Ga alloy and study the resultant effect on thermodynamic properties. The results show that all of the first neighbor Ga-Ga bonds are removed at all temperatures considered (200, 400 and 600 K) while the number of 2NN and 3NN bonds increase, and the number of 4NN bonds decreases. These results imply that Ga atoms develop strong short range ordering in the solid solution. The ordering causes an enthalpy decrease about similar to 3-4 meV/atom for different temperatures in the 5 at. % Ga alloy. This energy change is clearly important in the calculation of the Pu-Ga phase diagram. In addition, MC calculations at 200 K show pronounced Ga segregation.

We performed molecular dynamics (MD) simulations to study hydrogen effects on nanovoid nucleation at nickel grain boundaries using an embedded atom method (EAM) potential. Monte Carlo (MC) simulations were performed to introduce hydrogen atoms in low-angle and high-angle symmetrical [00 1] tilt boundaries at 300 K for analysis of plasticity and nanovoid nucleation. The simulation results show that hydrogen atoms were trapped at the grain boundaries and reduced the critical stresses and strains for nanovoid nucleation. The MD results also show that the effects of hydrogen on nanovoid nucleation depended on the grain-boundary hydrogen concentration regardless of the grain-boundary misorientations. The MD results were then inserted into a new hydrogen associated void nucleation model that operates as an internal state variable in the context of continuum thermodynamic plasticity. Published by Elsevier Ltd on behalf of Acta Materialia Inc.

The Modified Embedded Atom Method model for Pu metal is revised so that it more accurately captures the behavior of the Ziegler-Biersack-Littmark model of ion-ion interactions. Two revision are tested with somewhat different stiffnesses in the 2-1000 eV range. The revised models show higher damage levels at 20 KeV than an earlier model, suggesting that the behavior of the models above 100 eV is dominating damage production, at least in the earlier stages of the cascade.

Molecular dynamics (MD) simulations of spallation in single crystal nickel were performed for a range of system sizes and impact velocities. The initial compressive wave leaves a rich microstructure in its wake. The subsequent tensile waves create multiple grains and grain junctions between regions of differing crystal orientation. These grain junctions serve as void nucleation sites when the reflected tensile waves interact, leading to ductile failure. In this way, the mechanism for failure in an initially single-crystalline sample is similar to that seen experimentally in high-purity, poly-crystalline metals, in which grain boundaries are sites for void nucleation.

Monte Carlo (MC) and molecular dynamics (MD) simulations using embedded atom method (EAM) potentials were performed to study nanovoid nucleation in single-crystal nickel specimens in a hydrogen-precharged and a hydrogen dynamically-charged condition. In the hydrogen-precharged condition, MC simulations were performed to introduce hydrogen atoms in an unstressed specimen. MD simulations were then performed to study nanovoid nucleation and the associated plasticity. In the dynamically-charged condition, a novel coupled MD-MC process was used to introduce hydrogen into the specimen while the specimen was being strained until nanovoid nucleation occurred. The simulation results revealed that hydrogen only reduced the nanovoid nucleation stress in the precharged case slightly but caused a lower strain-hardening and a significant reduction in the nanovoid nucleation stress in the dynamically-charged case. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Basic concepts from the multi-reference state formalism for determining the functions for the modified embedded atom method (MEAM) are adopted to modeling elemental plutonium (Pu). In the case of elemental Pu, the focus is on the background electron density. Here we utilize a portion of the formalism that determines the structure of the background density necessary to capture correct phase ordering between fcc and ideal hcp crystal structures. The critical information comes from cold curves, that is the energy/volume relationships, for these phases. Practically speaking, the energy difference between these two phases determines the stacking fault energy of the material. At the same time, the simple monoclinic phase of elemental Pu also becomes higher in energy at the equilibrium volume of the fcc phase. The new model is based on first-principles electronic structure calculations and captures the basic phase ordering of those calculations. (C) 2011 Elsevier B.V. All rights reserved.