We have calculated the vacancy-assisted tracer diffusivities in these binary intermetallic alloys using on-the-fly kinetic monte carlo: Cu-Au, Au-Ag, and Cu-Ni. This set encompases behavior from weak segregation to moderately strong ordering. The energetics are based on the Embedded Atom Method, and the saddlepoints are found on-the-fly (that is, beginning from scratch with each valley.) Using this technique we have calculated the tracer diffusivities over a wide range of temperature and composition. We evaluate the strengths and weaknesses of the approach. Where possible we compare to experimental data.

The study of point defect clustering in hexagonal-close-packed (hcp) metals is dominated by a consideration of the geometry of the hcp lattice and lattice parameters ratio (c/a). Because of this crystallographic anisotropy, defect anisotropic diffusion is expected (jump distances and jump rates depend on jump directions). This study has focused on hcp alpha-Zirconium (c/a 1.594, lower than ideal 1.633 and similar to Titanium, c/a 1.587). We have created a new and original model for the understanding of the microscopic evolution (defect diffusion) in hcp metals, using a kinetic Monte Carlo (kMC) simulation technique. This technique allows us to understand the evolution of damage accumulation, due to either neutron or electron irradiation, for long times (hours-months). Several multiscale modelling simulations steps have been used in order to understand the microscopical fission reactor cladding behaviour. We have focused on zirconium alloys claddings (Zircaloy-4 and Zr-2.5%Nb). The first step we have simulated has been the neutron spectra. We have obtained spectra for current pressure water reactors (PWR) and high burn-up advanced reactors. We have also obtained neutron spectra in several burnt steps and we have represented the isotopic variation in the cladding (looking for Helium and Hydrogen apparition in the system). Taking these results as input data we have reproduced the Primary Knock-on Atom (PKA) spectra. From those data a systematic analysis of primary damage has been obtained using binary collisions code SRIM for high energy recoils, in order to get distribution of cascades and subcascades for these recoil energies. With these data we have studied the evolution of the microstructure during irradiation under environment conditions of 600K, dose rate 1e-6 dpa/s and final dose of 0.5 dpa. We have selected the molecular dynamics (MD) simulations cascades that fit best to PKA spectra and we have used them as input data on defect energetics and cascade damage. We have considered isotropic motion for vacancies and we have studied how the accumulation of damage is affected considering from one dimension to three dimension movement for interstitials.

Hydride formation and associated embrittlement is very important for the integrity of the nuclear fuel cladding and the safety of the nuclear power plants. However, the mechanism of the hydride formation ahead of a crack tip of the cladding has not been well understood especially at the atomic level. In the fuel cladding, relatively strong strain field is produced by not only the hoop stress due to internal pressure but also the hydrides or the stress concentration at the crack tip. Such a strain field should affect the diffusion of the point defects produced by irradiation and solute atoms such as hydrogen, and thus, the macroscopic phenomena such as hydride formation might be affected by the results of cumulative effect of migration energy changes.In this paper, diffusion of a hydrogen atom in a strain field in hcp zirconium crystal is studied by means of kinetic Monte Carlo (kMC) computer simulation method. In order to consider the strain effect, we first calculate migration energies of a hydrogen atom under a wide range of given uniform strain field using molecular dynamics. The computation box of the kMC simulations is subdivided into small (1nm cube) cells, and the strain field of the box is modeled by assigning an appropriate constant strain field to each of the small cells. Then, effect of strain field to hydrogen atom diffusion is considered by choosing appropriate migration energy for the hydrogen jump depending on the strain field of the cell in which the hydrogen is located.In the present simulations, we place edge dislocations as a source of strong strain field in the computation box where hydrogen atoms are also uniformly distributed in the box as an initial condition. Then the crystal is heated up to a given temperature (600K) to let the hydrogen atoms diffuse in the strain field in the crystal. No interaction between the hydrogen atoms is considered in the present simulations. General tendency of the hydrogen diffusion is that hydrogen atoms agglomerate in the tension strain region, and the population of the hydrogen in the compression region decreases. Steady state is reached after 1ms. This result clearly demonstrates that the strain fields ahead of crack tips or hydrides provide locations where hydrogen atoms prefer to agglomerate.

The in-diffusion of oxygen during the formation of Ag-MgO composites results in oxygen segregation to the metal-ceramic interface and subsequent weakening of the interface bonding. Removal of oxygen by out-diffusion can be achieved at high temperatures. These diffusion processes are simulated using finite element and Lattice Monte Carlo methods.

Predictive modeling of the early-time transient annealing of radiation damage in electronic devices requires a detailed, quantitative understanding of the behavior of the fundamental defects in the device material. The isolated self-interstitial in silicon is extremely difficult to observe experimentally, and therefore accurate theoretical results should be very valuable. We apply electronic structure calculations based on the Kohn-Sham Density Functional Theory (DFT) in concert with Kinetic Monte-Carlo (KMC) techniques to study diffusion of the silicon self-interstitial as a function of the majority and minority carrier populations. We perform carefully converged DFT calculations to identify the structures that are locally stable (stable or metastable) for each charge state and the reaction pathways and transition states for transformation between these configurations. The resulting transition barriers are incorporated into a KMC model to determine the diffusion rate as a function of temperature and carrier concentrations. For non-equilibrium carrier concentrations, fluctuations in the charge state of the defect lead to non-Arrhenius behavior via a modified Bourgoin-Corbett mechanism. The rather complex behavior of the calculated diffusion coefficient will be explained in terms of fundamental physical mechanisms. The effects of transient trapping of the silicon self-interstitial in a complex with the oxygen impurity will also be discussed.Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE- AC04-94AL85000.

Many industrial applications, such as automotive engines or high-pressure vessels for oil recovery, require strong materials to withstand high temperature or high pressure environments. Under these harsh conditions, corrosion of steel by various reactive agents is a great concern, even under reducing atmospheres. For example, attack by H₂S leads to incorporation of hydrogen into steel and subsequent embrittlement. Similarly, at high temperature, attack by CO and subsequent incorporation of C into steel leads to metal dusting and carburization. In this work, we investigate whether formation of an Fe alloy thin film on steel can curb corrosion in reducing atmospheres by limiting C and H diffusion into and through steel. In particular, we employ density functional theory to calculate rate constants and energy barriers for the diffusion of H and C through bulk FeAl and bulk Fe₃Si alloys. We also investigate the reaction pathways of H/C diffusing from the alloy surface to subsurface layers, in order to determine the overall kinetics of hydrogen and carbon incorporation into these Fe alloys.

Solid-state diffusion is a subject of great interest for its intellectual merit and practical applications in materials and coatings for advanced energy generation systems such as gas turbines and nuclear reactors. This talk will highlight the importance of multicomponent-multiphase interdiffusion with specific examples from materials and coatings used in gas turbine engines and metallic nuclear fuels. Results and analysis from both laboratory experiments and field applications are presented to emphasize the cross-fertilization of science and applications. Examples will include, for gas turbine applications, ternary interdiffusion in Ni3Al (L12) with Ir or Ta ternary alloying additions, ternary and quaternary interdiffusion in austenitic NiCr and FeNiCr alloys with Al, Si, Ge or Pd alloying additions for improved oxidation resistance, and interdiffusion between oxidation resistant coatings and superalloy substrates. Diffusion controlled degradation of metallic nuclear fuels due to interdiffusion with cladding and thermotransport will be presented with appropriate analysis for the determination of critical thermo-kinetic parameters that in turn are used for fuel performance prediction. This work was financially supported by CAREER award from National Science Foundation (NSF-CAREER DMR-0238356), U.S. Department of Energy (DE-FC26-02NT41431) subcontract through Clemson University (No. 01-01-SR103), and U.S. Department of Energy (DE-AC07-05ID14517) subcontract through Idaho National Laboratory (0051953) with technical assistance from Oak Ridge National Laboratory and National Institute of Materials Science (NIMS) of Japan.

Among all the fluorite-based electrolytes proposed for use in solid oxide fuel cells, δ-Bi2O3 has the highest oxygen ion conductivity. To achieve this fluorite crystal structure at relatively lower temperatures, δ-Bi2O3 is doped with different lanthanides [1]. However, doping decreases the oxygen conductivity; a direct proportional dependence of oxygen conductivity on atomic polarizability and radius are observed to be the two contributing factors [1, 2]. Here, the effects of ionic polarizability and ionic radius on oxygen conductivity are separated using molecular dynamics (MD) simulations. Using MD simulations, artificial cations are generated to separate the two factors’ effects by keeping one constant and varying the other. In the pure δ-Bi2O3, we find that high bismuth polarizability is important to achieve high oxygen conductivity; the ionic radius is not important. We have further observed that neither the oxygen radius nor its polarizability plays a role in its high diffusion. In doped δ-Bi2O3, elastic stresses are developed which effect the oxygen diffusion. Using both MD and density functional theory, we also present a detailed structural and oxygen diffusion mechanism comparison between the pure and doped δ-Bi2O3. This work is supported by NASA under the grant NAG3–2930 and by DOE through the High Temperature Electrochemistry Center (HiTeC) at the University of Florida, Contract No. DE-AC05-76RL01830. 1.Jiang N., Wachsman E. D., J. Am. Cer. Soc., 82 (11) (1999), 3057.2.Wachsman E. D., Ionics, 7 (2001), 1.

(La,Sr)MnO₃ (LSM) is the cathode catalyst generally used in commercial solid oxide fuel cells (SOFCs). However, SOFCs are significantly limited by the oxygen reduction reaction (ORR) kinetics on this material, particularly at lower temperatures. To understand the relationship between surface structure and ORR in LSM, we adopt an atomistic modeling approach based on ab initio density functiona