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Large-scale molecular dynamics simulations of shock-induced plasticity, phase transformations, and detonation

Description: Modern computers enable routine multimillion-atom molecular dynamics simulations of shock propagation in solids using realistic interatomic potentials, and offer a direct insight into the atomistic processes underlying plasticity, phase transformations, and the detonation of energetic materials. Past, present, and prospects for future simulations will be discussed in the context of prototypical systems for each of these three classes of problems. Initial samples ranging from perfect single crystals, to those with specific isolated defects, to full-fledged polycrystalline materials will be considered.
Date: June 1, 2001
Creator: Germann, T. C. (Timothy C.)
Partner: UNT Libraries Government Documents Department

Accelerating the dynamics of infrequent events: Combining hyperdynamics and parallel replica dynamics to treat epitaxial layer growth

Description: During the growth of a surface, morphology-controlling diffusion events occur over time scales that far exceed those accessible to molecular dynamics (MD) simulation. Kinetic Monte Carlo offers a way to reach much longer times, but suffers from the fact that the dynamics are correct only if all possible diffusion events are specified in advance. This is difficult due to the concerted nature of many of the recently discovered surface diffusion mechanisms and the complex configurations that arise during real growth. Here the authors describe two new approaches for this type of problem. The first, hyperdynamics, is an accelerated MD method, in which the trajectory is run on a modified potential energy surface and time is accumulated as a statistical property. Relative to regular MD, hyperdynamics can give computational gains of more than 10{sup 2}. The second method offers a way to parallelize the dynamics efficiently for systems too small for conventional parallel MD algorithms. Both methods exploit the infrequent-event nature of the diffusion process. After an introductory description of these methods, the authors present preliminary results from simulations combining the two approaches to reach near-millisecond time scales on systems relevant to epitaxial metal growth.
Date: Spring 1998
Creator: Voter, A. F. & Germann, T. C.
Partner: UNT Libraries Government Documents Department

Direct Observation of the alpha-epsilon Transition in Shock-compressed Iron via Nanosecond X-ray Diffraction

Description: In-situ x-ray diffraction studies of iron under shock conditions confirm unambiguously a phase change from the bcc ({alpha}) to hcp ({var_epsilon}) structure. Previous identification of this transition in shock-loaded iron has been inferred from the correlation between shock wave-profile analyses and static high-pressure x-ray measurements. This correlation is intrinsically limited because dynamic loading can markedly affect the structural modifications of solids. The in-situ measurements are consistent with a uniaxial collapse along the [001] direction and shuffling of alternate (110) planes of atoms, and in good agreement with large-scale non-equilibrium molecular dynamics simulations.
Date: March 21, 2005
Creator: Kalantar, D. H.; Belak, J. F.; Collins, G. W.; Colvin, J. D.; Davies, H. M.; Eggert, J. H. et al.
Partner: UNT Libraries Government Documents Department