Bulletin of the American Physical Society
16th APS Topical Conference on Shock Compression of Condensed Matter
Volume 54, Number 8
Sunday–Friday, June 28–July 3 2009; Nashville, Tennessee
Session H2: MD-2: Molecular Dynamics II |
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Chair: Ivan Oleynik, University of South Florida Room: Hermitage AB |
Tuesday, June 30, 2009 9:00AM - 9:15AM |
H2.00001: First-principles thermodynamics--specific heat of Mo from density functional theory molecular dynamics simulations Ann E. Mattsson, Thomas R. Mattsson, Nils Sandberg, Rickard Armiento A fundamental understanding of thermodynamical properties like specific heat is necessary in order to model shock compression of condensed matter to high fidelity. It is therefore interesting that also central issues remain unsatisfactorily understood for technologically important body centered cubic metals like Mo. For example the long-standing question whether the strong increase of the specific heat of Mo close to the melting point is caused by a high (several percent) concentration of vacancies or by anharmonic lattice and electronic effects. Here we show, through density functional theory (DFT) molecular dynamics simulations of vacancy motion in Mo close to the melting point, using the new AM05 density functional, that a low (fractions of percent) concentration of vacancies does explain experimental observations of specific heat and self-diffusion. We furthermore quantify and discuss the origin of the anharmonicity as well as implications for modeling of shock-processes from an atomistic point of view. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy National Nuclear Security Administration under Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, June 30, 2009 9:15AM - 9:30AM |
H2.00002: The onset of spallation in nanocrystalline copper: An atomic scale study Avinash Dongare, Arunachalam Rajendran, Bruce LaMattina, Mohammed Zikry, Donald Brenner Dynamic failure in nanocrystalline metals can be understood based on the mechanisms of plastic deformation and failure at high strain rates. The bulk of current research on nanocrystalline metals focuses on deformation mechanisms; however, the research on the failure mechanisms (spallation) at high strain rates is still at a stage of the initial exploration. We examine the micro-mechanisms related to dynamic failure of nanocrystalline Cu at high strain rates through a series of large-scale MD simulations. Void nucleation and growth is studied in nanocrystalline copper for conditions of deformation that lead to the onset of spallation during shock loading. The high tensile triaxial stress states result in the nucleation of nanocscale voids at the grain boundaries that grow and coalesce to form the microscopic crack. The effect of shock pressure, strain rates, and grain size on the spall strength and microscopic failure mechanisms as obtained from MD simulations will be discussed. The research is supported by the U. S. Army Research Office through the National Research Council Research Associateship Program. [Preview Abstract] |
Tuesday, June 30, 2009 9:30AM - 9:45AM |
H2.00003: Oxidation Phase Diagram of Small Aluminum Clusters Based on First-Principles Calculations Ligen Wang, Maija Kuklja It is important to understand the properties of individual nanometals before we can exploit their efficiency as energetic materials or as enhancement additives to other energetic formulations. In this paper, we construct the (p, T) phase diagram for the O/Al$_{13}$ system based on first-principles atomistic thermodynamics. The temperature and pressure is taken into account via the oxygen chemical potential. The optimized Al$_{13}$ cluster has an icosahedral shape. We find that O adsorption on the Al$_{13}$ surface is site-specific; in particular, O adsorption at the bridge sites is most stable, whereas adsorption at the hollow sites is slightly unfavorable. For various oxygen adsorption layers, we determine the adsorption configurations/patterns by performing Monte Carlo calculations. We assume that the metal cluster becomes completely oxidized and calculate formation enthalpies of various oxidized metal clusters. The obtained phase diagram shows that an intact Al$_{13}$ cluster is stable at the low O chemical potential range and the fully oxidized metal cluster is stable at the high O chemical potential range. However, the O adsorption phases are never thermodynamically stable. This study provides important insights into basic behavior of small aluminum clusters in the presence of oxygen, and may affect reliable predictions of behavior of Al-high explosive composites. [Preview Abstract] |
Tuesday, June 30, 2009 9:45AM - 10:00AM |
H2.00004: High velocity properties of the dynamic frictional force between ductile metals J.E. Hammerberg, B.L. Holian, T.C. Germann, R.J. Ravelo The high velocity properties of the tangential frictional force between ductile metal interfaces seen in large-scale NonEquilibrium Molecular Dynamics (NEMD) simulations are characterized by interesting scaling behavior. In many cases a power law decrease in the frictional force with increasing velocity is observed at high velocities. We discuss the velocity dependence of the high velocity branch of the tangential force in terms of structural transformation and ultimate transition, at the highest velocities, to confined fluid behavior characterized by a critical strain rate. The particular case of an Al/Al interface will be discussed. [Preview Abstract] |
Tuesday, June 30, 2009 10:00AM - 10:15AM |
H2.00005: Molecular dynamic modeling of plasticity of Al and Al-Cu alloys under dynamic loading Vladimir Stegailov, Alexey Kuksin, Genri Norman, Alexey Yanilkin The molecular dynamic (MD) simulations are carried out to study the mechanisms and kinetics of plasticity of Al and its alloys. The mobility of dislocations is evaluated from non-equilibrium MD simulations. The influence of temperature, presence of Cu precipitates and voids on dislocation motion is analyzed. Temperature dependence of the phonon drag coefficient on dislocations is calculated for Al. The drag coefficient increases linearly in all the temperature range (except the region near the melting point). It leads to an anomalous increase of the yield stress of single crystal Al with growing temperature. Critical resolved shear stresses required for dislocation to penetrate obstacle (precipitate or void) are calculated. Critical stress decreases with temperature, while pinning time increases. The following mechanism of the dislocation depinning is observed for cluster sizes (0.5--3 nm) and distances between them (10--60 nm) studied: strong bowing of the dislocation line with subsequent formation of the local shear plane cutting the cluster. A climb of the dislocation segment can be observed during depinning. The value of the dynamic yield stress of single crystal Al and Al-Cu alloy is estimated. Possible interpretation of the experimental data [G.I.Kanel et al., 2001,2008] on dynamic yield stress is discussed. [Preview Abstract] |
Tuesday, June 30, 2009 10:15AM - 10:30AM |
H2.00006: Influence of temperature on spall strength: atomistic simulation Peter Zhilyaev, Alexey Kuksin, Genri Norman, Vladimir Stegailov, Alexey Yanilkin The shock-wave experiments reveal different influence of temperature on spall strength depending on material microstructure. This work is devoted to study the effect in aluminum using large scale molecular dynamics. Simulations of triaxial deformation, corresponding to stretching in rarefaction waves, and the following fracture is considered. A single crystal with different defects (dislocations, stacking faults, voids) and a polycrystal are modeled. There is considerable difference between behavior of the single crystal and the polycrystal near melting curve. When approaching the melting curve amorphisation of grain boundaries starts. It provides a lower stress barrier to the microcrack propagation. In case of single crystal considerable overheating is observed even in the presence of stacking faults, dislocations and voids. Melting rate depends on temperature and the fracture can start in solid or liquid phase. The comparison with the experimental data on spall strength is carried out. This work was supported by the RAS programs {\#} 11, 12, and SNL under the US DOE/NNSA ASC program. [Preview Abstract] |
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