Bulletin of the American Physical Society
18th Biennial Intl. Conference of the APS Topical Group on Shock Compression of Condensed Matter held in conjunction with the 24th Biennial Intl. Conference of the Intl. Association for the Advancement of High Pressure Science and Technology (AIRAPT)
Volume 58, Number 7
Sunday–Friday, July 7–12, 2013; Seattle, Washington
Session Q6: TM Molecular Dynamics III |
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Chair: Roman Martonak, Comenius University Room: Cascade II |
Wednesday, July 10, 2013 1:45PM - 2:00PM |
Q6.00001: Evaluation of metastable region boundaries for liquid and solid states in MD simulations Gennady Ionov, Vladimir Dremov, Aleksey Karavaev, Sergey Protsenko, Vladimir Baidakov, Azat Tipeev An automatic method based on MD calculations was developed for detecting and tracing the boundaries of metastable states of superheated crystal and supercooled liquid. The main criterion of the detection of early nucleation of new phase is the self-diffusion coefficient temperature dependence. The scanning for nucleation events is performed at continuous temperature change. The set of independent nucleation events at a given pressure allows evaluation of temperature dependence of specific nucleation frequency. The collection of a large number of these calculations allows accurate approximation of the specific nucleation frequency surfaces in the both directions of phase transition. These surfaces provide an opportunity to estimate the behavior of the free energy in the region between overheating and overcooling curves. In addition, dependence of nucleation frequency on pressure and temperature provides an opportunity to integrate the probability of nucleation under dynamic loading and subsequent release and thus to determine the likelihood of the crystallization and melting. The technique was applied to argon, tin and beryllium. Tin is modeled with the EAM potential, well reproducing the properties of BCC phase. Beryllium is modeled with the GEAM/MEAM potential. [Preview Abstract] |
Wednesday, July 10, 2013 2:00PM - 2:15PM |
Q6.00002: The MD simulation on the micro-mechanism of micro-spallation of metal Pb under shock loading Jun Chen, Meizhen Xiang, Haibo Hu We study the micro-mechanism of crystal and nano-crystal metal Pb under shock loading by using the molecular dynamics method. A wide range of shock intensity is conducted with the lowest one just above the threshold of solid spallation, while the highest one higher than the threshold of shock melting. The spallation mechanism is dominated by cavitation, i.e., nucleation, growth and coalescence of voids, as well as the interplay of cavitation and melting. Our results discovered that the grain boundary plays an important effect in the case of releasing melting, while it is smaller effect on the cases of conventional spallation and shock melting. The cavitation and melting firstly form in the grain boundary, and they display mutual promotion: melting makes the void nucleation at smaller tensile stress; void growth speeds the melting. The spall strength dependence on the grain boundary, void and melting temperature is qualitatively discussed. Due to grain boundary effects, the spall strength of nano-crystalline Pb is less sensitive to shock intensity than single-crystalline Pb if cavitation occurs in solid state materials. If melting starts before cavitation, the spall strength of both nano-crystalline and single-crystalline Pb decreases dramatically as shock intensity increases. [Preview Abstract] |
Wednesday, July 10, 2013 2:15PM - 2:30PM |
Q6.00003: Atomistic models of plasticity of metals and alloys under dynamic loading Alexey Kuksin, Alexey Yanilkin The work presented is devoted to study the mechanisms and kinetics of plastic deformation of bcc and fcc metals and alloys under shock-wave loading (strain rates $>$ 10$^5$ s$^{-1}$). To study the behavior of metals under conditions described the two scale approach is developed. It comprises molecular dynamics (MD) calculations of dislocation mobility and dislocations nucleation rate and continuum mechanics model with equations for description of elastoplastic deformation, kinetics and dynamics of dislocations. Dislocation velocities as functions of applied shear stress are calculated in Al, Cu, Fe, Mo from MD in a wide temperature range up to the melting point. Velocity-dependent drag coefficient is introduced to approximate the data obtained. The influence of Guinier-Preston (GP) zones on dislocation motion is analyzed. The results obtained are used to evaluate temperature dependence of dynamic flow stress and the evolution of dislocations subsystem under shock loading. Data on the attenuation of the elastic precursor and rare surface velocity profiles calculated for Al are in good agreement with the experiments. Simulation of the free surface velocity profiles during shock-wave loading of AlCu alloys is carried out. [Preview Abstract] |
Wednesday, July 10, 2013 2:30PM - 2:45PM |
Q6.00004: Shock induced deformation twinning in tantalum MD simulations Matthew Suggit, Andrew Higginbotham, Gabriele Mogni, Justin Wark, Eduardo Bringa, Paul Erhart, James Hawreliak, Bruce Remington, Nigel Park Twinning is a potentially important deformation mechanism for shock compression of materials such as tantalum. We present large-scale molecular dynamics (MD) simulations of shock compression of tantalum employing an extended Finnis-Sinclair (EFS) potential\footnote{X. D. Dai et al. J. Phys.: Condensed Matter, \textbf{18}, 4527 (2006).}. For shock loading along the [100] axis, the plastic deformation mechanism is demonstrated to be twinning using the Fourier transform of the atomic positions, and a new per atom structure factor (PASF) method. Using this method, the atoms can be accurately separated into each twin variant and the stress and strain calculated individually. Locally, the individual twins support a large strain anisotropy and deviatoric stress, which is globally reduced towards the hydrostat. The mechanism of deformation twinning in the bcc structure is usually described by the successive displacement of $\left(112\right)$ planes. In these simulations, the twinning mechanism is identified as the short range shuffling of alternate $\left(112\right)$ planes after the large uniaxial compression due to the elastic precursor. [Preview Abstract] |
Wednesday, July 10, 2013 2:45PM - 3:00PM |
Q6.00005: Split and two-zone elastic-plastic shock waves in nickel: a molecular dynamics study Brian Demaske, Vasily Zhakhovsky, Nail Inogamov, Carter White, Ivan Oleynik Shock waves in \textless 110\textgreater and \textless 111\textgreater directions of single-crystal nickel samples were studied by molecular dynamics (MD) simulations. Standard piston-driven simulations were performed to investigate the split-wave regime, including an elastic precursor followed by a plastic wave both moving with different velocities. At moderate piston velocities, the material is initially in a metastable over-compressed elastic state. It later collapses into a plastic state resulting in a two-wave structure consisting of a slow plastic wave and fast elastic precursor. A single two-zone elastic-plastic shock-wave regime, appearing at higher piston velocities, was studied by a moving window MD technique. The plastic wave attains the same speed as the elastic precursor to form a single two-zone wave -- the simulated elastic zone width extending to hundreds nanometers. The orientation dependence of the shock-wave phenomena are also discussed. [Preview Abstract] |
Wednesday, July 10, 2013 3:00PM - 3:15PM |
Q6.00006: Molecular dynamics simulations of shock wave propagation in single crystal copper Romain Perriot, Vasily Zhakhovsky, Ivan Oleynik Various regimes of shock wave propagation, including both elastic-plastic split-shock waves and single two-zone elastic-plastic shock waves, were studied by molecular dynamics (MD) simulations in single crystal copper oriented along the \textless 100\textgreater , \textless 110\textgreater , and \textless 111\textgreater\ directions using both traditional piston-driven and the newly developed moving window MD techniques. The single two-zone elastic-plastic shock wave consists of the elastic zone followed by a plastic zone, where both elastic and plastic fronts move with the same speed, thus maintaining on average a constant separation. Although the properties of the leading elastic zone in both split-shock wave and single two-zone regimes are orientation-dependent, the thermodynamic properties of the plastic state are not once the steady-state regime is achieved in micrometer-thick films. The orientation-independent plastic Hugoniot obtained in our MD simulations agree with experimental observations of orientation-independent shock-wave propagation in single crystal copper [1].\\[4pt] [1] R. Chau, J. St\"{o}lken, P. Asoka-Kumar, M. Kumar, and N. C. Holmes, J. Appl. Phys. 107, 023506 (2010). [Preview Abstract] |
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