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 W4: TM Molecular Dynamics IV |
Hide Abstracts |
Chair: Sandro Scandolo, International Centre for Theoretical Physics Room: Vashon |
Thursday, July 11, 2013 4:00PM - 4:15PM |
W4.00001: Unrevealing transition mechanism: novel carbons, metallic germaniums, and low-temperature galliums Daniele Selli, Igor A. Baburin, Roman Marton\'ak, Stefano Leoni The quest for novel carbon-based materials is a topic of high priority. Using accelerated molecular dynamics techniques we investigated low-temperature compression of graphite into novel carbon modifications with odd-even topological pattern. At room temperature germanium modifications shows semiconducting properties, while metallicity and superconductivity have been found, so far, only in high pressure modifications. By means of different theoretical methodology, we are able to predict new semiconducting and low pressure metallic Ge phases together with a clearer picture of particular transformation paths and specific indication of possible synthesizabilities. Gallium is among the most challenging elements of the periodic systems. Its polymorphs are structurally very peculiar, characterized by unusual open ground-state crystal structures. While high-pressure promotes close-packed galliums, low-temperature, and the use of mild oxidative chemical approaches, is a way of affecting nucleation patterns towards novel open, clathrate-like compounds. [Preview Abstract] |
Thursday, July 11, 2013 4:15PM - 4:30PM |
W4.00002: Frictional Interactions at High Velocity Polycrystalline Ductile Metal Interfaces James Hammerberg, Jacqueline Milhans, Ramon Ravelo, Timothy Germann We have examined the effect of evolution of grain morphology on the frictional force at polycrystalline Al-Al and Al-Ta interfaces as a function of grain size and sliding velocity. We present the results of 8M, 26M and 138M particle NonEquilibrium Molecular Dynamics (NEMD) simulations for grain sizes of 13.5, 19.3 and 20 nm. Sample sizes consisted of 3x3x3 and 5x5x5 grains on each side of a sliding interface. We have considered sliding velocities of 42, 50, 100, 140, and 240 m/s. For velocities below a size dependent critical velocity above which a fluid layer forms, we find enhanced grain coarsening leading to a highly strained, graded final steady state microstructure that exhibits a dynamic morphology for times greater than 5-10 ns. We find that the frictional force is insensitive to the initial grain size distribution due to the evolution of the initial distribution to a new nonequilibrium steady state. We discuss the relationship of these results to single crystal interfaces and the mechanisms for grain size and shape evolution. [Preview Abstract] |
Thursday, July 11, 2013 4:30PM - 4:45PM |
W4.00003: Large-scale Molecular Dynamics Simulations of Shock-induced Plasticity and Twinning in bcc Nb and Ta Timothy Germann, Ruifeng Zhang, Ramon Ravelo Large-scale classical molecular dynamics (MD) simulations are used to investigate dislocation slip and twinning activity in bcc metals under shock compression. We will discuss both the orientation-dependent response of Nb and Ta single crystals, as well as the more complex response of nanocrystalline samples. Of particular importance as MD simulations are becoming applied to model more complex materials, we will discuss issues related to the interatomic potential description and the analysis of the deformation response. Embedded atom method (EAM) potentials for shock compression studies must properly describe the energy landscape under the pressure range of interest; and an orientation imaging map technique is described for following the plastic response of fcc and bcc metals. [Preview Abstract] |
Thursday, July 11, 2013 4:45PM - 5:00PM |
W4.00004: Shock waves in polycrystalline iron: plasticity and phase transitions Eduardo Bringa, Nina Gunkelmann, Carlos Ruestes, Herbert Urbassek Iron undergoes a bcc to close-packed structural phase transition under pressure, at around 13 GPa. Atomistic simulations have been able to provide insights into the transition, but without dislocation plasticity occurring before the phase change, while experiments in polycrystals do show clear evidence for dislocation plasticity. Here we study shock waves in polycrystalline Fe using two different interatomic potentials, below and above the phase transition pressure. We show that it is essential to employ a finite ramp time of the shock wave in the crystal in order to give dislocations sufficient time for nucleation. For grain sizes below 10 nm, where a significant fraction of the plastic activity can occur by grain boundary sliding, dislocation nucleation still is a relatively small contribution to shear stress relaxation. [Preview Abstract] |
Thursday, July 11, 2013 5:00PM - 5:15PM |
W4.00005: Role of Interface on Shock Response of Cu-Nb Nanolaminate Composites Ruifeng Zhang, T.C. Germann, I.J. Beyerlein, X.Y. Liu, Jian Wang Using newly constructed interatomic potential for Cu-Nb system, large-scale molecular dynamics simulations are performed on two interfaces, Kurdjumov-Sachs \textbraceleft 111\textbraceright Cu\textbar \textbar \textbraceleft 110\textbraceright Nb (KS) and \textbraceleft 112\textbraceright KS orientation relationship, to provide insight into the role of interface structure on the nucleation, transmission, absorption, and storage of dislocations during shock compression. We found that Shockley partials prefer to nucleate from the interface and then transmit into the neighboring layers when the layer thicknesses are lower than 10 nm, controlling their plasticity. The preferred nucleation sites are found to be closely associated with the interface misfit dislocation structures, and dislocation transmission abides by the geometrical compatibility of pairs of slip systems of adjoining crystals. Critical shock pressures to nucleate from and transmit dislocations across the atomically flat interface are shown to be substantially higher than those for the faceted interface. We discuss the atomic-level interface characteristics that cause these two types of interfaces to nucleate and transmit dislocations by significantly different mechanisms. [Preview Abstract] |
Thursday, July 11, 2013 5:15PM - 5:30PM |
W4.00006: Plasticity mechanisms in nanovoided b.c.c. metals under high strain rate compression Carlos J. Ruestes, Eduardo M. Bringa, Alexander Stukowski, Joaquin F. Rodr\'Iguez Nieva, Graciela Bertolino, Yizhe Tang, Marc A. Meyers Atomistic-scale simulations provide unique insights to plasticity mechanisms arising under extreme conditions where its relative nanoscopic length and time scales render experiments almost impossible. Our studies explore the mechanical response and plasticity effects under uniaxial high strain rate compression for a Ta single crystal with a collection of spherical nanovoids, with a radius of 3-4 nm, providing an initial porosity of 5{\%}-20{\%}. We examine strain rate effects, from 10$^{7}$/s to 10$^{10}$/s, in the dislocation density and dislocation-induced heating. The resulting dislocation densities are in good agreement with experimental results for shock-recovered samples. [Preview Abstract] |
Thursday, July 11, 2013 5:30PM - 5:45PM |
W4.00007: MD-generated molecular volumes and their mechanistic applications Noham Weinberg Experimentally, the effects of pressure on reaction rates are described by their pressure derivatives, known as volumes of activation. Transition state theory directly links activation volumes to partial molar volumes of reactant and transition states. Traditionally, the experimentally measured effects of pressure on reaction rates are expressed in terms of so-called volumes of activation, which represent the difference in volumes of transition states and reactants. Since the volumes of reactants are readily available experimentally, the volumes of activation provide a direct measure of the transition state volumes. In this context, a reliable technique is required for relating the volume of a microscopic system to its geometrical parameters. Until recently, such a technique did not exist. Over the past five years we developed a precise method based on molecular dynamics simulations and applied it to calculation of molecular volumes and volumes of activation. The results of calculations closely matched the experimental values. We are now extending and refining this method in its application to a wider range of reactions, including biochemical processes and processes at extreme pressures. [Preview Abstract] |
Thursday, July 11, 2013 5:45PM - 6:00PM |
W4.00008: Equation of state and stability of metal crystals at high pressure by DFT calculations Dmitry Minakov, Pavel Levashov In this work we present \textit{ab initio} equation-of-state calculations for crystals of some metals. Density functional theory at finite temperature (VASP code) is used to obatin the properties of electrons; lattice is simulated in quasi-harmonic approximation at non-zero temperature of electrons. Anharmonic effects are taken into account by the thermal expansion of a crystal. All calculations were performed for aluminum, copper and gold. We compare our results with available shock-wave data in crystal phase, including isentropic expansion. The melting curves are calculated by different criteria; the effect of different temperatures of electrons and ions is taken into account. Also we determine thermodynamic and kinetic boundaries of stability of crystals. Our calculations demonstrate that \textit{ab initio} approaches can be used to theoretically reconstruct thermodynamically complete EOS of metallic crystals. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700