Bulletin of the American Physical Society
17th Biennial International Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 56, Number 6
Sunday–Friday, June 26–July 1 2011; Chicago, Illinois
Session M3: First-Principles and Molecular Dynamics Calculations VIII: Metals III |
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Chair: Betsy Rice, Army Research Laboratory Room: Renaissance Ballroom AB |
Wednesday, June 29, 2011 11:00AM - 11:15AM |
M3.00001: Shock Response of Cu-Nb Nanolayer Composites Ruifeng Zhang, Jian Wang, Xiang-Yang Liu, Shengnian Luo, Timothy C. Germann Large-scale classical molecular dynamics (MD) simulations are used to study the shock response of Cu-Nb nanolayered composites. We describe the development of an interatomic potential which provides an accurate description of deformation twinning in bcc Nb under compression, slip in fcc Cu, and the interface structure of Cu-Nb interfaces with the Kurdjumov-Sachs (KS) orientation relationship. The MD simulations provide insight into the role of atomic Cu-Nb interface structures on the nucleation, transmission, absorption, and storage of dislocations during shock compression, and their role as dislocation sinks upon release. This, together with the effects of confined layer slip and twinning, leads to a greater degree of recovery as compared to either constituent Cu or Nb single crystal for layer thicknesses down to 5 nm, an effect seen both in our simulations and in companion shock experiments. [Preview Abstract] |
Wednesday, June 29, 2011 11:15AM - 11:30AM |
M3.00002: Growth and Collapse of Nanovoids in Tantalum Monocrystals Loaded at High Strain Rate Yizhe Tang, Eduard Bringa, Bruce Remington, Marc Meyers Shock-induced spall in ductile metals is known to occur by the sequence of nucleation, growth and coalescence of voids, even in high purity monocrystals, but the atomistic mechanisms involved are still not completely understood. The growth and collapse of nanoscale voids in tantalum are investigated under different stress states and strain rates by molecular dynamics simulations. Three principal mechanisms of deformation are identified and quantitatively evaluated: shear loop emission, prismatic loop formation, and twinning. Dislocation shear loops expand as expected from a crystallographic analysis, and their extremities remain attached to the void surface in tension (a requisite for void growth), but can detach in compression. Prismatic loops that detach from the void are generated only during hydrostatic loading, due to the equal resolved shear stress components. Nanotwins form preferably due to tensile stress both uniaxial and hydrostatic. There is a slip-to-twinning transition as the strain rate exceeds 108/s, and a simplified constitutive description is presented to explain this transition. Comparison with recent laser-shock experiments will be discussed. [Preview Abstract] |
Wednesday, June 29, 2011 11:30AM - 11:45AM |
M3.00003: Shock compression and spallation of tantalum: Molecular dynamics simulations S.N. Luo, Q. An, R. Ravelo, T.C. Germann, D.L. Tonks, W.A. Goddard III We perform large-scale molecular dynamics simulations of shock wave compression and spallation of Ta single crystals with different potentials including embedded-atom method (EAM), first-principles-based EAM (qEAM) and reactive forcefield (ReaxFF). Shock loading is applied along $\langle 100 \rangle$, $\langle 110\rangle$ and $\langle111\rangle$. Hugoniot states are obtained from direct shock or Hugoniostat simulations. Anisotropic behaviors are observed in plasticity (including twinning) during compression/tension and in spallation. We present detailed analysis of dislocations, twins and void nucleation and growth, and their implications for the mechanisms of plasticity and spall damage in Ta. [Preview Abstract] |
Wednesday, June 29, 2011 11:45AM - 12:00PM |
M3.00004: Atomistic simulation of shocks in single crystal and polycrystalline Ta E.M. Bringa, A. Higginbotham, N. Park, Y. Tang, M. Suggit, G. Mogni, C.J. Ruestes, J. Hawreliak, P. Erhart, M.A. Meyers, J.S. Wark Non-equilibrium molecular dynamics (MD) simulations of shocks in Ta single crystals and polycrystals were carried out using up to 360 million atoms. Several EAM and FS type potentials were tested up to 150 GPa, with varying success reproducing the Hugoniot and the behavior of elastic constants under pressure. Phonon modes were studied to exclude possible plasticity nucleation by soft-phonon modes, as observed in MD simulations of Cu crystals. The effect of loading rise time in the resulting microstructure was studied for ramps up to 0.2 ns long. Dislocation activity was not observed in single crystals, unless there were defects acting as dislocation sources above a certain pressure. [Preview Abstract] |
Wednesday, June 29, 2011 12:00PM - 12:15PM |
M3.00005: Large-Scale Molecular Dynamics Simulations of Shock-Induced Plasticity in Tantalum Single Crystals R. Ravelo, Q. An, T.C. Germann, B.L. Holian We report on large-scale non-equilibrium molecular dynamics (NEMD) simulations of shock wave compression in Ta single crystals. The atomic interactions are modeled via a recently developed and optimized embedded-atom method (EAM) potential for Ta, which reproduces the equation of state up to 200 GPa. We examined the elastic-plastic transition and shock wave structure for wave propagation along the low index directions: (100), (110) and (111). Shock waves along (100) and (111) exhibit an elastic precursor followed by a plastic wave for particle velocities below 1.1 km/s for (100) and 1.4 km/s for (111). The nature of the plastic deformation along (110) is dominated by twinning for pressures above 41 GPa. [Preview Abstract] |
Wednesday, June 29, 2011 12:15PM - 12:30PM |
M3.00006: Shock-induced phase transitions in metals: recrystallization of supercooled melt and melting of an overheated solid Vasily Zhakhovsky, Mikalai Budzevich, Carter White, Ivan Oleynik Steady melting shock waves in aluminum and nickel were studied using a novel moving window molecular dynamics (MW-MD) technique. It was found that shock compression in the [100] crystallographic direction leads to the formation of an overheated metastable solid state within the shock front. This state is located on an extension of the solid branch of the T-P Hugoniot above the melting line. Such an overheated crystal melts behind the shock front accompanied by a temperature decrease. By contrast, the shock compression in the [110] and [111] directions results in a so-called ``cold'' melting that takes place at a temperature/pressure range below the melting line. Such unusual melting occurs because large shear stresses within the shock front induce an enormous overproduction of defects that transform the crystal to a highly amorphous, liquid-like state. This metastable state, lying on the extension of liquid branch of the T-P Hugoniot below the equilibrium melting line, eventually undergoes a recrystallization associated with a temperature increase and growth of crystal grains in the after-shock flow. [Preview Abstract] |
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