Bulletin of the American Physical Society
2005 14th APS Topical Conference on Shock Compression of Condensed Matter
Sunday–Friday, July 31–August 5 2005; Baltimore, MD
Session U5: First-Principles & Molecular Dynamics Calculations V |
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Chair: M. Riad Manaa, Lawrence Livermore National Laboratory Room: Hyatt Regency Constellation F |
Thursday, August 4, 2005 3:00PM - 3:30PM |
U5.00001: Quantum-Based Atomistic Simulation of Transition Metals Invited Speaker: First-principles generalized pseudopotential theory (GPT) provides a fundamental basis for \textit{transferable} multi-ion interatomic potentials in transition metals and alloys within density-functional quantum mechanics. In central bcc transition metals, where multi-ion angular forces are important to structural properties, simplified \textit{model} GPT or MGPT potentials have been developed based on canonical $d$ bands to allow analytic forms and large-scale atomistic simulations. Robust, advanced-generation MGPT potentials have now been obtained for Ta and Mo and successfully applied to a wide range of structural, thermodynamic, defect and mechanical properties at both ambient and extreme conditions of pressure and temperature, including multiphase equation of state, melting and rapid resolidification, thermoelasticity and the detailed atomistic simulation of point defects, dislocations and grain boundaries needed for the multiscale modeling of plasticity and strength. Recent algorithm improvements have also allowed the implementation of a more general matrix representation of MGPT beyond canonical bands for increased accuracy and extension to $f$-electron actinide metals plus an order of magnitude increase in computational speed. An important further advance still in progress is the development of temperature-dependent MGPT potentials that subsume electron-thermal as well as ion-thermal contributions to high-temperature properties. [Preview Abstract] |
Thursday, August 4, 2005 3:30PM - 3:45PM |
U5.00002: Modeling Rapid Resolidification of Ta on BlueGene/L Frederick H. Streitz, Mehul V. Patel, James N. Glosli We investigate the rapid, pressure-induced solidification of molten Ta using the ddcMD code on BlueGene/L. Access to this massively parallel computer (the world's largest) has enabled us to investigate solidification at a size scale which is unprecedented - we will directly compare results obtained from simulations ranging from 64,000 atoms to over 16 million atoms. Solidification is indeed found to be ``rapid,'' occuring on a time scale of 100 ps in these simulations before grain coalescence occurs and a coarsening process begins. Finite size effects apparent in simulations of less than 1 million atoms had a dramatic impact on not only the final microstructure but on the the approach to final microstucture as well, even at early times. These results punctuate the need for very large simulations to explore the resolidification process. \vskip2em\noindent Work performed under the auspices of the U.S. DOE at the University of California/Lawrence Livermore National Laboratory under contract W-7405-ENG-48 [Preview Abstract] |
Thursday, August 4, 2005 3:45PM - 4:00PM |
U5.00003: Multibillion-atom Molecular Dynamics Simulations on BlueGene/L Peter S. Lomdahl, Timothy C. Germann, Kai Kadau The IBM BlueGene/L supercomputer at Lawrence Livermore National Laboratory, with 65,536 CPU processors connected by multiple high-performance networks, enables a completely new class of physical problems to be investigated. Using either pairwise interactions such as the Lennard-Jones potential, or the embedded atom method (EAM) potential for simple metals, system sizes up to 160 billion atoms (or a cube of copper a micron on each side) can be modeled. In order to obtain any new physical insights, however, it is equally important that the analysis of such systems be tractable. This is in fact possible, in large part due to our highly efficient parallel visualization code, which enables the rendering of atomic spheres, Eulerian cells, and other geometric objects in a matter of minutes, even for tens of thousands of processors and billions of atoms. We will describe the performance scaling and initial results obtained for shock compression and release of a defective EAM Cu sample, illustrating the plastic deformation accompanying void collapse as well as the subsequent void growth and linkup upon release. [Preview Abstract] |
Thursday, August 4, 2005 4:00PM - 4:15PM |
U5.00004: Theoretical Spall Strength and EOS of Ti, Zr and Hf in the Negative Pressure Region. K.D. Joshi, Satish C. Gupta Determination of tensile stresses from the particle velocity histories in materials unloaded from the shocked state has opened the possibility of understanding the material behaviour in the negative pressure regimes. In the present work, we have determined the theoretical spall strength ($\sigma _{s})$ along [0001] direction, and also the EOS in the negative pressure regime, for transition metals Ti, Zr and Hf from first principles using FP-LAPW method. The $\sigma _{s}$ for hcp ($\alpha )$ and $\omega $ phases of these metals have been derived from the calculated total energy versus uniaxial strain along [0001] direction. The calculated $\sigma _{s}$ for $\alpha $ phase of Ti, Zr and Hf are 22, 18 and 20 GPa, and for the $\omega $ phase are 24.2, 19.5 and 23.6 GPa, respectively. The $\omega $ phase is found to be harder than $\alpha $ phase in agreement with available experimental results. The trend in the group IV B indicates that $\sigma _{s}$ for Ti is largest followed by Hf and then Zr for both $\alpha $ and $\omega $ structures. The theoretical $\sigma _{s}$ for Ti is much higher than $\sim $ 4.2 GPa measured at strain rates of $\sim $10$^{6}$/s. This discrepancy could be associated with the material defects, which dominantly control the spalling at such strain rates. For determination of ideal $\sigma _{s}$, experiment should be performed at still higher stresses (and higher strain rates). The bulk modulus of 110, 96 and 115 GPa, respectively for Ti, Zr and Hf, determined from the theoretical EOS in the negative pressure region, are in good agreement with experiments. [Preview Abstract] |
Thursday, August 4, 2005 4:15PM - 4:30PM |
U5.00005: Surface Roughness Effects on Ejecta Production Timothy C. Germann, James E. Hammerberg, Brad Lee Holian, Ramon Ravelo We utilize large-scale classical molecular dynamics simulations with embedded atom method (EAM) potentials to investigate the effects of surface roughness on the ejection of material when a shock wave releases from a free surface. There are (at least) three regimes which, in principle, can lead to different mechanisms of ejecta production: (1) shock compression and release both occur in the solid phase; (2) the shocked state is solid, but melts upon release; and (3) the shocked state is liquid (or mixed-phase). For a perfect Cu surface (Germann, Hammerberg, and Holian, SCCM 03), a continuous increase in ejecta mass is seen, from single atoms and clusters at dislocation intersections in the plastic regime (1), to on the order of a monolayer of ejecta upon shock melting (case 3). With surface imperfections such as machining grooves, ejecta production is completely dominated by hydrodynamic jetting phenomena in both regimes (2) and (3). [Preview Abstract] |
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