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 J5: First-Principles & Molecular Dynamics Calculations IV |
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Chair: Timothy Germann, Los Alamos National Laboratory Room: Hyatt Regency Constellation F |
Tuesday, August 2, 2005 11:00AM - 11:15AM |
J5.00001: Modeling the Effects of Quasistatic and Dynamic Stress on Extended Defects in FCC Al and Cu Alison Kubota, Wilhelm Wolfer There has been much interest in understanding the effect of extended defects on the static and dynamic responses of metals to applied stress. These aging effects, in the form of dislocation loops, voids, and He bubbles can possibly lead to changes in properties such as the equation-of-state of materials. In this work, we focus on the use of large-scale molecular dynamics simulations to model the loading response of voids and He bubbles in both low- and high-stacking-fault energy (Cu and Al) face-centered-cubic metals. In both quasistatic and dynamic loading conditions, both voids and He bubbles are found to initiate collapse by emission of stacking faults from the defect edge. However, we show the significant differences in the intermediate and end-state structures due to the presence of He. In addition, we discuss the effect of short-pulse shock waves on the unfaulting of dislocation loops and net change in Burgers vector through complex dislocation reaction pathways. These simulations using pulsed loading conditions are able to capture the details of these processes that would otherwise remain hidden from observation by electron microscopy. This work was performed under the auspices of the U. S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. [Preview Abstract] |
Tuesday, August 2, 2005 11:15AM - 11:30AM |
J5.00002: Molecular-dynamics simulation of highly-symmetric grain boundary shock interaction Vladimir Dremov, Philipp Sapozhnikov, Eduardo Bringa The defects present in a material (vacancies and interstitial atoms, dislocations, or grain boundaries) are of significant effect upon its properties including elastic-plastic ones. Here we present results of the molecular-dynamics simulations of the interaction of a shock wave with a highly-symmetric grain boundary in copper. The calculations were carried out for five slopes of the grain boundary (110) $\Sigma $5 to the shock front and for two levels of loading: above the Hugoniot elastic limit but below the critical level at which the homogeneous dislocation production occurs and above that critical level. Shock-induced defects and their role in the shear stress relaxation have been analyzed. It has been shown that there are two competing shear stress relaxation mechanisms as a result of the grain boundary shock interaction. Calculations of the temperature evolution in the vicinity of the grain boundary make evidence for the possibility of local melting far below the intersection of the Hugoniot and the melting line. [Preview Abstract] |
Tuesday, August 2, 2005 11:30AM - 12:00PM |
J5.00003: Atomistic shock simulations in defective metals Invited Speaker: I will present molecular dynamics (MD) simulations of shocks in embedded atom method (EAM) metals with pre-existing defects. Samples with prismatic loops and voids have been studied, and analytic dislocation-based models have been used to accurately predict their Hugoniot elastic limit. On the other hand, understanding of nanocrystalline samples is more challenging. Nanocrystals are of great interest due to a number of unique properties, including higher strength and hardness than larger polycrystaline materials. I will show simulations of shock waves in nanocrystals, where grain boundary sliding is reduced and ``harder'' nanocrystals are observed. Related shock-recovery experiments (details presented in another talk) are being carried out at LLNL, and could open up new applications for nanocrystalline materials. *Numerous people contributed to this work, especially A. Caro and M. Victoria. The work at LLNL was performed under the auspices of the U.S. Department of Energy and Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48, LDRD 04-ERD-021. [Preview Abstract] |
Tuesday, August 2, 2005 12:00PM - 12:15PM |
J5.00004: Nanoscale molecular dynamics simulation of shock compression of silicon Douglas Lovelady, Ivan Oleynik, Sergey Zybin, Mark Elert, Carter White Shock waves in solids provide a unique opportunity to study the fundamental physics and chemistry of matter at extreme pressures and temperatures. In spite of substantial theoretical and experimental efforts a full understanding of shock-induced elastic and plastic responses and polymorphic phase transitions is still far from complete. These phenomena often occur at the nanometer size and picosecond time scales which makes molecular dynamics simulations an ideal tool for exploring nanoscale mechanisms of shock induced processes such as chemical reactions and phase transitions. We report results of molecular dynamics simulation of shock wave propagation in silicon in [100], [110], and [111] directions obtained using a classical interatomic potential. Several regimes of materials response are classified as a function of shock wave intensity and crystalline orientation of shock wave propagation using calculated shock Hugoniot. The shock induced chemistry and shock wave splitting are discussed in relation to recent experimental results [1] that indicate anomalous elastic response of the lattice at high compression ratios. [1] A. Loveridge-Smith, Phys. Rev. Let. \textbf{86}, 2349 (2001). [Preview Abstract] |
Tuesday, August 2, 2005 12:15PM - 12:30PM |
J5.00005: MD Simulation of Dislocation Behavior in KCl under Shock Compression Takahiro Kinoshita, Tsutomu Mashimo, Katsuyuki Kawamura MD simulations of dislocation behavior in KCl under uniaxial compression are carried out to discuss the elastoplastic transition under shock compression. The dislocations proceeded along to 45 degrees to the uniaxialy compressed direction due to the displacement of atomic lines. This result is consistent with the VonMises hypothesis that the shear stress along 45 degrees to uniaxial stress is maximum. Simulation results also showed that the stress, which dislocations started to move under the uniaxial compression along the $<$111$>$ axis direction, was larger than other ones along the $<$100$>$ and $<$110$>$ axis directions. These results are qualitatively consistent with the experimental ones that the Hugoniot-elastic limit along the $<$111$>$ axis direction was larger than the others. [Preview Abstract] |
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