Bulletin of the American Physical Society
20th Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 62, Number 9
Sunday–Friday, July 9–14, 2017; St. Louis, Missouri
Session J5: First Principles and Molecular Dynamics IV |
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Chair: Saryu Fensin, Los Alamos National Laboratory Room: Regency Ballroom B |
Tuesday, July 11, 2017 11:15AM - 11:30AM |
J5.00001: Large-scale molecular dynamics studies of sliding friction in nanocrystalline aluminum Timothy Germann, Ramon Ravelo, James Hammerberg We present the results of 138 million-atom and 1.8 billion-atom non-equilibrium molecular dynamics (NEMD) simulations for Al-Al sliding friction at pressures of 15 GPa. Three-dimensional samples comprised of 4 nm, 20 nm and 50 nm grains were studied to times of 100 ns for the largest systems. We discuss the evolution of the initial grain size distribution to a steady state distribution that is statistically similar for all initial grain sizes. We compare the results for the frictional force to a rate dependent model that incorporates plasticity and discuss the relationships among grain size, grain morphology, dislocations and other defect structures, and plasticity. [Preview Abstract] |
Tuesday, July 11, 2017 11:30AM - 11:45AM |
J5.00002: The role of reducing agents in the nucleation and growth of Al metalloid clusters: Ab initio molecular dynamic study. Sufian Alnemrat Ab initio simulations are used to study the growth of metalloid aluminum clusters from their monohalide (AlCl) precursors. Molecular dynamics (MD) simulation is used to study the role of reducing agents in the growth process of Al metalloid clusters. Car-Parrinello MD simulations of AlCl liquid and Lithium-Aluminum Hydride reducing agent (LiAlH$_{\mathrm{4}})$ show spontaneous metalloid cluster growth. The growth process is initiated by transferring a proton to a nearby Al atom that helps forming trivalent impurities (AlCl$_{\mathrm{3}})$ in the solution. Growth towards larger metalloid clusters then proceeds via repeated insertion of AlCl into Al--Cl bonds as well as elimination of AlCl$_{\mathrm{3}}$ species. The transferred proton plays a significant role in reducing additional monohalide species from the solution. The energy barrier associated with the Al-Cl bond is dropped from 7.8 eV to 4 eV via proton-hopping between Al centers. However, this process is completely prohibited in the case of sodium borohydride (NaBH$_{\mathrm{4}})$ reducing agent due to strong Coulomb interactions between Na and B centers. Repeated insertion of additional AlCl monomers towards larger clusters was not observed within the same time scale of the previous simulations. [Preview Abstract] |
Tuesday, July 11, 2017 11:45AM - 12:15PM |
J5.00003: Atomistic simulation approach to constructing models of phase transition kinetics for hydrocodes Invited Speaker: Jonathan Belof Phase transformations under dynamic compression, of great interest due to their importance in interpreting high pressure experiments relevant to planetary physics, present several unique challenges to existing theoretical and computational methodologies. The wide disparity between the length and time scales of atomistic simulation (molecular dynamics) in comparison with the scale of experimental measurements results in a mismatch of theories: accurate models of phase kinetics are required at the hydrodynamic scale for experimental analysis, but our direct knowledge of the phase transition process is limited to the atomistic regime. Focusing on compressive solidification of water and copper, we will highlight atomistic methods for closing a kinetic theory derived from a classical nucleation approach. New techniques, based on the Gibbs-Thomson condition and free energy sampling, for the determination of the nucleation free energy barriers and growth rates at high pressure, will be presented. In addition to these quantities, it has been found that, for dynamic solidification, the nucleation kinetic prefactor and transient induction time play a crucial role in explaining the homogeneous nucleation behavior of water at high pressure. We will also highlight how classical nucleation theory predicts a cross-over from a heterogeneous to homogeneous nucleation mode depending upon the magnitude of the driving force (\emph{i.e.}, shock strength). [Preview Abstract] |
Tuesday, July 11, 2017 12:15PM - 12:30PM |
J5.00004: Molecular Dynamics Simulation of the Shock Response of Nanocrystalline Cu-Ta Systems Jie Chen, Mark Tschopp, Avinash Dongare Nanocrystalline (nc) metal alloys comprising of second phase solutes show promise towards the design of high strength materials as compared to their coarse-grained counterparts. One such system is the high strength nc-Cu-Ta alloy. The improved deformation response is attributed to grain boundary pining due to the presence of Ta precipitates resulting in limited grain boundary sliding and rotation at high temperatures. A thorough understanding of the role of microstructure and chemistry on the nucleation and evolution of defects under shock loading conditions is crucial for the design and optimization of Cu-Ta alloys for dynamic loading environments. Large-scale molecular dynamics (MD) simulations are therefore performed to examine the deformation and failure behavior of Ta solute strengthened bulk nc-Cu systems under shock loading conditions. The dynamic evolution of defects (dislocations and twinning behavior) is investigated for variations in microstructure (grain size of nc-Cu and Ta distribution in the form of solute atoms as well as precipitates) during shock compression and spall failure. The MD simulations suggest that the spall strengths of the metal are largely influenced by the distribution of the Ta solute in the nc-Cu matrix. The effect of Ta distribution at the grain boundaries, grain interior and as precipitates on the evolution of dislocation densities and the spall strength of the alloy will be presented. [Preview Abstract] |
Tuesday, July 11, 2017 12:30PM - 12:45PM |
J5.00005: The Role of Interfaces in Nucleation of Dynamic Damage in BCC Materials Saryu Fensin, Eric Hahn, Timothy Germann, George Gray III For ductile metals, the process of dynamic fracture occurs through nucleation, growth and coalescence of voids. For high purity single-phase metals, it has been observed by numerous investigators that voids tend to heterogeneously nucleate at grain boundaries and all grain boundaries are \textit{not} equally susceptible to void nucleation. Several factors can affect the failure stress of a grain boundary, such as grain boundary structure, energy and excess volume, in addition to its interactions with dislocations. Flyer plate simulations were carried out for four boundary types with different energies and excess volumes in both materials. These boundaries were chosen as model systems to represent various boundaries observed in ``real'' materials. In this work, we investigate the role of interfaces in BCC (Ta) in void nucleation. The simulation results will be compared with bi-crystal gas-gun experiments. We will also explore the influence of grain boundary energy, excess volume and plasticity at the boundary on the failure stress of a grain boundary. [Preview Abstract] |
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