Bulletin of the American Physical Society
19th Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 60, Number 8
Sunday–Friday, June 14–19, 2015; Tampa, Florida
Session K5: Phase Transitions I: Molecular Dynamics |
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Chair: Carl Greeff, Los Alamos National Laboratory, Paulo Rigg, Washington State University Room: Grand I/J |
Tuesday, June 16, 2015 2:15PM - 2:30PM |
K5.00001: A molecular-dynamics study of shock-induced $\alpha$-$\omega$ phase transformation of Ti Babak Sadigh, Luis Zepeda-Ruiz, Thomas Lenosky, Tomas Oppelstrup, Jon Belof We present molecular-dynamics simulations of structural phase transformations in single-crystal hcp-Ti shocked along the (0001)-axis. The interatomic interaction potential is modeled within the Modified Embedded-Atom Method (MEAM) framework. Above a critical shock strength $\sigma_{th}$, a displacive structural transition is observed from the hcp phase into the $\omega$-structure, whereupon a splitting of the shock-wave occurs and a two-wave shock structure is observed. We investigate the kinetics of this structural transition as a function of increasing shock strength beyond $\sigma_{th}$. We identify the atomistic mechanisms underlying the $\alpha$-$\omega$ martensitic transformation and study the role of the pressure waves in driving the phase change by analyzing the evolution of local stress and temperature behind the shock fronts as well as in the interfacial regions between the two phases. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Tuesday, June 16, 2015 2:30PM - 2:45PM |
K5.00002: Non-equilibrium Molecular Dynamics simulations of Cu solidification Luis Zepeda-Ruiz, Babak Sadigh, Jon Belof The development of predictive theories for the study of non-equilibrium phase transitions occurring in dynamic materials processes, such as shocks, requires a multi-scale approach that spans from the atomistic to the continuum scale. For this purpose, we present results of Non-equilibrium Molecular Dynamics (NEMD) simulations of solidification of Cu from the melt for both, thermal cooling and compression conditions. Our simulations show a transition from spinodal-like to nucleation-dominated phase transition as a function of both cooling and compression rates. In addition, the appearance and evolution of different Cu phases are presented. This work is performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Tuesday, June 16, 2015 2:45PM - 3:00PM |
K5.00003: Shock response of nanoporous Cu---A molecular dynamics simulation FengPeng Zhao Shock response of porous materials can be of crucial significance for shock physics and bears many practical applications in materials synthesis and engineering. Molecular dynamics simulations are carried out to investigate shock response of nanoporous metal materials, including elastic-plastic deformation, Hugoniot states, shock-induced melting, partial or complete void collapse, hotspot formation, nanojetting, and vaporization. A model nanoporous Cu with cylindrical voids and a high porosity under shocking is established to investigate such physical properties as velocity, temperature, density, stress and von Mises stress at different stages of compression and release. The elastic-plastic and overtaking shocks are observed at different shock strengths. A modified power-law P-$\alpha $ model is proposed to describe the Hugoniot states. The Gr\"{u}neisen equation of state is validated. Shock-induced melting shows no clear signs of bulk premelting or superheating. Void collapse via plastic flow nucleated from voids, and the exact processes are shock strength dependent. With increasing shock strengths, void collapse transits from the ``geometrical'' mode (collapse of a void is dominated by crystallography and void geometry and can be different from that of one another) to ``hydrodynamic'' mode (collapse of a void is similar to one another). The collapse may be achieved predominantly by plastic flows along the \textbraceleft 111\textbraceright slip planes, by way of alternating compression and tension zones, by means of transverse flows, via forward and transverse flows, or through forward nano-jetting. The internal jetting induces pronounced shock front roughening, leading to internal hotspot formation and sizable high speed jets on atomically flat free surfaces. [Preview Abstract] |
Tuesday, June 16, 2015 3:00PM - 3:15PM |
K5.00004: MD simulation of steady shock waves with bcc-to-hcp phase transition in single-crystal Iron Vasily Zhakhovsky, Kirill Migdal, Nail Inogamov, Sergey Anisimov Overdriven shock waves propagating along the main crystallographic directions of single-crystal bcc iron were studied with moving-window molecular dynamics (MD) technique. To simulate correctly the shock-induced bcc-to-hcp phase transition a new EAM potential fitted to the cold pressure curves and pressure transition at about 13 GPa was developed for iron by the stress matching method. We demonstrate that structure of shock fronts depends on orientation of crystal. A peculiar structure of steady shock-wave front in [100] direction is observed. While a single-wave plastic shock front in [100] direction has no elastic precursor, a single two-zone elastic-plastic shock wave with highly-overcompressed elastic zone ahead of a plastic front propagates in other directions. The mechanisms and interplay of elastic-plastic transformation and bcc-to-hcp phase transition induced by steady shock waves in iron are discussed. [Preview Abstract] |
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