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 V4: Phase Transitions V: Metals II |
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Chair: Malcolm McMahon, University of Edinburgh, Turab Lookman, Los Alamos National Laboratory Room: Grand H |
Thursday, June 18, 2015 3:45PM - 4:00PM |
V4.00001: Hydrocode and Molecular Dynamics modelling of uniaxial shock wave experiments on Silicon Paul Stubley, David McGonegle, Shamim Patel, Matthew Suggit, Justin Wark, Andrew Higginbotham, Andrew Comley, John Foster, Steve Rothman, Jon Eggert, Dan Kalantar, Ray Smith Recent experiments have provided further evidence that the response of silicon to shock compression has anomalous properties, not described by the usual two-wave elastic-plastic response. A recent experimental campaign on the Orion laser in particular has indicated a complex multi-wave response. While Molecular Dynamics (MD) simulations can offer a detailed insight into the response of crystals to uniaxial compression, they are extremely computationally expensive. For this reason, we are adapting a simple quasi-2D hydrodynamics code to capture phase change under uniaxial compression, and the intervening mixed phase region, keeping track of the stresses and strains in each of the phases. This strain information is of such importance because a large number of shock experiments use diffraction as a key diagnostic, and these diffraction patterns depend solely on the elastic strains in the sample. We present here a comparison of the new hydrodynamics code with MD simulations, and show that the simulated diffraction taken from the code agrees qualitatively with measured diffraction from our recent Orion campaign. [Preview Abstract] |
Thursday, June 18, 2015 4:00PM - 4:15PM |
V4.00002: Structural changes in shock compressed silicon observed using time-resolved x-ray diffraction at the Dynamic Compression Sector Stefan Turneaure, E. Zdanowicz, N. Sinclair, T. Graber, Y.M. Gupta Structural changes in shock compressed silicon were observed directly using time-resolved x-ray diffraction (XRD) measurements at the Dynamic Compression Sector at the Advanced Photon Source. The silicon samples were impacted by polycarbonate impactors accelerated to velocities greater than 5 km/s using a two-stage light gas gun resulting in impact stresses of about 25 GPa. The 23.5 keV synchrotron x-ray beam passed through the polycarbonate impactor, the silicon sample, and an x-ray window (polycarbonate or LiF) at an angle of 30 degrees relative to the impact plane. Four XRD frames ($\sim$ 100 ps snapshots) were obtained with 153.4 ns between frames near the time of impact. The XRD measurements indicate that in the peak shocked state, the silicon samples completely transformed to a high-pressure phase. XRD results for both shocked polycrystalline silicon and single crystal silicon will be presented and compared. Work supported by DOE/NNSA. [Preview Abstract] |
Thursday, June 18, 2015 4:15PM - 4:45PM |
V4.00003: Contemporary Research of Dynamically Induced Phase Transitions Invited Speaker: Lawrence Hull Dynamically induced phase transitions in metals, within the present discussion, are those that take place within a time scale characteristic of the shock waves and any reflections or rarefactions involved in the loading structure along with associated plastic flow. Contemporary topics of interest include the influence of loading wave shape, the effect of shear produced by directionality of the loading relative to the sample dimensions and initial velocity field, and the loading duration (kinetic effects, hysteresis) on the appearance and longevity of a transformed phase. These topics often arise while considering the loading of parts of various shapes with high explosives, are typically two or three-dimensional, and are often selected because of the potential of the transformed phase to significantly modify the motion. In this paper, we look at current work on phase transitions in metals influenced by shear reported in the literature, and relate recent work conducted at Los Alamos on iron's epsilon phase transition that indicates a significant response to shear produced by reflected elastic waves. A brief discussion of criteria for the occurrence of stress induced phase transitions is provided. Closing remarks regard certain physical processes, such as fragmentation and jet formation, which may be strongly influenced by phase transitions. [Preview Abstract] |
Thursday, June 18, 2015 4:45PM - 5:00PM |
V4.00004: Analysis of PDV velocity fluctuations in presence of phase transition for Bi and Ce Roger Minich, Ricky Chau Particle velocity fluctuations from PDV velocimetry of Bismuth and Cerium are analyzed to probe the dynamics of phase transitions in Bismuth and Cerium. Wavelet analysis is used to study the velocity dispersion as it evolves in time. Also, phase portraits, (parametric plots of particle acceleration versus particle velocity) are studied and results suggest that the phase transition behaves like a driven nonlinear dissipative dynamical system. An effective second order equation is extracted from the data. Surprisingly, the equation can be shown to be derived from the hydrodynamic equations when the bulk modulus has a dependence on the phase fraction. In addition, the velocity time history exhibits a discrete hopping between average velocity states reminiscent of a driven bistable oscillator. Finally, the phase portraits suggest how to study phase hysteresis scaling in real time. [Preview Abstract] |
Thursday, June 18, 2015 5:00PM - 5:15PM |
V4.00005: Role of plastic deformation in shock-induced phase transitions Punam Ghimire, T.C. Germann, R. Ravelo Non-equilibrium molecular dynamics (NEMD) simulations of shock-wave propagation in fcc single crystals exhibit high elastic limits and large anisotropies in the yield strength. They can be used to explore the role of plastic deformation in the morphology and kinetics of solid-solid phase transformations. We report on large-scale atomistic simulations of defect-mediated phase transformations under shock and quasi-isentropic compression (QIC). An analytical embedded atom method (EAM) description is used to model a fcc-bcc phase transition (PT) boundary fitted to occur below or above the elastic-plastic threshold in order to model systems undergoing a PT with and without plasticity. For cases where plastic deformation precedes the phase transformation, the defect-mediated PT proceeds at faster rates than the defect-free ones. The bcc fraction growth rate can be correlated with a sharp decrease in the dislocation densities originally present in the parent phase. [Preview Abstract] |
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