Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session R42: Focus Session: Plasticity and Phase Transitions |
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Sponsoring Units: DCMP GSCCM Chair: Ricky Chau, Lawrence Livermore National Laboratory Room: Baltimore Convention Center 345 |
Wednesday, March 15, 2006 2:30PM - 3:06PM |
R42.00001: Multiphase Equations of State for Materials with Solid-Solid and Solid-Liquid Phase Transitions Invited Speaker: Many materials undergo a solid-solid phase transition where the crystal structure changes with a change in pressure or temperature. In addition materials melt, which is another phase transition. Across a phase boundary there are discontinuities in density and internal energy (latent heat). Although the volume change is small, even a small volume change may cause significant effects. For example under shock loading a phase transition may cause the shock to split into two distinct shocks. The equation of state (EoS) of a material with a phase transition may be modelled by determining the EoS for each individual phase separately. The phase that exists at any pressure and temperature is the phase with the lowest Gibb's free energy. Then the phase boundaries between phases are the loci of points in the pressure/temperature plane where two phases have the same Gibb's free energy. Along a phase boundary the EoS may be determined by assuming that the material consists of a mixture of two phases. Then the EoS in the mixed-phase region is completely determined by the EoS of the single phases. It can be shown that the bulk sound speed always falls when passing from a single-phase region into a mixed-phase region. Therefore, under isentropic compression through a phase boundary a shock will form. When implementing the EoS into hydrocodes an iterative method may be used to determine the solution to the equations: An initial guess is made of which phases exist at the required specific volume and internal energy and then the pressure, temperature and mass fractions are calculated. If the chosen phases have the lowest Gibb's free energy the solution has been found, otherwise another guess of the stable phases is made, and the iteration is repeated. [Preview Abstract] |
Wednesday, March 15, 2006 3:06PM - 3:18PM |
R42.00002: First-principles study of high-pressure phase transitions in Si$_{3}$N$_{4}$ Bin Xu, Jianjun Dong, Harold Stokes, Dorian Hatch We have performed a systematic theoretical study of phase transitions in Si$_{3}$N$_{4}$ at high-pressure. We calculated the Gibbs free energies of the ground state $\beta $ phase and the high-pressure $\gamma $ phase at various temperatures and pressures based on the first- principles density functional theory and the statistical quasi-harmonic theory. The thermal equations of state and the Clapeyron slope of the $\beta $-to- $\gamma $ transition at high-pressure are determined and compared with available experimental data. In addition to the recently discovered high- pressure $\gamma $-phase, we predict a new high-pressure phase of hexagonal symmetry. This new high-pressure phase is likely to exist metastably at the high pressure \textit{and room temperature}, and it is associated with a non \AA-point soft phonon distortion. The structural and elastic properties and the stability condition of this predicted metastable high-pressure phase will be discussed. [Preview Abstract] |
Wednesday, March 15, 2006 3:18PM - 3:30PM |
R42.00003: Pressure-induced B1-to-B2 phase transition in AgSbSe$_2$ Eunja Kim, Ravhi Kumar, Andrew Cornelius, Malcolm Nicol We have investigated the pressure-induced B1-to-B2 phase transitions in AgSbSe$_{2}$. High pressure x-ray diffraction experiments clearly show that the process starts at 20 GPa and completes at 55 GPa. Our density-functional calculations are in good agreement with the experiment and confirm the B1 to B2 transition which is similar to AgSbTe$_{2}$. The possible intermediate structure for B1-to-B2 transition in AgSbSe$_{2}$ occurring at 21-54 GPa is under investigation. Its similarity and difference to AgSbTe$_{2}$ will be discussed in this study. [Preview Abstract] |
Wednesday, March 15, 2006 3:30PM - 3:42PM |
R42.00004: High Pressure-Induced Phase Transition In $\beta $-Ga$_{2}$O$_{3}$: in situ Synchrotron X-Ray Diffraction Studies up to 70 Gpa Kristina E. Lipinska-Kalita, Patricia E. Kalita, Russell J. Hemley, Cedric L. Gobin A renewed interest in $\beta $-Ga$_{2}$O$_{3}$ has arisen since it has potential applications in optoelectronic devices. We performed \textit{in situ} synchrotron radiation x-ray diffraction studies in a diamond anvil cell on $\beta $-Ga$_{2}$O$_{3}$ on compression up to 70 GPa and on successive decompression. The pressure-evolution of x-ray diffraction patterns was consistent with a low-to-high density phase transition. A thermodynamically stable \textit{$\beta $}-Ga$_{2}$0$_{3}$ phase was converted into the \textit{$\alpha $}-Ga$_{2}$0$_{3}$ phase, which is unstable at ambient conditions. The effect of hydrostatic and non-hydrostatic compression conditions on the evolution of the phase transition was also investigated in compression and decompression cycles. This work is the first report of high-pressure investigations of Ga$_{2}$O$_{3}$ on compression up to 70 GPa. [Preview Abstract] |
Wednesday, March 15, 2006 3:42PM - 3:54PM |
R42.00005: X-ray diffraction study of elemental thulium to 86 GPa Michael Pravica, Edward Romano, Zachary Quine, Walter Pravica We have studied the structures and equation of state of elemental thulium up to 86 GPa in a diamond anvil cell using angular-dispersive x-ray powder diffraction methods at the Advanced Photon Source. This is part of a study of phase transitions in the lanthanide-series metals using cyclohexane as a quasi-hydrostatic medium. We present evidence of a series of phase transitions that appear to follow the anticipated hcp $\to $Sm-type $\to $ dhcp $\to $ distorted fcc sequence of transitions and show the equation of state derived from the x-ray fit data. [Preview Abstract] |
Wednesday, March 15, 2006 3:54PM - 4:06PM |
R42.00006: Fermi surface nesting and pre-martensitic softening in V-Nb system under high pressure Alexander Landa, John Klepeis, Per Soderlind, Babak Sadigh, Ivan Naumov, Oleg Velikokhatnyi, Levente Vitos, Andrei Ruban First-principles total energy calculation based on the exact muffin-tin orbital and full potential linear muffin-tin orbital methods were used to calculate the equation of state and shear elastic constants of bcc V, Nb, and the V$_{95}$Nb$_{05}$ disordered alloy as a function of pressure up to 6 Mbar. We found a mechanical instability in C$_{44}$ and a corresponding softening in C$^{'}$ at pressures $\sim $ 2 Mbar for V. Both shear elastic constants show softening at pressures $\sim $ 0.5 Mbar for Nb. Substitution of 5 at. {\%} of V with Nb removes the instability of V with respect to trigonal distortions in the vicinity of 2 Mbar pressure, but still leaves the softening of C$_{44}$ in this pressure region. We argue that the pressure induced softening in the shear elastic constants of V and Nb can be attributed simultaneously to three different electronic structure peculiarities, namely to the Fermi Surface nesting, electronic topological transition, and the band Jahn-Teller effect. [Preview Abstract] |
Wednesday, March 15, 2006 4:06PM - 4:18PM |
R42.00007: Pressure-Induced Antifluorite-to-Anticotunnite Phase Transition in Lithium Oxide Amy Lazicki, Choong-Shik Yoo, William Evans, Warren Pickett, Richard Scalettar Using synchrotron angle-dispersive x-ray diffraction (ADXD) and Raman spectroscopy on samples of Li$_2$O pressurized in a diamond anvil cell, we observed a reversible phase change from the cubic antifluorite ($\alpha$, Fm3m) to orthorhombic anticotunnite ($\beta$, Pnma) phase at 50($\pm$5) GPa at ambient temperature. This transition is accompanied by a moderate volume collapse of 5.4 ($\pm$0.8)\% and large hysteresis upon pressure reversal (P$_{down}$ at $\sim$25 GPa). Contrary to a recent study, our data suggest that the high-pressure $\beta$-phase (B$_o$ = 188 $\pm$12 GPa) is substantially stiffer than the low-pressure $\alpha$-phase (B$_o$ = 90$\pm$1 GPa). A relatively strong and pressure-dependent preferred orientation in $\beta$-Li$_2$O, resulting in changes diffraction intensities, is observed. The present result is in accordance with the systematic behavior of antifluorite-to-anticotunnite phase transitions occurring in the alkali-metal sulfides. This work has been supported by LLNL, University of California, under the auspices of the U.S. DOE under Contract No. W-7405- ENG-48 and by the Stockpile Stewardship Academic Alliances Program under grant DOE DE-FG03-03NA00071, and by the NSF(ITR 031339) at UCD. [Preview Abstract] |
Wednesday, March 15, 2006 4:18PM - 4:30PM |
R42.00008: Novel Features in the Mixed-Valence State of TmTe at High Pressure: A Resonant Inelastic X-ray Scattering Study Ignace Jarrige, Yong Cai, Hirofumi Ishii, Nozomu Hiraoka, Chien-Te Chen, Jean-Pascal Rueff, Sean Shieh, Ching-Pao Wang, Takeshi Matsumura Using resonant inelastic x-ray scattering, the mixed-valence state of TmTe under pressure is accurately determined up to 10.6 GPa. A remarkable interplay between electronic, magnetic and structural properties in TmTe under pressure is inferred throughout the considered pressure range. A nearly linear correlation is found between the pressure dependences of the valence and the volume in the metallic regime of TmTe. In the framework of the RKKY theory, the pressure dependence of the valence is found to explain both the steady decrease of the ferromagnetic state with increasing pressure in the 2-6 GPa range and the formation of the antiferromagnetic state above 6 GPa. [Preview Abstract] |
Wednesday, March 15, 2006 4:30PM - 4:42PM |
R42.00009: Elastic Stability Analysis of the Structural Response of Cubic Crystals to Hydrostatic Loading Hadrian Djohari, Frederick Milstein, Dimitrios Maroudas We report results of systematic elastic stability analyses in metallic crystals based on isobaric molecular-dynamics simulations that capture the mechanical, geometric, and kinetic aspects of instabilities induced under hydrostatic loading. Our analysis emphasizes bifurcations in the crystal structural response as the applied load is varied and atomic pattern formation characteristics beyond the instability onset. Results are presented for various cases of structural evolution under hydrostatic loading for model crystals that have a cubic lattice structure at equilibrium, including crystals with structural heterogeneities such as internal nanovoids and free surfaces. The corresponding structural responses range from inhomogeneous structural transitions to fracture through decohesion and voiding. The observed instabilities are thermally activated and associated with vanishing or diminishing combinations of elastic moduli. [Preview Abstract] |
Wednesday, March 15, 2006 4:42PM - 5:18PM |
R42.00010: Dynamic Strength of Metals at High Pressure and Strain Rate Invited Speaker: A new approach to materials science at very high pressures and strain rates has been developed on the Omega laser, using a ramped plasma piston drive. A laser drives an ablative shock through a solid plastic reservoir where it unloads at the rear free surface, expands across a vacuum gap, and stagnates on the metal sample under study. This produces a gently increasing ram pressure, compressing the sample nearly isentropically. The peak pressure on the sample, diagnosed with VISAR measurements, can be varied by adjusting the laser energy and pulse length, gap size, and reservoir density, and obeys a simple scaling relation.$^{1}$ This has been demonstrated at OMEGA at pressures to 200 GPa in Al foils. In an important application, using in-flight x-ray radiography, the material strength of solid-state samples at high pressure can be inferred by measuring the reductions in the growth rates (stabilization) of Rayleigh-Taylor (RT) unstable interfaces. RT instability measurements of solid of Al-6061-T6 $^{2}$ and vanadium, at pressures of 20-100 GPa, and strain rates of 10$^{6}$ to 10$^{8}$ s$^{-1}$, show clear material strength effects. Modelling results for two constitutive strength models -- Steinberg-Guinan and Preston-Tonks-Wallace, show enhanced dynamic strength that may be correlated with a high-strain-rate, phono-drag mechanism. Data, modeling details and future prospects for this project using the National Ignition Facility laser, will be presented. [1] J. Edwards et al., Phys. Rev. Lett., \textbf{92}, 075002 (2004). [2] K. T. Lorenz et al., Phys. Plasmas \textbf{12}, 056309 (2005). 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 W-7405-Eng-48. [Preview Abstract] |
Wednesday, March 15, 2006 5:18PM - 5:30PM |
R42.00011: Atomistic simulations of quasi-static and shockwave loading of HMX crystals Thomas Sewell, Eugenio Jaramillo, Alejandro Strachan We have recently undertaken non-equilibrium molecular dynamics studies to aid our understanding of dynamical processes in the high explosive HMX, in particular the inelastic, anisotropic response of crystalline HMX subjected to quasi-static and shock loading. The same force field used by Sewell and co-workers in preceding simulations of HMX equilibrium properties is employed for the present research. The overarching goal of this work is to provide information that can serve as a foundation in basic science for the formulation of improved mesoscale constitutive models for the constituent materials in selected energetic formulations. The medium-term scientific challenge that stands as a prerequisite to this larger objective is to carefully identify, characterize, and quantify the dominant mechanisms of localization and dissipation in such materials, under a variety of prescribed quasi-static and dynamic loading scenarios. The focus of the present talk will be the shock response of structurally perfect, but properly thermalized, $\alpha $-HMX crystal shocked in the (100) direction, with specific discussion of results for loading below the hugoniot elastic limit, in the two-wave elastic-plastic region, and for overdriven shocks. [Preview Abstract] |
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