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 K6: Phase Transitions III |
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Chair: Kai Kadau, Los Alamos National Laboratory Room: Hyatt Regency Chesapeake A/B |
Tuesday, August 2, 2005 1:30PM - 1:45PM |
K6.00001: Direct observation of the alpha-epsilon phase transformation in shocked single crystal iron Daniel H. Kalantar, G. Collins, J.D. Colvin, J.H. Eggert, H.E. Lorenzana, J.S. Stolken, J. Hawreliak, K. Rosolankova, J.S. Wark, M. Schneider, M.A. Meyers The technique of in-situ wide angle diffraction provides direct access to study the dynamic deformation of a material during the shock process. This technique has been used to study the shock response of single crystal Fe on a ns time-scale. Single crystals of Fe (001) were shock compressed by direct laser irradiation using the OMEGA, Janus, and Vulcan lasers. The lattice was probed by in-situ diffraction during shock loading at pressures spanning the alpha-epsilon phase transformation at 13 GPa. Simultaneous diffraction from multiple lattice planes provides the first direct evidence that the Fe transforms to an hcp structure on the nanosecond time-scale. The Fe was observed to respond with a uniaxial compression to 6{\%}, with an additional collapse by 15-18{\%} and transformation to the hcp phase. Details on the technique, diffraction wave profile measurements, and methods of analysis will be discussed. [Preview Abstract] |
Tuesday, August 2, 2005 1:45PM - 2:00PM |
K6.00002: Scaling of Pressure with Intensity in Laser-driven Shocks and Effects of Hot X-ray Preheat Jeffrey Colvin, Daniel Kalantar To drive shocks into solids with a laser we either illuminate the material directly, or to get higher pressures, illuminate a plastic ablator that overlays the material of interest. In both cases the illumination intensity is low, $<<$10$^{13}$ W/cm$^{2}$, compared to that for traditional laser fusion targets, so the laser beam creates and interacts with a collisional, rather than a collisionless, plasma. We present scaling relationships for shock pressure with intensity for this low-intensity collisional plasma regime derived from simulations. In addition, sometimes the plastic-ablator targets have a thin flash-coating of Al on the plastic surface as a shine-through barrier; this Al layer can be a source of hot x-ray preheat. We discuss how the preheat affects the shock pressure, with particular application to simulating Visar measurements from a set of experiments conducted on the Omega laser on shock compression of Fe. [Preview Abstract] |
Tuesday, August 2, 2005 2:00PM - 2:30PM |
K6.00003: Picosecond X-ray Diffraction from Laser-Shocked Copper and Iron Invited Speaker: In situ X-ray diffraction allows the determination of the structure of transient states of matter. We have used laser-plasma generated X-rays to study how single crystals of metals (copper and iron) react to uniaxial shock compression. We find that copper, as a face-centred-cubic material, allows rapid generation and motion of dislocations, allowing close to hydrostatic conditions to be achieved on sub-nanosecond timescales. Detailed molecular dynamics calculations provide novel information about the process, and point towards methods whereby the dislocation density might be measured during the passage of the shock wave itself. We also report on recent experiments where we have obtained diffraction images from shock-compressed single-crystal iron. The single crystal sample transforms to the hcp phase above a critical pressure, below which it appears to be uniaxially compressed bcc, with no evidence of plasticity. Above the transition threshold, clear evidence for the hcp phase can be seen in the diffraction images, and via a mechanism that is also consistent with recent multi- million atom molecular dynamics simulations that use the Voter- Chen potential.\footnote[1] { 1 Kai Kadau, Timothy C. Germann, Peter S. Lomdahl, Brad Lee Holian, Science 296, 1681 (2002). } We believe these data to be of import, in that it is the first conclusive in situ evidence of the transformed structure of iron during the passage of a shock wave. [Preview Abstract] |
Tuesday, August 2, 2005 2:30PM - 2:45PM |
K6.00004: Dynamic Compression of Iron Single Crystals B.J. Jensen, W.W. Anderson, R.S. Hixson, P.A. Rigg, F.J. Cherne, III, G.T. Gray, III Iron undergoes a polymorphic phase transformation from alpha phase (bcc) to the epsilon phase (hcp) when compressed to stresses exceeding 130 kbar. Because the epsilon phase is denser than the alpha phase, a single shock wave is unstable and breaks up into an elastic wave, a plastic wave, and a phase transition wave known as the P2 wave. Although there exists a large body of continuum data related to the phase transitions of shocked polycrystalline iron, data for single crystal iron is lacking. Such data are required for a more complete understanding of the response of iron subjected to dynamic loading conditions. The objective of the current work was to examine wave profiles for iron single crystals, oriented along the [100], [110], and [111] directions, subjected to quasi-isentropic loading using the Sandia Z-machine. By comparing the experimentally obtained wave profiles with similar profiles for polycrystalline iron, the orientation dependence of the phase transition stress is determined. Results and implications will be presented. [Preview Abstract] |
Tuesday, August 2, 2005 2:45PM - 3:00PM |
K6.00005: A Critical Criterion for Martensitic Phase Transformation Considering both Hydrostatic Pressure and Deviatoric Stress Songlin Xu, Yangbo Guo, Zhiping Tang In this article, from the point of view of thermodynamic and energy, criterions for stress-induced and strain-induced phase transformation are established considering both hydrostatic pressure and the deviatoric stress. The critical surface for phase transformation predicted by the criterion is a conic surface in the principal stress space, demonstrating asymmetric property in tension and compression states. The conic critical surface of phase transition may intersect the cylindrical yield surface in the principal stress space, such intersection may induce two abnormal phenomena: (1) unloading phase transformation and (2) the transition between the stress-induced and the strain-induced phase transformations. These predictions need to be verified experimentally. Using the criterion established, the initial critical stresses of transformation for TiNi alloy and Fe-Ni alloy are predicted, the predictions are in good agreement with the experimental results, but the predicted volumetric strain for phase transformation is larger than the experimental result. [Preview Abstract] |
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