Bulletin of the American Physical Society
17th Biennial International Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 56, Number 6
Sunday–Friday, June 26–July 1 2011; Chicago, Illinois
Session V2: High Pressure Strength V |
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Chair: Scott Alexander, Sandia National Laboratories Room: Grand Ballroom IV-V |
Thursday, June 30, 2011 4:00PM - 4:15PM |
V2.00001: Shock Response of Body Centred Cubic Metals Jeremy Millett, Glenn Whiteman, Neil Bourne, Nigel Park, Matthew Cotton Over the past few years, a research programme has been in place to examine the shock response of body centred cubic metals such as tantalum and tungsten. Examination of the development of shear strength behind the shock front has shown common behaviour in that a marked decrease has been noted, both in the pure metals and their simple alloys. This has been ascribed to the low generation of new dislocation line length due to the high Peierls stresses found in these metals. However more recent work in niobium and molybdenum has shown a more constant response in shear strength due to either a much lower Peierls stress (niobium) or the possibility of twin formation (molybdenum). We now extend this work to investigate the role of initial dislocation density in tantalum, and broaden the mechanical study to include the spallation response in these metals. [Preview Abstract] |
Thursday, June 30, 2011 4:15PM - 4:30PM |
V2.00002: Experimental results of Ta material strength and grain size effect at high pressure and high strain rate Hye-Sook Park, N. Barton, R. Cavallo, B. Maddox, M. May, S. Pollaine, S. Prisbrey, B. Remington, R. Rudd We are studying material strength under high pressures ($>$1 Mbar) and high strain rates (10$^{6}$ - 10$^{8}$ sec$^{-1})$ in Ta using Omega lasers. The Ta sample is maintained in the solid state throughout a quasi-isentropic ramped drive using a reservoir-gap-sample configuration. The strength is inferred from the growth measurements of the pre-imposed sinusoidal ripples on the sample via Rayleigh-Taylor (RT) instability properties. The material strength can greatly suppress RT growth rate via an effective lattice viscosity [1]. Our recent experiments include the study of any grain size dependence of strength under these high pressures and strain rates. The conventional Hall-Petch effect predicts that smaller grain sizes correspond to stronger materials. There are neither existing experimental data nor theoretical predictions of the expected Hall-Petch effect under the extreme conditions of our RT experiments. Three different samples of 0.25 $\mu $m, 15 $\mu $m and 90 $\mu $m average grain sizes are fabricated and their corresponding RT-induced ripple growth factors are measured. The details of the measurements, target characteristics, analysis, and final results will be presented. Designs that extend this experiment by an order of magnitude in pressure on NIF will also be shown [1] H. S. Park et al., PRL. 104, 135504 (2010). [Preview Abstract] |
Thursday, June 30, 2011 4:30PM - 4:45PM |
V2.00003: High pressure, high strain rate material strength experiments in Ta and V using the Rayleigh-Taylor instability Bruce A. Remington Constitutive models for material strength are currently being tested at 1 Mbar pressures by comparing 2D simulations with experiments measuring the Rayleigh-Taylor (RT) instability evolution in solid state samples of vanadium (V) and tantalum (Ta) [1,2]. The multiscale strength models being tested combine molecular dynamics, dislocation dynamics, and continuum simulations. Our analysis for the V experiments suggests that the material deformation at these conditions falls into the phonon drag regime, whereas for Ta, the deformation is due to a mix of phonon drag and thermal activation. Using the Ta multiscale model, we decompose the strength as a function of strain rate into its components of thermal activation, phonon drag, and work hardening, and show where each mechanism is predicted to dominate. This predicted decomposition becomes particularly interesting at the 5 Mbar conditions that correspond to the first NIF experiments, that will commence in the near future. [1] H.S. Park et al., PRL [3] 104, 135504 (2010); PoP 17, 056314 (2010). [2] N.R. Barton et al., JAP, in press (2011). [Preview Abstract] |
Thursday, June 30, 2011 4:45PM - 5:00PM |
V2.00004: Rayleigh-Taylor Strength Experiments of the Pressure-Induced $\alpha \rightarrow \epsilon$ Phase Transition in Iron Jonathan Belof, Robert Cavallo, Russell Olson, Peter Vitello, Dana Rowley HE-driven experiments to shocklessly cross the pressure-induced martensitic $\alpha$ (bcc) to $\epsilon$ (hcp) phase boundary in iron have been designed and preliminary data will be presented. The quasi-isentropic drive conditions result in peak pressures of 120-250 kbar and strain rates on the order of $10^6$ sec$^{-1}$. The target samples under study have been fabricated containing a single-mode perturbation such that the resulting Rayleigh-Taylor growth may be measured using the 800 MeV proton radiography facility at LANL. Simultaneous Photon Doppler Velocimetry can provide insight into the EP/P1/P2/PIR waves and allows for validation of the high-explosive drive conditions. Having designed four distinct assembly geometries, the goal of the experiment is to measure the dynamic strength of the bcc/hcp phases accurately and to provide experimental data that will allow further development of material strength models for this classical system. With both RT growth factors and target velocimetry, we may infer the effects of material strength for not only $\alpha \rightarrow \epsilon$ iron, but also reverted $\epsilon \rightarrow \alpha$ iron containing the dislocations generated and stored from the $\epsilon$ phase. [Preview Abstract] |
Thursday, June 30, 2011 5:00PM - 5:15PM |
V2.00005: Shock-induced phase transitions and their effects on the dynamic strength in tin and tantalum Jianbo Hu, Chengda Dai, Hua Tan, Yuying Yu, Xianming Zhou, Lingcang Cai, Qiang Wu We report our observations of shock-induced structural transformations in tin and tantalum based on precise sound velocity measurements. The measured sound velocity against shock pressure showed that there exist discontinuities in both tin and tantalum. Two observed discontinuities in the sound velocity of shocked tin are assigned to the bct-bcc transition and shock melting, respectively, which are confirmed by diamond anvil cell experiments after taking the temperature effect into account. The discontinuity of shocked tantalum, however, has no counterpart in diamond anvil cell experiments. We infer that tantalum under shock loading will undergo a shear stressinduced reversible Martensitic transition, according to the reported results of the shock recovery experiments. This inference may be helpful to explain the long-standing debate on the high-pressure melting curve of tantalum. For both metals, the deduced strength properties demonstrated significant changes around transition pressures, although the associated volume changes are slight, or even negligible. Therefore, it is easy to conclude that the strength of materials strongly depends on its high-pressure crystal structure. [Preview Abstract] |
Thursday, June 30, 2011 5:15PM - 5:30PM |
V2.00006: Simulation and theory of high-rate plastic deformation of polycrystalline bcc metals Robert Rudd High-rate plastic deformation is the subject of increasing experimental activity. High power laser platforms such as the NIF offer the possibility to study plasticity at extremely high rates in shock waves and shockless ramp waves. Here we report on the results of molecular dynamics (MD) simulations of these processes at the atomistic level and related analytic theory. These theories are compared with experiment, the MD simulations and other plasticity models. In the MD simulations we focus on the high-rate deformation of multi-million atom (nanoscale) polycrystalline tantalum and vanadium systems at pressures up to a few Mbar. The simulations span several orders of magnitude in strain rate, allowing us to analyze the rate dependence and the effect of the rate on the mechanisms of plasticity. We also compare the simulations to single crystal simulations. The vanadium simulations explore the pressure range of a recently reported rhombohedral phase. Rhombohedral variant formation contributes to the plastic flow, and we show the interaction of variant nucleation and growth with dislocation flow. We also report an analytic theory of the rate of plastic relaxation associate with dislocation flow and compare it to the MD simulations. [Preview Abstract] |
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