Bulletin of the American Physical Society
20th Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 62, Number 9
Sunday–Friday, July 9–14, 2017; St. Louis, Missouri
Session J7: Phase Transitions III |
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Chair: Brittany Branch, Los Alamos National Laboratory Room: Regency Ballroom F |
Tuesday, July 11, 2017 11:15AM - 11:30AM |
J7.00001: Metastability of the alpha and omega phases in shock-loaded titanium David Jones, Ben Morrow, Ellen Cerreta Under shock-loading titanium will go through a solid-solid phase transition from the alpha phase (HCP) to the omega phase (simple hexagonal). The onset of this transition is not well-defined, as there is a large scatter in the published data. We present a series of experiments where high purity titanium (99.999\%) was shock-loaded in gas-gun driven flyer plate impacts. Rear free-surface velocity measurements reveal the transition begins at a stress of 10.1~GPa when loaded to a Hugoniot state of 15~GPa. The aforementioned scatter in existing data around this stress and the slow transformation kinetics in the velocity data suggests that at this state both phases are metastable. Samples were loaded to the same conditions and soft recovered for post-mortem microscopy. With electron backscatter diffraction techniques we observed that the recovered samples contain approximately 65\% omega phase. Through texture analysis of both the alpha and omega regions we show that all remaining alpha is the original material, i.e. the sample never reached 100\% omega and there is no phase-reversion during release. Further experiments are also presented where this pre-shocked, omega-heavy material was shocked again, showing how this changes the free-surface velocity profiles and kinetics. [Preview Abstract] |
Tuesday, July 11, 2017 11:30AM - 11:45AM |
J7.00002: In-situ experiments at DCS to capture the alpha-omega phase transformation in titanium during dynamic loading Benjamin Morrow, David Jones, Paulo Rigg, Ellen Cerreta Under sufficient stresses, such as during dynamic loading, titanium experiences a phase transformation from hcp alpha phase to hexagonal omega phase. Omega phase is often retained in the microstructure after unloading, and has a strong influence on subsequent mechanical properties. Simulations suggest there are multiple pathways and underlying mechanisms for this transformation. Due to the incredibly short timescales involved, experimental measurements for model validation have been difficult. However, new capabilities at the Advanced Photon Source have enabled diffraction measurements during plate impact experiments to study the microstructural evolution of titanium during transformation. These high-rate data allow us to probe the mechanism and kinetics of phase transformations in new ways. Recent results from the Dynamic Compression Sector (DCS) on shock compression in Ti will be presented and compared to similar ex-situ dynamic tests. Advantages of the new technique and challenges associated with quantification of such phase data will also be discussed. [Preview Abstract] |
Tuesday, July 11, 2017 11:45AM - 12:00PM |
J7.00003: Phase transitions of titanium under dynamic loading. Alexey Kovalev, Mikhail Zhernokletov, Oleg Aprelkov, Alexey Podurets, Viktor Skokov, Dmitry Zamotaev Information on sound velocity, which characterizes substance behavior under conditions of shock compression followed by release, is required for formulation of substance equations of state. The kink in the dependence of sound velocity on pressure is associated with structural transitions including its melting in shock-compressed substance. Investigations of the ($\alpha $-$\omega )$ titanium phase transition revealed significant discrepancy in the measured values of the transition. Pressures of phase transition completion varied from $\approx $17.5 to 22~GPa under dynamic compression. Interaction of Ti with majority of elements gives opportunity to produce many alloys with various properties. In structure, which is formed when annealing, titanium alloy VT-20 is classified as a pseudo $\alpha $-alloy with its structure presented by the $\alpha $-phase and insignificant quantity of the $\beta $-phase. The authors present results of sound velocity measurement in shock-compressed samples of VT1-0 titanium and VT-20. In titanium, kinks were recorded at the dependence of sound velocity on pressure at the pressures of 20-40 and 60-90~GPa. These kinks can be explained by phase transitions. X-ray structural analysis revealed presence of the $\omega $-phase in the samples, which had been recovered after loading by pressures in steel ampoules in the range from 9 to 23~GPa. Beginning of VT-20 alloy melting relates to pressure of 130 GPa at shock adiabat. [Preview Abstract] |
Tuesday, July 11, 2017 12:00PM - 12:15PM |
J7.00004: Phase Transitions in Dynamically Compressed Bi Martin Gorman, Richard Briggs, Amy Coleman, Stewart McWilliams, Emma McBride, David McGonegle, Justin Wark, Cindy Bolme, Arianna Gleason, Gilbert Collins, Jon Eggert, Dayne Fratanduono, Ray Smith, Eric Galtier, Hae Ja Lee, Eduardo Grandos, Bob Nagler, Zhou Xing, Malcolm McMahon, N/A N/A The ability to characterise atomic structure at extreme conditions and on the timescale of laser-driven shock experiments is vital for our understanding of how materials behave under rapid pressure loading. A key finding in recent static high-pressure studies has been that many materials adopt complex crystal structures at high-pressure such as incommensurate host-guest structures. However, it is uncertain whether such complex structures are able to form on the timescales of laser shock experiments due to the highly reconstructive nature of the phase transformation mechanisms, leading to the possibility of non-equilibrium phases forming. We present X-ray diffraction measurements that characterise the structure of several solid phases of Bi including one new phase, which is not reported in the equilibrium phase diagram. Diffraction measurements on molten Bi will also be presented and the prospect of extracting quantitative density information from the liquid diffraction data will be discussed. [Preview Abstract] |
Tuesday, July 11, 2017 12:15PM - 12:30PM |
J7.00005: Shock wave experiments on gallium Brian Jensen, Brittany Branch, Frank Cherne Gallium exhibits a complex phase diagram with multiple solid phases, an anomalous melt boundary, and a low-temperature melt transition making it a suitable material for shock wave studies focused on multiphase properties including kinetics and strength. Apart from high-pressure shock wave data that exists for the liquid phase, there is a clear lack of data in the low-pressure regime where much of the complexity in the phase diagram exists. In this work, a series of shock wave experiments were performed to begin examining the low-pressure region of the phase diagram. Additional data on a gallium alloy, which remains liquid at room temperature, will be presented and compared to data available for pure gallium (LA-UR-17-21449). [Preview Abstract] |
Tuesday, July 11, 2017 12:30PM - 12:45PM |
J7.00006: Temperature and pressure determination of the tin melt boundary from a combination of pyrometry, spectral reflectance, and velocity measurements along release paths Brandon La Lone, Paul Asimow, Oleg Fatyanov, Robert Hixson, Gerald Stevens Plate impact experiments were conducted on tin samples backed by LiF windows to determine the tin melt curve. Thin copper flyers were used so that a release wave followed the 30-40 GPa shock wave in the tin. The release wave at the tin-LiF interface was about 300 ns long. Two sets of experiments were conducted. In one set, spectral emissivity was measured at six wavelengths using a flashlamp illuminated integrating sphere. In the other set, thermal radiance was measured at two wavelengths. The emissivity and thermal radiance measurements were combined to obtain temperature histories of the tin-LiF interface during the release. PDV was used to obtain stress histories. All measurements were combined to obtain temperature vs. stress release paths. A kink or steepening in the release paths indicate where the releases merge onto the melt boundary, and release paths originating from different shock stresses overlap on the melt boundary. Our temperature-stress release path measurements provide a continuous segment of the tin melt boundary that is in good agreement with some of the published melt curves. [Preview Abstract] |
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