Session Z2: PS-3: Phase Transitions

Structural Phase Stability in Group IV Metals Under Static High Pressure

In group IV metals (Ti, Zr, and Hf) room temperature compression leads to a martensitic transformation from a ductile $\alpha $ to a brittle $\omega $ phase. $\alpha -\omega $ phase boundary decreases to lower pressure at high temperature and can limit the use of group IV metals in industrial applications. There is a large discrepancy in the transition pressure reported in literature, with some of the variation attributed to experimental conditions (i.e. hydrostatic vs. non-hydrostatic). Shear deformation in non-hydrostatic experiments drives $\alpha \to \omega $ transition and decreases transition pressure. Impurities can also aid or suppress $\alpha \to \omega $ transition. By performing x-ray diffraction experiments on samples in a diamond anvil cell we show that interstitial impurities, such as C, N, and O can obstruct $\alpha \to \omega $ transition and stabilize $\alpha $ phase to higher pressure. We also show that reduction in grain size can also influence $\alpha -\omega $ phase boundary and help stabilize $\alpha $ phase to higher pressure under non-hydrostatic conditions.   

In-situ probing of lattice response in shock compressed polycrystalline iron through the $\alpha$-$\epsilon$ transition using single shot x-ray diffraction

Diagnostics which probe the lattice response during shock compression offer insight into many fundamental physical phenomena which govern material response. While diffraction from shocked loaded single crystals has been demonstrated for several years the recent development of in-situ polycrystalline diffraction techniques offers similar levels of insight into materials which are more complicated by nature of having grains of multiple orientations, but are more representative of commonly used materials. We will exam both single and polycrystalline iron shock loaded through the $\alpha$-$\epsilon$ transition. This work was conducted under the auspices of the U.S. DOE by the UC LLNL and LANL under Contract No. W-7405-Eng-48. Additional support was provided by LDRD program Project No. 06-SI-004 at LLNL.   

Exotic Behavior of Materials at Ultra-High Densities

New compression and diagnostic techniques reveal that matter at ultra-high densities can exhibit quite exotic behavior. For example, shock compressed He, precompressed in a diamond anvil cell, shows He transforms to a metal at $\sim $2.5 g/cc. Single and multiple shock compression data for diamond shows the melting temperature is constant from 0.6 TPa to 2 TPa. Ramp wave compression now allows us to explore solid-state properties into the TPa regime and reveals that at low temperatures the diamond phase of carbon is stable and strong to 0.8 TPa. By varying the ramp compression time we can explore the kinetics of phase transitions and strength. For example, above a material dependent critical strain rate for Bi or Fe, the pressure required to cross a given phase boundary increases logarithmically with strain rate. Over the next few years, these techniques will allow us to explore the nature of solids to several TPa, complex chemistry to 100 TPa (1 Gbar), and the nature of helium and hydrogen at interatomic spacings comparable to their DeBroglie wavelength.\\[4pt] In collaboration with J. Eggert, R. Smith, D. Hicks, P. Celliers, M. Bastea, D. Bradley, R. McWilliams, R. Rygg, and D. Braun, Lawrence Livermore National Laboratory; P. Loubeyre and S. Brygoo, Commissariat \`a l'EnergieAtomique, 91680, France; D. Spalding and R. Jeanloz, University of California, Berkeley; and T. Boehly, Laboratory for Laser Energetics, University of Rochester, Rochester NY.   

Detailed dynamic behavior and phase transition of CaF$_{2}$ under shock loading

Dynamic behavior and phase transition of CaF$_{2}$ under shock loading are not well understood in spite of its important application as optical material. There are several static compression data available and the data indicate a high-pressure transition at a pressure of 8-9 GPa. Our recent spectroscopic study on Eu-doped CaF$_{2}$ single crystals suggests a high-pressure phase transition around 15 GPa, based on the luminescence wavelength shift as a function of pressure. In order to confirm it, we carried out the wave profile measurements on single crystals CaF$_{2}$ with two different crystal directions. We determined the correction factor for VISAR window. The obtained wave profiles show clear and distinct HEL behaviors and a phase transition through the observed two-wave and three-wave structures. The two crystal directions displayed variations which will related to the kinetics. The high-pressure phase transition mechanism will be discussed by the obtained wave profile.   

Shock-induced hcp to bcc phase transition in polycrystalline Mg

We report on recent dynamic x-ray diffraction measurements of shocked polycrystalline Mg between 10-50GPa. The experiment was performed at the JANUS laser facility at LLNL utilizing its two high-energy beam capability for nanosecond x-ray production and laser-driven shocks. Samples of polycrystalline rolled Mg foil, 50um thick, were probed by 4.7keV, 3ns x-ray pulses while shocked by 532nm, 20-200J, 6ns laser pulses, over 1mm$^{2}$. A cylindrical x-ray pinhole camera was fielded to measure lattice structure simultaneously with line-imaging velocimetry of the free surface. We present diffraction data suggesting the onset of the hcp to bcc phase transition at 28+/-2GPa on the principal shock Hugoniot, in agreement with the computed phase boundary for Mg. Observation of shocked diffraction lines solely in the bcc phase above 30GPa indicates a subnanosecond phase transition timescale. In addition, the absence of a double shock wave structure in the velocimetry data is attributed to the small volume change expected in the hcp to bcc phase transition in Mg.   

Measurement of sound velocities in shock-compressed tin under pressures up to 150~GPa

Tin has a complex phase diagram, which can be explained by presence of structural phase transitions. The fracture in the dependence of sound velocity on pressure is caused by structural transitions in shock-compressed substance. Therefore, basing on measurement of sound velocities, it is possible to reveal phase transitions of substance along shock adiabat, including its melting. The results of different authors give the melting range of tin from 35 up to 93~GPa. In this work tin samples with initial density of 7.28~g/cm$^{3}$ were loaded with use of HE-based generators of shock waves. In the pressure range of 30-150~GPa, sound velocity in tin was measured by the method of overtaking release with use of the optical gauges and the indicator liquids: carbogal, tetrachlormethane, and bromoform. Up to shock compression pressures of about 35~GPa, sound velocity was measured by the method of oncoming release with use of piezoresistive manganin-based gauges. The obtained data testifies that the melting range of tin is $\sim $63$\div $90~GPa.   

Phase Transitions in Simulation of Hypervelocity Impact Experiments

Hypervelocity impact experiments can give us additional information about thermodynamical properties of matter in extreme state. In this work we simulate shock--induced melting, fragmentation and vaporization in aluminum and zinc targets. A tantalum impactor strikes zinc and aluminum targets at a velocity of 10 km/s and causes melting of these materials in a shock wave. Then under intensive rarefaction the thermodynamic path crosses the liquid--vapor coexistence boundary and enters into a metastable liquid state. Liquid in a metastable state undergoes either liquid--vapor phase separation or mechanical spallation. The theory of homogeneous nucleation as well as mechanical fragmentation criterion are used to control the kinetics of these processes in our model. The first effect dominates in the vicinity of the critical point, the second one at lower temperatures and negative pressure. Phase transitions and kinetics of phase separations are taken into account using a thermodynamically complete equation of state in tabular form with stable and metastable states for all materials under consideration. It is shown that liquid--vapor properties are very important for adequate description of experiment.