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The phase transition of iron under shock compression has attracted much attention in recent decades because of its importance in fields such as condensed matter physics, geophysics, and metallurgy. At room temperature, the transition of iron from the α-phase (bcc) to the ε-phase (hpc) occurs at a stress of 13 GPa. At high temperature, a triple point followed by transformation to the γ-phase (fcc) is expected. However, the details of the high-temperature phase transitions of iron are still under debate. Here, we investigate the phase-transition behavior of polycrystalline iron under compression from room temperature to 820 K. The results show that the shock-induced phase transition is determined unequivocally from the measured three-wave-structure profiles, which clearly consist of an elastic wave, a plastic wave, and a phase-transition wave. The phase transition is temperature-dependent, with an average rate Δσtr/ΔT of -6.91 MPa/K below 700 K and -34.7 MPa/K at higher temperatures. The shock α-ε and α-γ phase boundaries intersect at 10.6 ± 0.53 GPa and 763 K, which agrees with the α-ε-γ triple point from early shock wave experiments and recent laser-heated diamond-anvil cell resistivity and in situ X-ray diffraction data but disagrees with the shock pressure-temperature phase diagram reported in 2009 by Zaretsky. [ABSTRACT FROM AUTHOR]

The phase transition of iron under shock compression has attracted much attention in recent decades because of its importance in fields such as condensed matter physics, geophysics, and metallurgy. At room temperature, the transition of iron from the α-phase (bcc) to the ε-phase (hpc) occurs at a stress of 13 GPa. At high temperature, a triple point followed by transformation to the γ-phase (fcc) is expected. However, the details of the high-temperature phase transitions of iron are still under debate. Here, we investigate the phase-transition behavior of polycrystalline iron under compression from room temperature to 820 K. The results show that the shock-induced phase transition is determined unequivocally from the measured three-wave-structure profiles, which clearly consist of an elastic wave, a plastic wave, and a phase-transition wave. The phase transition is temperature-dependent, with an average rate Δσtr/ΔT of −6.91 MPa/K below 700 K and −34.7 MPa/K at higher temperatures. The shock α-ε and α-γ phase boundaries intersect at 10.6 ± 0.53 GPa and 763 K, which agrees with the α-ε-γ triple point from early shock wave experiments and recent laser-heated diamond-anvil cell resistivity and in situ X-ray diffraction data but disagrees with the shock pressure–temperature phase diagram reported in 2009 by Zaretsky [J. Appl. Phys. 106, 023510 (2009)]. [ABSTRACT FROM AUTHOR]

The pressure dependence of the yield strength of tantalum was investigated experimentally up to 101 GPa at room temperature using a diamond anvil cell. A yield strength softening is observed between 52 and 84 GPa, whereas a normal trend is observed below 52 GPa and above 84 GPa. The onset pressure of the softening is in agreement with previous results obtained by the pressure gradient method and shock wave experiments. This unusual strength softening in tantalum is not related with structural transformation, preferred orientation, or material damage. Our measurements indicate that microscopic deviatoric strain is the major reason for the observed strength softening in tantalum. [ABSTRACT FROM AUTHOR]

A thermodynamic equation of state (EOS) for thermoelectrons is derived which is appropriate for investigating the thermodynamic variations along isobaric paths. By using this EOS and the WuJing (WJ) model, [Q. Wu and F. Jing J. Appl. Phys. 80, 4343 (1996)] an extended Hugoniot EOS model is developed which can predict the compression behavior of highly porous materials. Theoretical relationships for the shock temperature, bulk sound velocity, and the isentrope are developed. This method has the advantage of being able to model the behavior of porous metals over the full range of applicability of pressure and porosity, whereas methods proposed in the past have been limited in their applicability. [ABSTRACT FROM AUTHOR]

In order to verify and validate the newly developed thermoelectron equation of state (EOS) model that is based on the Wu-Jing EOS [J. Appl. Phys. 80, 4343 (1996); Appl. Phys. Lett. 67, 49 (1995)], calculations of shock compression behavior have been made on five different porous metals: iron, copper, lead, tungsten, and aluminum which are commonly used as standards. The model was used to calculate the Hugoniot, shock temperature, sound velocity, and unloading isentrope for these materials and comparisons were made to previous calculations and available data. Based on these comparisons, it is felt that the model provides information in good agreement with the corresponding experimental and theoretical data published previously. This suggests that the new model can satisfactorily describe the properties of shocked porous materials over a wide range of pressure and porosity. [ABSTRACT FROM AUTHOR]

The pressure-volume-temperature (P-V-T) equation of state (EOS) of MgO is widely used as a pressure scale in static compression experiments. However, there are remarkable inconsistencies among different previously proposed MgO pressure scales. We calculated the P-V-T EOS of MgO up to 300 GPa and 3000 K based on experimental shock Hugoniot data and a simple thermal pressure model within the Mie-Grüneisen-type analysis framework. All of the parameters used can be measured experimentally with high accuracies. We found that, in overall, the calculated P-V-T EOS of MgO has excellent agreement with the available volume compression data over a wide range of pressure and temperature. A comparison of our results with the previous theoretical investigations has also been performed and confirms that our calculated P-V-T EOS of MgO can be used as a reliable pressure scale for static experiments at high pressures and high temperatures. [ABSTRACT FROM AUTHOR]

A novel explanation of the quasi-elastic release phenomenon in shock-compressed aluminum is presented. A dislocation-based model, taking into account dislocation substructures and evolution, is applied to simulate the elastic-plastic response of both single-crystal and polycrystalline aluminum. The calculated results are in good agreement with experimental results from not only the velocity profiles but also the shear strength and dislocation density, which demonstrate the accuracy of our simulations. Simulated results indicate that dislocation immobilization during dynamic deformation results in a smooth increase in the yield stress, which leads to the quasielastic release, while the generation of dislocations caused by the plastic release wave results in the appearance of a transition point between the quasi-elastic release and the plastic release in the profile. [ABSTRACT FROM AUTHOR]

Strong oxides at high shock pressures have broad crossovers from elastic solids at ambient to failure by plastic deformation, to heterogeneous deformation to weak solids, to fluid-like solids that equilibrate thermally in a few ns, to melting and, at sufficiently high shock pressures and temperatures, to metallic fluid oxides. This sequence of crossovers in single-crystal cubic Gd3Ga5O12 (Gd-Ga Garnet-GGG) has been diagnosed by fast emission spectroscopy using a 16-channel optical pyrometer in the spectral range 400-800 nm with bandwidths per channel of 10 nm, a writing time of ~1000 ns and time resolution of 3 ns. Spectra were measured at shock pressures from 40 to 290 GPa (100 GPa=1 Mbar) with corresponding gray-body temperatures from 3000 to 8000 K. Experimental lifetimes were a few 100 ns. Below 130 GPa, emission is heterogeneous and measured temperatures are indicative of melting temperatures in grain boundary regions rather than bulk temperatures. At 130 GPa and 2200 K, GGG equilibrates thermally and homogeneously in a thin opaque shock front. This crossover has a characteristic spectral signature in going from partially transmitting shock-heated material behind the shock front to an opaque shock front. Opacity is caused by optical scattering and absorption of light generated by fast compression. GGG melts at ~5000K in a two-phase region at shock pressures in the range 200 GPa to 217 GPa. Hugoniot equation-of-state data were measured by a Doppler Pin SystemDPS with ps time resolution and are generally consistent with previous data. Extrapolation of previous electrical conductivity measurements indicates that GGG becomes a poor metal at a shock pressure above ~400 GPa. Because the shock impedance of GGG is higher than that of Al2O3 used previously to make metallic fluid H (MFH), the use of GGG to make MFH will achieve higher pressures and lower temperatures than use of Al2O3. However, maximum dynamic pressures at which emission temperatures of fluid hydrogen made by shock reverberation between GGG anvils could be measured remains limited to ~130 GPa, as for Al2O3 anvils. [ABSTRACT FROM AUTHOR]