Chengda Dai

Hugoniot temperatures and melting of tantalum under shock compression determined by optical pyrometry

LiF single crystal was used as transparent window (anvil) to tamp the shock-induced free surface expansion of Ta specimen, and the Ta/LiF interface temperature was measured under shock compression using optical pyrometry technique. The shock temperatures and/or melting temperatures of Ta up to ∼400 GPa were extracted from the observed interface temperatures based on the Tan–Ahrens’ model for one-dimensional heat conduction across metal/window ideal interface in which initial melting and subsequent solidification were considered under shock loading. The obtained data within the experimental uncertainties are consistent with the results from high-pressure sound velocity measurements. The temperature of the partial melting on Ta Hugoniot is estimated to be ∼9700 K at 300 GPa, supported by available results from theoretical calculations.

Model for assessing the melting on Hugoniots of metals: Al, Pb, Cu, Mo, Fe, and U

A shock-release model is proposed to calculate shock temperatures along the principal Hugoniot, and to relate initial shock (Hugoniot) to release melting (off-Hugoniot). Thus a melting point crossing the Hugoniot can be defined. For the shock-release model, an assumed configuration of baseplate(metal-sample)/window is modeled to formulate the initial shock and interfacial release processes, the Lindemann law is used to describe the melting behavior of metals at high pressure, and the Gruneisen equation of state is applied to express the relation between pressure and specific internal energy. The effect of melting on the Hugoniot temperature is considered in this model, so the pressure–temperature regime of solid–liquid mixed phase region can be outlined. Aluminum, lead, copper, molybdenum, iron, and uranium were chosen for modeling calculations. It was found from the comparison of calculated results with their high-pressure melting data from experimental measurements that for most investigated metals γ=γ0...

Method to determine the melting temperatures of metals under megabar shock pressures

Based on the model that the high-pressure melting temperatures of metals approximately equal the experimentally measured interface temperatures between the metallic plate sample and the transparent window when shock- and/or release-induced melting falls into the mixed phase region, we proposed a method to determine the melting temperatures of metals under megabars of shock compression. Experiments were conducted by using the oxygen-free high-conductivity copper, and pure iron plate sample with single-crystal lithium fluoride windows. Results showed that the measured melting temperatures are in good agreement with reported theoretical calculations.

Shock-induced bct-bcc transition and melting of tin identified by sound velocity measurements

A direct reverse-impact configuration together with the velocity interferometer system for any reflector was utilized to accurately measure longitudinal and bulk sound velocities of tin under shock compression. Shock-induced bct-bcc transition and melting of tin were identified based on the discontinuity of the longitudinal sound velocity against shock pressure, which were not previously revealed by Hugoniot and wave profile measurements. The transition pressures for bct to bcc phase and bcc to liquid phase were constrained to be ∼34±2 and ∼39±2 GPa, respectively. The shear modulus and yield strength at loaded shock stresses were extracted from the measured sound velocities. A tentative phase diagram of tin was constructed by available experimental data and thermodynamic calculations, which was consistent with results from diamond anvil cell experiments and inconsistent with those from shock temperature measurements and molecular dynamic simulations.

Sound velocity measurements of tantalum under shock compression in the 10-110 GPa range

The high-pressure melting curve of tantalum (Ta) has been the center of a long-standing controversy. Sound velocities along the Hugoniot curve are expected to help in understanding this issue. To that end, we employed a direct-reverse impact technique and velocity interferometry to determine sound velocities of Ta under shock compression in the 10-110 GPa pressure range. The measured longitudinal sound velocities show an obvious kink at ∼60 GPa as a function of shock pressure, while the bulk sound velocities show no discontinuity. Such observation could result from a structural transformation associated with a negligible volume change or an electronic topological transition.

Successive phase transitions of tin under shock compression

Longitudinal and bulk sound velocities of tin in the shock pressure range from ∼25to∼80GPa were measured using a direct reverse-impact method. The bct to bcc phase transition along the Hugoniot was identified by the discontinuity of the longitudinal sound velocity against shock pressure. The incipient melting on the Hugoniot was also revealed by the transition from longitudinal to bulk sound velocity. The shock pressure for bct-bcc phase transition and incipient melting were constrained to be ∼35 and ∼45GPa, respectively. It is inferred that the bcc phase possesses higher shear modulus than the bct phase.

Shock compression response of a Zr-based bulk metallic glass up to 110 GPa

Shock wave compression experiments were conducted on a Zr-based bulk metallic glass (BMG, Zr51Ti5Ni10Cu25Al9 in atomic percent) up to 110 GPa. Time-resolved free-surface velocity profiles were measured in a shock stress range from 18 to 28 GPa with velocity interferometer techniques. The shock Hugoniot data in a shock stress range from 53 to 110 GPa were obtained by using electric pin techniques. The time-resolved wave profiles showed a distinct two-wave structure consisting of an elastic precursor followed by a plastic wave. Based on the obtained wave profiles, the Hugoniot elastic limits were determined to be 6.9 to 9.6 GPa. The shock wave velocity (Ds) vs. particle velocity (up) Hugoniot data in a shock stress range from 18 to 110 GPa were linearly fitted by Ds=(4.241±0.035)+(1.015±0.024)up. No evidence of phase transition was found in the performed shock experiments of the Zr-based BMG.

Determination of effective shear modulus of shock-compressed LY12 Al from particle velocity profile measurements

Unloading wave profile measurements using the velocity interference system for any reflector technique were performed on LY12 Al over shock stress ranging from ∼20to∼100GPa, from which longitudinal and bulk sound velocities along the quasielastic release path were evaluated. Based on the intrinsic relations under uniaxial strain conditions, the effective shear modulus defined by Cochran and Guinan (Lawrence Livermore National Laboratory Report No. UCID-17105, 1976) was correlated to the longitudinal and bulk sound velocities. Results show that the effective shear modulus calculated from the measured sound velocities decreases rapidly with the release stress and can be expressed approximately as a linear function of the release stress; the slope of the linear function depends the initial shock-loading stress. By using this linear function of the effective shear modulus, the performed numerical simulations well reproduce the release wave traces of the Al alloys observed in the present work and reported in l...

Phase transition and strength of vanadium under shock compression up to 88 GPa

A series of reverse-impact experiments were performed on vanadium at shock pressure ranging from 32 GPa to 88 GPa. Particle velocity profiles measured at sample/LiF window interface were used to estimate the sound velocities, shear modulus, and yield stress in shocked vanadium. A phase transition at ∼60.5 GPa that may be the body-centered cubic (BCC) to rhombohedral structure was identified by the discontinuity of the sound velocity against shock pressure. This transition pressure is consistent with the results from diamond anvil cell (DAC) experiments and first-principle calculations. However, present results show that the rhombohedral phase has higher strength and shear modulus than the BCC phase, which is contrast to the findings from DAC experiments and theoretical work.

Hugoniot and sound velocity measurements of bismuth in the range of 11–70 GPa

Plate impact experiments in backward-impact geometry were performed on bismuth (Bi) in the pressure range of 11–70 GPa. The bismuth sample used as flyer impacted a LiF window, and the impact velocity and particle velocity at interface were simultaneously measured by a distance interferometer system for any reflector. Hugoniot and sound velocity data were extracted from the observed particle velocity profiles. The obtained plot of shock velocity (D) versus particle velocity (u) showed a discontinuity at u ≈ 0.9 km/s, corresponding to a pressure of ∼27 GPa. Furthermore, plate impact experiments in forward-impact geometry were conducted to measure sound velocities of bismuth. The extracted sound velocity data from backward and forward-impact experiments showed a transition from longitudinal to bulk sound velocity (18 GPa–27 GPa), and the pressure of transition to bulk sound velocity is consistent with the pressure of D-u knee at u ≈ 0.9 km/s. This D-u discontinuity at u ≈ 0.9 km/s is attributed to shock indu...