Real-time (nanoseconds) determination of liquid phase growth during shock-induced melting.

Melting of solids is a fundamental natural phenomenon whose pressure dependence has been of interest for nearly a century. However, the temporal evolution of the molten phase under pressure has eluded measurements because of experimental challenges. By using the shock front as a fiducial, we investigated the time-dependent growth of the molten phase in shock-compressed germanium. In situ x-ray diffraction measurements at different times (1 to 6 nanoseconds) behind the shock front quantified the real-time growth of the liquid phase at several peak stresses. These results show that the characteristic time for melting in shock-compressed germanium decreases from ~7.2 nanoseconds at 35 gigapascals to less than 1 nanosecond at 42 gigapascals. Our melting kinetics results suggest the need to consider heterogeneous nucleation as a mechanism for shock-induced melting and provide an approach to measuring melting kinetics in shock-compressed solids.

Crystal Structure and Melting of Fe Shock Compressed to 273 GPa: In Situ X-Ray Diffraction.

Despite extensive shock wave and static compression experiments and corresponding theoretical work, consensus on the crystal structure and the melt boundary of Fe at Earth's core conditions is lacking. We present in situ x-ray diffraction measurements in laser-shock compressed Fe that establish the stability of the hexagonal-close-packed (hcp) structure along the Hugoniot through shock melting, which occurs between ∼242 to ∼247 GPa. Using previously reported hcp Fe Hugoniot temperatures, the melt temperature is estimated to be 5560(360) K at 242 GPa, consistent with several reported Fe melt curves. Extrapolation of this value suggests ∼6400 K melt temperature at Earth's inner core boundary pressure.

Single-pulse (100 ps) extended x-ray absorption fine structure capability at the Dynamic Compression Sector.

Determining real-time changes in the local atomistic order is important for a mechanistic understanding of shock wave induced structural and chemical changes. However, the single event and short duration (nanosecond times) nature of shock experiments pose challenges in obtaining Extended X-ray Absorption Fine Structure (EXAFS) measurements-typically used for monitoring local order changes. Here, we report on a new single pulse (∼100 ps duration) transmission geometry EXAFS capability for use in laser shock-compression experiments at the Dynamic Compression Sector (DCS), Advanced Photon Source. We used a flat plate of highly oriented pyrolytic graphite (HOPG) as the spectrometer element to energy disperse x rays transmitted through the sample. It provided high efficiency with ∼15% of the x rays incident on the HOPG reaching an x-ray area detector with high quantum efficiency. This combination resulted in a good signal-to-noise ratio (∼103), an energy resolution of ∼10 eV at 10 keV, EXAFS spectra covering 100 s of eV, and a good pulse to pulse reproducibility of our single pulse measurements. Ambient EXAFS spectra for Cu and Au are compared to the reference spectra, validating our measurement system. Comparison of single pulse EXAFS results for ambient and laser shocked Ge(100) shows large changes in the local structure of the short lived state of shocked Ge. The current DCS EXAFS capability can be used to perform single pulse measurements in laser shocked materials from ∼9 keV to 13 keV. These EXAFS developments will be available to all users of the DCS.

Structural response of α-quartz under plate-impact shock compression.

Because of its far-reaching applications in geophysics and materials science, quartz has been one of the most extensively examined materials under dynamic compression. Despite 50 years of active research, questions remain concerning the structure and transformation of SiO2 under shock compression. Continuum gas-gun studies have established that under shock loading quartz transforms through an assumed mixed-phase region to a dense high-pressure phase. While it has often been assumed that this high-pressure phase corresponds to the stishovite structure observed in static experiments, there have been no crystal structure data confirming this. In this study, we use gas-gun shock compression coupled with in situ synchrotron x-ray diffraction to interrogate the crystal structure of shock-compressed α-quartz up to 65 GPa. Our results reveal that α-quartz undergoes a phase transformation to a disordered metastable phase as opposed to crystalline stishovite or an amorphous structure, challenging long-standing assumptions about the dynamic response of this fundamental material.

What Determines the fcc-bcc Structural Transformation in Shock Compressed Noble Metals?

High pressure structural transformations are typically characterized by the thermodynamic state (pressure-volume-temperature) of the material. We present in situ x-ray diffraction measurements on laser-shock compressed silver and platinum to determine the role of deformation-induced lattice defects on high pressure phase transformations in noble metals. Results for shocked Ag show a copious increase in stacking faults (SFs) before transformation to the body-centered-cubic (bcc) structure at 144-158 GPa. In contrast, shock compressed Pt remains largely free of SFs and retains the fcc structure to over 380 GPa. These findings, along with recent results for shock compressed gold, show that SF formation promotes high pressure structural transformations in shocked noble metals that are not observed under static compression. Potential SF-related mechanisms for fcc-bcc transformations are discussed.

Structural Transformation and Melting in Gold Shock Compressed to 355 GPa.

Gold is believed to retain its ambient crystal structure at very high pressures under static and shock compression, enabling its wide use as a pressure marker. Our in situ x-ray diffraction measurements on shock-compressed gold show that it transforms to the body-centered-cubic (bcc) phase, with an onset pressure between 150 and 176 GPa. A liquid-bcc coexistence was observed between 220 and 302 GPa and complete melting occurs by 355 GPa. Our observation of the lower coordination bcc structure in shocked gold is in marked contrast to theoretical predictions and the reported observation of the hexagonal-close-packed structure under static compression.

Nanosecond Melting and Recrystallization in Shock-Compressed Silicon.

In situ, time-resolved, x-ray diffraction and simultaneous continuum measurements were used to examine structural changes in Si shock compressed to 54 GPa. Shock melting was unambiguously established above ∼31-33 GPa, through the vanishing of all sharp crystalline diffraction peaks and the emergence of a single broad diffraction ring. Reshock from the melt boundary results in rapid (nanosecond) recrystallization to the hexagonal-close-packed Si phase and further supports melting. Our results also provide new constraints on the high-temperature, high-pressure Si phase diagram.

Twinning and Dislocation Evolution during Shock Compression and Release of Single Crystals: Real-Time X-Ray Diffraction.

Determining the temporal evolution of twinning and/or dislocation slip, in real-time (nanoseconds), in single crystals subjected to plane shock wave loading is a long-standing scientific need. Noncubic crystals pose special challenges because they have many competing slip and twinning systems. Here, we report on time-resolved, in situ, synchrotron Laue x-ray diffraction measurements during shock compression and release of magnesium single crystals that are subjected to compression along the c axis. Significant twinning was observed directly during stress release following shock compression; during compression, only dislocation slip was observed. Our measurements unambiguously distinguish between twinning and dislocation slip on nanosecond timescales in a shocked hexagonal-close-packed metal.

In situ X-Ray Diffraction of Shock-Compressed Fused Silica.

Because of its widespread applications in materials science and geophysics, SiO_{2} has been extensively examined under shock compression. Both quartz and fused silica transform through a so-called "mixed-phase region" to a dense, low compressibility high-pressure phase. For decades, the nature of this phase has been a subject of debate. Proposed structures include crystalline stishovite, another high-pressure crystalline phase, or a dense amorphous phase. Here we use plate-impact experiments and pulsed synchrotron x-ray diffraction to examine the structure of fused silica shock compressed to 63 GPa. In contrast to recent laser-driven compression experiments, we find that fused silica adopts a dense amorphous structure at 34 GPa and below. When compressed above 34 GPa, fused silica transforms to untextured polycrystalline stishovite. Our results can explain previously ambiguous features of the shock-compression behavior of fused silica and are consistent with recent molecular dynamics simulations. Stishovite grain sizes are estimated to be ∼5-30 nm for compression over a few hundred nanosecond time scale.

Transformation of shock-compressed graphite to hexagonal diamond in nanoseconds.

The graphite-to-diamond transformation under shock compression has been of broad scientific interest since 1961. The formation of hexagonal diamond (HD) is of particular interest because it is expected to be harder than cubic diamond and due to its use in terrestrial sciences as a marker at meteorite impact sites. However, the formation of diamond having a fully hexagonal structure continues to be questioned and remains unresolved. Using real-time (nanosecond), in situ x-ray diffraction measurements, we show unequivocally that highly oriented pyrolytic graphite, shock-compressed along the c axis to 50 GPa, transforms to highly oriented elastically strained HD with the (100)HD plane parallel to the graphite basal plane. These findings contradict recent molecular dynamics simulation results for the shock-induced graphite-to-diamond transformation and provide a benchmark for future theoretical simulations. Additionally, our results show that an earlier report of HD forming only above 170 GPa for shocked pyrolytic graphite may lead to incorrect interpretations of meteorite impact events.

Real-Time Examination of Atomistic Mechanisms during Shock-Induced Structural Transformation in Silicon.

The experimental determination of atomistic mechanisms linking crystal structures during a compression-driven solid-solid phase transformation is a long-standing and challenging scientific objective. Using new capabilities at the Dynamic Compression Sector at the Advanced Photon Source, the structure of shocked Si at 19 GPa was identified as simple hexagonal, and the lattice orientations between ambient cubic diamond and simple hexagonal structures were related. The approach is general and provides a powerful new method for examining atomistic mechanisms during stress-induced structural changes.

Real-time, high-resolution x-ray diffraction measurements on shocked crystals at a synchrotron facility.

The Advanced Photon Source (APS) at Argonne National Laboratory was used to obtain real-time, high-resolution x-ray diffraction measurements to determine the microscopic response of shock-compressed single crystals. Disk shaped samples were subjected to plane shock wave compression by impacting them with half-inch diameter, flat-faced projectiles. The projectiles were accelerated to velocities ranging between 300 and 1200 m/s using a compact powder gun designed specifically for use at a synchrotron facility. The experiments were designed to keep the sample probed volume under uniaxial strain and constant stress for a duration longer than the 153.4 ns spacing between x-ray bunches. X-rays from a single pulse (<100 ps duration) out of the periodic x-ray pulses emitted by the synchrotron were used for the diffraction measurements. A synchronization and x-ray detection technique was developed to ensure that the measured signal was obtained unambiguously from the desired x-ray pulse incident on the sample while the sample was in a constant uniaxial strain state. The synchronization and x-ray detection techniques described can be used for a variety of x-ray measurements on shock compressed solids and liquids at the APS. Detailed procedures for applying the Bragg-Brentano parafocusing approach to single crystals at the APS are presented. Analytic developments to determine the effects of crystal substructure and non-ideal geometry on the diffraction pattern position and shape are presented. Representative real-time x-ray diffraction data, indicating shock-induced microstructural changes, are presented for a shock-compressed Al(111) sample. The experimental developments presented here provided, in part, the impetus for the Dynamic Compression Sector (DCS) currently under development at the APS. Both the synchronization∕x-ray detection methods and the analysis equations for high-resolution single crystal x-ray diffraction can be used at the DCS.