Session Q24: Matter at Extreme Conditions: Dynamic Compression II

Extraordinary performance of the stable, nanocrystalline Cu-Ta system under extreme dynamic mechanical load

The influence of shock loading on nanocrystalline materials has heretofore not been examined in large part due to the instability of nanocrystalline structures under extreme mechanical and thermal loads. However, a recently developed binary alloy of nanocrystalline Cu and Ta has proven to remain nanocrystalline up to temperatures of 800 °C and, under uniaxial stress, up to strain rates of 10⁴ s^-1. Here, the performance of a nanocrystalline Cu-Ta alloy is investigated as both temperature and strain rate increase to the extremes under shock compression up to approximately 15 GPa. Post deformed micrographs reveal a shift from full dislocation to twin dominated deformation mechanisms above 10³ s^-1. In the shock regime, however, no significant microstructure changes are observed between the unshocked and shock recovered samples. Further, despite this apparent lack of change under shock loading, spall strength of the material is only slightly improved over polycrystalline Cu. While the refined grain size appears to contribute to arresting nucleated spall cracks, the plethora of grain boundaries and Ta clusters may act as potential void nucleation sites for spall crack initiation reducing the benefits of grain boundary strengthening.   

Phase Transition Mechanisms in Cadmium Sulfide from X-ray Diffraction Comparisons of High-pressure Experiments and Molecular Dynamics Simulation

We use simulated X-ray diffraction (XRD) from atomistic molecular dynamics simulations to interpret recent shock and ramp compression experiments carried out at the Dynamic Compression Sector at the Advanced Photon Source, and the Thor driver at Sandia National Labs.  Cadmium Sulfide (CdS) single crystal is a well-studied material which undergoes a stress-induced solid-solid phase transition from wurtzite to rock salt structures at a few GPa.  The time scales, and perhaps mechanisms, of this transition have been found to depend on the orientation of the compression wave relative to the initial crystal lattice (i.e. a-axis vs c-axis).  We use simulation to help separate XRD pattern contributions from multiple final orientation domains and use these orientations to distinguish between several possible Martensitic phase transition mechanisms.   

Investigating the effect of shock-compression on glass transition in Germanium-Selenium glass Systems

The nature of the rigid (stiffness) to floppy (flexible) phase (intermediate phase) transition in network glasses continues to be one of the central issues in glass science.  we have successfully observed intermediate phase in Ge_xSe_1-x (x = 0.225) glass samples at a glass transition temperature (Tg) of 220^o C. In this work, we report postmortem studies on shock-compressed Ge-Se glasses. The dynamic compression experiments were performed using a laser-driven flyer plate system. This high-throughput technique can incorporate in situ, real time spectroscopy and high speed, high resolution images collected during shock wave propagation through the glasses. To investigate any permanent changes due to compression, glass samples were characterized before and after the experiments. The flyers had impact velocities of 3.5 km/s, corresponding to pressures of ~ 24 GPa. The impacted glass samples were recovered after shock for postmortem analysis with modulated differential scanning calorimeter and x-ray diffraction at room temperature. The results have shown changes in glass transition temperatures, evidence of permanent changes in the crystal structures of the compressed glasses. The study is extremely important in order to develop new glass materials with advanced properties.   

High pressure-temperature phase diagram of silicon carbide from quantum molecular dynamics

High pressure-temperature behavior of SiC is being actively investigated with the goal of delivering key data for developing interior models of carbon-rich exoplanets. Recent experimental static  and dynamic compression studies of phase transformations in SiC have uncovered substantial differences between static and dynamic transition pressures of solid-solid phase transition from the ambient pressure zinc-blende or hexagonal to high pressure rocksalt phases. The solid-liquid melting phase transformations in SiC are also poorly understood. Quantum molecular dynamics (QMD) simulations of the SiC phase diagram and shock Hugoniots are performed within a broad range of pressures (up to 1 TPa) and temperatures (up to 50,000 K). The QMD results allow to reconcile the puzzling difference between the static and dynamic shock compression experiments while delivering accurate equation of state of the solid and liquid phases as well as the SiC melting lines.   

Measurement of high-pressure crystal structure and the pressure-temperature melt conditions in shock-compressed silicon carbide

Silicon Carbide (SiC) has many attractive properties including low density, high strength, high melting point, low wear coefficient and high chemical stability that lead to its extensive use in a wide range of industrial applications including as abrasives, shielding material on space craft,^, personal armor and as a potential ablator material for fusion capsules. Silicon carbide is also important in geology and planetary science. SiC grains - found in meteorites and impact sites – have an unusual isotopic signature which indicate that they are pre-solar in origin and provide constraints on stellar nucleosynthesis and on the stellar sources for the origin of the solar system. Several studies have also explored possible interior structure of carbon-rich planets in which SiC is a likely main constituent, but experimental data for the high-pressure properties of SiC is currently unavailable to test these models. Here we report on laser-driven nanosecond x-ray diffraction and shock-decay experiments on the Omega-EP laser facility located at the Laboratory for Laser Energetics (NY, USA). We present new data on the high-pressure crystal structure, and pressure-temperature conditions for melt in single-crystal SiC samples.

    

In situ x-ray diffraction of sapphire single crystals during laser compression and release

Sapphire is an important earth mineral notable for its high compressive strength and hardness. Static compression experiments have found evidence of phase transformations from the α-corundum phase to a Rh₂O₃ (II)-type structure at ~80 GPa (Funamori & Jeanloz, 1997, Lin et al., 2004), and then to the post-perovskite structure at ~130 GPa (Oganov & Ono, 2005).

In this work, we describe laser-driven shock compression experiments on a-cut and c-cut sapphire to 150 GPa conducted at the Matter in Extreme Conditions endstation of the Linac Coherent Light Source. Ultrafast x-ray pulses (50 fs, 10¹² photons/pulse) were used to probe the lattice-level response as a function of time during and after shock propagation. VISAR velocimetry was used to measure free-surface particle velocity, from which pressure was calculated. Through in situ x-ray diffraction, we observe evidence of residual elastic strain, twinning and crystal break-up during and after compression. After release to ambient pressure, measured temperatures far exceed those predicted by isentropic release, indicating plastic work heating. We will discuss the effect of crystal orientation on shock-induced phase transformation.   

Thermal Emission and Reflectivity of Shocked SiO2 Aerogel for Broadband Optical Probing

The thermodynamic behavior of shock-compressed SiO₂ was studied over a broad density and temperature range using low-density aerogels. Temperature and reflectance measurements were performed on singly shocked SiO₂ at initial densities of 0.3 (silica aerogel), 0.2, and 0.11 g/cm³. This work generated a radiance model for pragmatic experimental design and extends the capability of SiO₂ as a thermodynamic calibrant for high-energy-density–physics experiments.   

Shock melting of tin

An accurate understanding of the Sn phase diagram is needed to correctly interpret experiments examining ejecta, cavitation, and multiphase strength. A recently published multiphase equation of state (Rehn et al.) has suggested that δ-Sn (bcc) should form prior to incipient melt along the principal Hugoniot, with the two-phase region extending from nominally 57-73 GPa. This two-phase region lies at significantly higher pressures than previously anticipated based on previously reported shock release experiments. In this work, X-ray diffraction (XRD) was used to examine the phase evolution along the Sn Hugoniot from 30-85 GPa. The results of these experiments are presented and discussed in the context of the Sn phase diagram with an emphasis on the extent of the solid-liquid two-phase region and whether Sn melts from the δ phase along the principal Hugoniot.   

Temperature measurements on shocked two-phase Ce

The ability to understand and predict the response of matter at extreme conditions requires knowledge of a materials equation-of-state (EOS) including the location of phase boundaries, transitions kinetics, and accurate measurements of temperature at pressure.  Recent developments in optical pyrometry have provided methods for measuring temperature in the shocked state, and these have been used to study the phase diagram for some metals including cerium.  Cerium has received significant attention because it exhibits a rich phase diagram that includes an anomalous melt boundary and a low-pressure shock-melt transition.   In this work, we continue our study of shock-melting using optical pyrometry to measure the temperature and stress states for two-phase cerium (Υ-Ce and β-Ce) shocked into the high-pressure liquid phase. These results will be compared to the previously measured results for single phase Ce (γ-Ce) with the goal of gaining further insight into the shock melt transition and associated kinetics.  Details of the experimental methods, analysis, and results will be presented (LA-UR-21-30567).    

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The performance of additively manufactured (AM) composites subjected to dynamic loading significantly depends on their micro- and meso-scale structures. The versatility offered by AM opens new ways to control performance via design of structure. AM composites have a wide range of process-inherent heterogeneities with a hierarchy of length scales such as non-uniform constituent distribution, interfaces, and porosity. In this work, temporally- and spatially-resolved measurements of macro-, micro-, and meso-scale heterogeneities were performed to investigate the shock response of an AM-fabricated high-solids-loaded polymer composite. Samples with pre-identified heterogeneities in the form of pores were impacted in different orientations relative to the print direction of the filaments. X-ray phase contrast imaging (PCI) was used to study the void collapse process and determine the shock and particle velocities upon arrival of shock wave front(s) via feature tracking. Photon Doppler velocimetry (PDV) was simultaneously used to measure the sample free surface velocity to determine the effects of local directional porosity on the equation of state, which in previous work was found to be independent of orientation effects in a similar material with a more uniform porosity distribution.   

Shockcompression of metal-organic frameworks

To better protect personnel and equipment from injuries and damages caused by shock waves generated from explosions and high-speed impacts, we need improved methods to absorb shock waves and new methods to measure shock wave absorption. Metal-organic framework (MOF), a nanoporous crystalline material, caught our attention for its multiple responses to mechanical compressions and considerable mechanical energy absorbing capability in previous static high-pressure studies. In this work, we have developed a method to measure shock absorption using a laser-driven-flyer shock compression microscope and we have studied the shock wave absorption in MOF (denoted ZIF-8) films. We monitored the shock wave breaking out of the MOF films of different thicknesses with a photon-Doppler velocimeter (PDV). After shock we were able to collect the ZIF-8 films for post-mortem analysis.

We show that the ZIF-8 film absorbs 7 times more energy than an equivalent mass of PMMA. Our post-mortem electronic and Raman microscopy analyses reveal that the high efficiency of shock energy absorption in MOF film is resulted from the incorporation of multiple shock absorbing mechanisms, including crystal powder compaction, nanopore collapse, and endothermic bond breaking. The PDV data show that shock wave energy follows saturable absorption trend in MOFs. This study suggests new ways for fundamental shock investigations and illustrates how to improve the shock absorbing efficiency by engineering multiple shock absorbing mechanisms into armor materials.