H. E. Lorenzana

Material dynamics under extreme conditions of pressure and strain rate

Abstract Solid state experiments at extreme pressures (10–100 GPa) and strain rates (106–108s−1) are being developed on high energy laser facilities, and offer the possibility for exploring new regimes of materials science. These extreme solid state conditions can be accessed with either shock loading or with a quasi-isentropic ramped pressure drive. Velocity interferometer measurements establish the high pressure conditions. Constitutive models for solid state strength under these conditions are tested by comparing 2D continuum simulations with experiments measuring perturbation growth from the Rayleigh–Taylor instability in solid state samples. Lattice compression, phase and temperature are deduced from extended X-ray absorption fine structure (EXAFS) measurements, from which the shock induced α–ω phase transition in Ti and the α–ϵ phase transition in Fe, are inferred to occur on subnanosec time scales. Time resolved lattice response and phase can also be measured with dynamic X-ray diffraction measurements, where the elastic–plastic (1D–3D) lattice relaxation in shocked Cu is shown to occur promptly (<1 ns). Subsequent large scale molecular dynamics (MD) simulations elucidate the microscopic dislocation dynamics that underlies this 1D–3D lattice relaxation. Deformation mechanisms are identified by examining the residual microstructure in recovered samples. The slip-twinning threshold in single crystal Cu shocked along the [001] direction is shown to occur at shock strengths of ∼20 GPa, whereas the corresponding transition for Cu shocked along the [134] direction occurs at higher shock strengths. This slip twinning threshold also depends on the stacking fault energy (SFE), being lower for low SFE materials. Designs have been developed for achieving much higher pressures, P>1000 GPa, in the solid state on the National Ignition Facility (NIF) laser.

Pseudotachylites generated in shock experiments : implications for impact cratering products and processes

Laboratory hypervelocity impact experiments in which quartz was shock-loaded from 42 to 56 gigapascals imply that type A pseudotachylites form by strain heating and contribute to the loss of strength of rocks in the central uplift of large impact structures. Shock impedance-matched aluminum sample containers, in contrast to steel containers, produced nearly single-wave pressure loading, and enhanced deformation, of silicate samples. Strain heating may act with shock heating to devolatilize planetary materials and destroy extraterrestrial organic material in an impact.

Analysis of the x-ray diffraction signal for the {alpha}-{epsilon} transition in shock-compressed iron: Simulation and experiment

Recent published work has shown that the phase change of shock-compressed iron along the [001] direction does transform to the {epsilon} [hexagonal close-packed (hcp)] phase similar to the case for static measurements. This article provides an in-depth analysis of the experiment and nonequilibrium molecular dynamics simulations, using x-ray diffraction in both cases to study the crystal structure upon transition. Both simulation and experiment are consistent with a compression and shuffle mechanism responsible for the phase change from body-centered cubic to hcp. Also both show a polycrystalline structure upon the phase transition, due to the four degenerate directions in which the phase change can occur.

CARBON MONOXIDE : SPECTROSCOPIC CHARACTERIZATION OF THE HIGH-PRESSURE POLYMERIZED PHASE

AbstractSolid carbon monoxide transforms to the δ–phase at about 48 kbar at room temperature. In this pressure regime (50 kbar and greater), carbon monoxide undergoes a transformation at room temperature to a light–pink solid, which has not been studied in detail and may be different from the δ–phase. Exposure to moderate light intensities at these P–T conditions converts the system to a dark red material. We report visible and infrared absorption as well as Raman investigations of this dark red substance, which likely contains a polymeric product of the photochemical reaction. This material is stable upon pressure release down to ambient conditions. Previous studies speculated that the dark red product was a mixture of poly–carbonsuboxide (C3O2) and oxalic anhydride (C2O3). In contrast, we present evidence that this material is composed of graphitic–like carbon, carbon dioxide, and possibly a polymerized network containing

Nanosecond x-ray diffraction from polycrystalline and amorphous materials in a pinhole camera geometry suitable for laser shock compression experiments.

- \left( {{\text{C = O}}} \right) - {\text{O}} - \left( {{\text{C}} - } \right) = {\text{C}} <

Simulating picosecond x-ray diffraction from shocked crystals using post-processing molecular dynamics calculations

as a repeating unit.

Extended x-ray absorption fine structure measurements of quasi-isentropically compressed vanadium targets on the OMEGA laser

Small holes are drilled in diamond anvil cell gaskets to contain and pressurize samples. As high‐pressure technology pushes the multimegabar regime, smaller‐tipped diamond anvils are being increasingly utilized. Consequently, well‐centered holes with diameters smaller than 100 μm need to be routinely produced in these gaskets made of exceedingly hard metals. We describe the construction of an inexpensive electric discharge machine that can drill metals with holes as small as 25 μm in diameter. This method of drilling is easy to use, far less expensive than other commonly used techniques, and has the advantage of being effective on extremely difficult to machine metals such as rhenium.

Extended x-ray absorption fine structure measurement of phase transformation in iron shocked by nanosecond laser

Nanosecond pulses of quasimonochromatic x-rays emitted from the K shell of ions within a laser-produced plasma are of sufficient spectral brightness to allow single-shot recording of powder diffraction patterns from thin foils of order millimeter diameter. Strong diffraction signals have been observed in a cylindrical pinhole camera arrangement from both polycrystalline and amorphous foils, and the experimental arrangement and foil dimensions are such that they allow for laser shocking or quasi-isentropic loading of the foil during the diffraction process.