J. Reed Patterson

Ramp compression of iron to 273 GPa

Multiple thickness Fe foils were ramp compressed over several nanoseconds to pressure conditions relevant to the Earths core. Using wave-profile analysis, the sound speed and the stress-density response were determined to a peak longitudinal stress of 273 GPa. The measured stress-density states lie between shock compression and 300-K static data, and are consistent with relatively low temperatures being achieved in these experiments. Phase transitions generally display time-dependent material response and generate a growing shock. We demonstrate for the first time that a low-pressure phase transformation (α-Fe to e-Fe) can be overdriven by an initial steady shock to avoid both the time-dependent response and the growing shock that has previously limited ramp-wave-loading experiments. In addition, the initial steady shock pre-compresses the Fe and allows different thermodynamic compression paths to be explored.

X-ray diffraction and nanoindentation studies of nanocrystalline graphite at high pressures

We report energy dispersive x-ray diffraction studies on nanocrystalline hexagonal graphite samples (average grain size=12 nm) in a diamond-anvil cell to 65 GPa at room temperature. A structural phase transition to a hexagonal diamond phase beginning at 15 GPa is completed at 55 GPa, and is reversible on decompression. The x-ray diffraction studies were followed by nanoindentation hardness measurements on the pressure treated samples. The obtained hardness values are in the range of 1–2 GPa. Unlike fullerenes, the room-temperature compression of nanocrystalline graphite to 65 GPa did not produce a superhard carbon material.

Application of tape-cast graded impedance impactors for light-gas gun experiments

Fabrication of compositionally graded structures for use as light-gas gun impactors has been demonstrated using a tape casting technique. Mixtures of metal powders in the Mg-Cu system were cast into a series of 19 tapes with uniform compositions ranging from 100% Mg to 100% Cu. The individual compositions were fabricated into monolithic pellets for characterization of microstructure, density, and sound wave velocity. Graded impactors were fabricated by stacking layers of different compositions in a sequence calculated to yield a tailored acoustic impedance profile, and were characterized by ultrasonic C-scan and white light interferometry. The graded impactors were launched into stationary Al targets using a two-stage light-gas gun, and the resulting wave profiles were measured with either VISAR or Photonic Doppler Velocimetry. For an impactor using only seven compositions ranging from Mg to Cu, the composition steps are visible in the wave profiles. An impactor utilizing the full series of 19 composition...

A unified approach for extracting strength information from nonsimple compression waves. Part II. Experiment and comparison with simulation

We apply general thermodynamics-based wave analysis methods to a gas-gun-driven plate impact experiment designed to derive strength information from tantalum at pressures of 10–25 GPa. The analysis provides estimates of the complete deformation paths in terms of the coupled evolution of mean stress, deviatoric stress, plastic strain, and plastic strain rate, yielding detailed information for direct comparison to strength models. This inverse analysis (deriving estimates of strength behavior directly from the measurements, with no strength model assumed) is compared to forward analysis (hydrodynamic simulations with specific strength models, in general adjusting parameters to optimally match the experiment). This comparison fulfills three goals. (1) To determine the parameter sensitivity and overall stability of the inverse analysis by analyzing simulated data as if it were experimental data. We find that, in reasonably favorable cases, precision to ∼10% is possible for the flow curve during loading and ∼3...

Crystallographic Anisotropy in Compression of Uranium Metal to 100 GPa

New high-pressure x-ray diffraction data on uranium metal (99.9 %) in a diamond anvil cell is presented to 100 GPa (Volume compression V/V o = 0.700) at room temperature using a variety of pressure markers like ruby, copper, and platinum. The diffraction patterns are carefully indexed allowing for reversal of peak positions based on anisotropic compression. We report anisotropic compression of the orthorhombic unit cell with the axial ratio b/a increasing initially to 40 GPa followed by a rapid decrease at higher pressure. On the other hand, axial ratio c/a shows a rapid increase with increasing pressure followed by saturation at megabar pressures. The most recent full potential electronic structure calculations reproduce the increasing tend of axial ratio c/a to 100 GPa but do not explain the variation in the b/a ratio. Our detailed analysis of all available experimental data also indicates that the observed anisotropic effects are intrinsic to Uranium and are independent of the pressure medium used in the high-pressure experiments.