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Quasi-isentropic and shock compression of FCC and BCC metals : effects of grain size and stacking-fault energy


Quasi-isentropic compression experiments (ICE) of monocrystalline copper and nanocrystalline nickel and nickel-tungsten were carried out. The ICE process allows higher pressures to be accessed while minimizing the associated temperature rise. Monocrystalline copper was subjected to pressures between 18 GPa and 52 GPa, and the deformation substructure was studies via transmission electron microscopy (TEM). Current experimental evidence suggests a deformation substructure that transitions from slip to twinning, where twinning occurs at the higher pressures (5̃2 GPa), and heavily dislocated laths and dislocation cells take place at the intermediate and lower pressures. Evidence of stacking faults at the intermediate pressures was also found. The Preston-Tonks-Wallace constitutive description was used to model both quasi- isentropic and shock compression experiments and predict the pressure at which the slip-twinning transition occurs in both cases. Nanocrystalline nickel and nickel-tungsten, 13 at. % (G S between 10 and 50nm), subjected to pressures between 20 and 70 GPa, were also analyzed. Shock compression of mono and nanocrystalline nickel is simulated over a range of pressures (10-80 GPa) and compared with experimental results. Contributions to the net strain from the various mechanisms of plastic deformation such as partial dislocations, perfect dislocations, and twins are quantified in the nanocrystalline samples. The effect of stress unloading, a phenomenon often neglected in MD simulations, on dislocation behavior is computed. It is shown that a large fraction of the dislocations generated during compression is annihilated upon unloading. The present analysis resolves a disagreement consistently observed between MD computations and experimental results. Analytical models are applied to predict the critical pressures for the cell -to-stacking-fault transition and the onset of twinning as a function of grain-size and stacking-fault energy (through the addition of tungsten). These predictions are successfully compared with experimental results. Polycrystalline vanadium was subjected to shock compression followed by tensile wave release to study spall and fragmentation behavior. These experiments are part of an effort to help predict and minimize damage to diagnostic tools and protective shields of high-powered laser facilities such as the National Ignition Facility, NIF. The shock pulse was generated by a direct laser drive at energy levels ranging between 160 J and 440 J. Glass shields placed at a specific distance behind the Va targets were used to collect and analyze the ejected fragments in order to evaluate and quantify the extent of damage. The effects of target thickness, laser energy, and pulse duration were studied. Calculations show melting at a pressure threshold of 1̃50 GPa, which corresponds to a laser energy level of ̃200 J. The recovered specimens and fragments show evidence of melting at the higher energy levels, consistent with the analytical predictions. Spalling occurred by a ductile tearing mechanism that favored grain boundaries. Experimentally obtained fragment sizes were compared with predictions from the Grady-Kipp model, and a good agreement was obtained. The spall strength of vanadium under laser loading conditions was calculated from the spall thickness and found to be in the 9-18 GPa range

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