Laser Compression of Nanocrystalline Tantalum

Nanocrystalline tantalum (g.s. $\sim$70 nm) prepared by severe plastic deformation (HPT) from monocrystalline [100] stock was subjected to high energy laser driven shock compression (up to $\sim$850 J), generating a pressure pulse with initial duration of ? 3 ns and amplitude of up to $\sim$145 GPa. TEM revealed few dislocations within the grains and an absence of twins at the highest shock strengths, in contrast with monocrystalline tantalum, which exhibited twinning at P \textgreater\ $\sim$45 GPa. Hardness measurements were conducted and show a rise as the energy deposition surface is approached, evidence of shock-induced defects. The grain size was found to increase at a distance of 100 $\mu$m from the energy deposition surface as a result of thermally induced grain growth. Calculations using the Hu-Rath analysis are consistent with the experimental results. The experimentally measured dislocation densities and threshold stress for twinning are compared with predictions using analyses based on physically-based constitutive models. The predicted threshold stress for twinning increases from $\sim$45 GPa for the monocrystalline to $\sim$165 GPa for the nanocrystalline tantalum.