Laser compression of nanocrystalline tantalum

Nanocrystalline tantalum (grain size ∼70 nm) prepared by severe plastic deformation (high-pressure torsion) from monocrystalline [1 0 0] stock was subjected to shock compression generated by high-energy laser (∼350–850 J), creating pressure pulses with initial duration of ∼3 ns and amplitudes of up to ∼145 GPa. The laser beam, with a spot radius of ∼1 mm, created a crater of significant depth (∼135 μm). Transmission electron microscopy revealed few dislocations within the grains and an absence of twins at the highest shock pressure, in contrast with monocrystalline tantalum. 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 μm from the energy deposition surface as a result of thermally induced grain growth. The experimentally measured dislocation densities are compared with predictions using analyses based on physically based constitutive models, and the similarities and differences are discussed in terms of the mechanisms of defect generation. A constitutive model for the onset of twinning, based on a critical shear stress level, is applied to the shock compression configuration. The predicted threshold pressure at which the deviatoric component of stress for slip exceeds the one for twinning is calculated and it is shown that it is increased from ∼24 GPa for the monocrystalline to ∼150 GPa for the nanocrystalline tantalum (above the range of the present experiments). Calculations using the Hu–Rath analysis show that grain growth induced by the post shock-induced temperature rise is consistent with the experimental results: grains grow from 70 to 800 nm within the post-shock cooling regime when subjected to a laser pulse with energy of 684 J.