UC San Diego

Monocrystalline ([100], [110], [111] and [123]) and nanocrystalline (grain size ̃ 70 nm) tantalum were subjected to shock compression generated by high energy laser (up to ̃ 850 J), creating pressure pulses with initial duration of ̃3 ns and amplitudes of up to ̃ 145 GPa in the Omega facility (LLE, U. of Rochester). The laser beam, with a spot radius of ̃ 1 mm, created a crater of significant depth (̃ 200 [mu]m). Twins were observed beneath the crater surface (̃ 42 [mu]m) by back-scattered SEM in monocrystalline Ta. Transmission electron microscopy revealed profuse mechanical twinning within about 1.5 mm from the energy deposition surface at 684 J laser energy, corresponding to an approximate pressure of 35 GPa. Profuse omega (HCP) phase structures also been observed in [110] and [123] monocrystalline Ta at approximate pressure of 70 GPa. For nanocrystalline Ta, TEM revealed few dislocations within the grains and an absence of twins at the highest shock pressure (842 J). The decay of the pulse through the specimens was accompanied by an attendant decrease in the density of shock-generated dislocations. Dislocation densities as a function of pressure were calculated for the case of homogeneous nucleation and for Orowan hardening. The observed results are compared with predictions. Constitutive models for both homogeneous dislocation generation and Orowan dislocation multiplication were used. Nanocrystalline tantalum was prepared by severe plastic deformation (high pressure torsion) from [001] monocrystalline Ta. 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 as a result of thermally- induced grain growth. The experimentally measured dislocation densities and threshold stress for twinning 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. The predicted threshold pressure for twinning increases from 24 GPa for monocrystalline to 150 GPa for nanocrystalline tantalum. 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 to 800 nm.