Lattice-level observation of the elastic-to-plastic relaxation process with subnanosecond resolution in shock-compressed Ta using time-resolved in situ Laue diffraction

We report direct lattice-level measurements of plastic relaxation kinetics through time-resolved, in situ Laue diffraction of shock-compressed single-crystal [001] Ta at pressures of 27-210 GPa. For a 50-GPa shock, a range of shear strains is observed extending up to the uniaxial limit for early data points (<0.6 ns), and the average shear strain relaxes to a near steady state over similar to 1 ns. For 80- and 125-GPa shocks, the measured shear strains are fully relaxed already at 200 ps, consistent with rapid relaxation associated with the predicted threshold for homogeneous nucleation of dislocations occurring at shock pressure similar to 65 GPa. The relaxation rate and shear stresses are used to estimate the dislocation density, and these quantities are compared to the results of other high-pressure work, flow stress models, and molecular dynamics simulations.