Plastic flow, inferred strength, and incipient failure in BCC metals at high pressures, strains, and strain rates

We present our extensive experimental results from the Omega laser to test models of high pressure, high strain rate strength at $\sim$ 1 Mbar peak pressures, strains \textgreater 10{\%}, and strain rates of $\sim$ 10$^{7}$ s$^{-1}$ in Ta, V, and Fe, using plastic flows driven by the Rayleigh-Taylor instability. The observed time evolution of the plastic deformation is compared with 2D simulations incorporating a strength model. This methodology allows average values of strength at peak pressure and peak strain rate conditions to be inferred. The observed values of strength are typically factors of 5-10 higher than ambient strength, with contributions coming from pressure hardening (via the shear modulus), and strain rate hardening. For Fe, there is the added contribution from the alpha-epsilon phase transition. Ta has been studied as a function of grain size, and at the high strain rates and short durations of the experiments, no grain size dependence was observed; the observed deformation and inferred strength were independent of grain size. Both Ta and V have been driven to large enough strains that incipient failure (softening) has been observed. Both the Ta and V experiments were compared favorably with multiscale strength models, with the conclusion that the Ta deformation was in the thermal activation regime, whereas the V deformation was in the phonon drag regime. Finally, preliminary results of new iron RT strength experiments done at $\sim$ 1 Mbar pressures, and $\sim$ 10$^{7}$ s$^{-1}$ strain rates, well beyond the alpha-epsilon phase transition, will be given.