Ramp compression of tantalum to 330 GPa

We report on the stress-density and rate-dependent response for Ta, ramp compressed to 330GPa with strain rates up to 5x10(8)s(-1). We employ temporally shaped laser drives to compress Ta stepped foils over several to tens of nanoseconds. Lagrangian wave-profile analysis reveals a stress-density relationship which falls below the Hugoniot, above the hydrostat, and is consistent with ramp-compression experiments at lower strain rates. We also report on the peak elastic stress prior to plastic deformation as a function of strain rate for laser-driven ramp and shock-compression data spanning the 1-50x10(7)s(-1) strain-rate range. When combined with previously published lower strain data (10(1)-10(7)s(-1)), we observe a change in rate dependence, suggesting a transition from thermally activated to defect-limited (phonon drag) dislocation motion occurring at a strain rate of about 10(5)s(-1).