Yielding of tantalum at strain rates up to 109 s-1

We have used a 45 mu J laser pulse to accelerate the free surface of fine-grained tantalum films up to peak velocities of similar to 1.2 km s(-1). The films had thicknesses of similar to 1-2 mu m and in-plane grain widths of similar to 75-150 nm. Using ultrafast interferometry, we have measured the time history of the velocity of the surface at different spatial positions across the accelerated region. The initial part of the histories (assumed to correspond to the elastic precursor observed previously) exhibited measured strain rates of similar to 0.6 to similar to 3.2 x 10(9) s(-1) and stresses of similar to 4 to similar to 22GPa. Importantly, we find that elastic amplitudes exhibit little variation with strain rate for a constant peak surface velocity, even though, via covariation of the strain rate with peak surface velocity, they vary with strain rate. Furthermore, by comparison with data obtained at lower strain rates, we find that amplitudes are much better predicted by peak velocities rather than by either strain rate or sample thickness. Published by AIP Publishing.