Picosecond X-ray diffraction studies of shocked crystals

Summary form only given. We present experimental time-resolved X-ray diffraction data that provide firm evidence that the response of single crystal silicon to nanosecond timescale uniaxial shock compression along the [400] axis is anomalous in that it is purely elastic. Despite compressions along this axis of nearly 11% - more than twice the previously accepted steady-state Hugoniot elastic limit (HEL) - the [040] diffraction data, which measure the unit cell spacing orthogonal to the shock propagation direction, confirm the lack of detectable atomic motion perpendicular to the shock front. For similar shock strengths, however, single crystal Cu shocked along the [200] axis exhibits compression of the unit cells along both [200] and [020], indicating a prompt transition to plastic flow. Furthermore, we present the results of a molecular-dynamics simulations of the diffraction of X-rays from shocked samples, which indicate that it may be possible to directly measure dislocation densities by observing the angular shifts associated with stacking faults within face-centred-cubic metals.