Professor Justin Wark

Abstract:

We report experiments in which powder-diffraction data were recorded from polycrystalline vanadium foils, shock-compressed to pressures in the range 10 – 70 GPa. Anisotropic strain in the compressed material is inferred from the asymmetry of Debye-Scherrer diffraction images, and used to infer residual strain and yield strength (residual von Mises stress) of the vanadium sample material. We find residual anisotropic strain corresponding to yield strength in the range 1.2 GPa – 1.8 GPa for shock pressures below 30 GPa, but significantly less anisotropy of strain in the range of shock pressures above this. This is in contrast to our simulations of the experimental data using a multi-scale crystal plasticity strength model, where significant yield strength persists up to the highest pressures we access in the experiment. Possible mechanisms that could contribute to the dynamic response of vanadium that we observe for shock pressures ≥ 30 GPa are discussed.

Abstract:

Using x-ray diffraction at the LCLS x-ray free electron laser, we have determined simultaneously and self-consistently the phase transitions and equation-of-state of the lightest transition metal, scandium, under shock compression. On compression scandium undergoes a structural phase transition between 32 and 35 GPa to the same bcc structure seen at high temperatures at ambient pressures, and then a further transition at 46 GPa to the incommensurate host-guest polymorph found above 21 GPa in static compression at room temperature. Shock melting of the host-guest phase is observed between 53 and 72 GPa with the disappearance of Bragg scattering and the growth of a broad asymmetric diffraction peak from the high-density liquid.

Abstract:

Recent experiments employing nanosecond white-light x-ray di↵raction have demonstrated a complex response of pure, single crystal silicon to shock compression on ultra-fast timescales. We present here details of a Lagrangian code which tracks both longitudinal and transverse strains, and successfully reproduces the experimental response by incorporating a model of the shock-induced, yet kinetically inhibited, phase transition. This model is also shown to reproduce results of classical molecular dynamics simulations of shock compressed silicon.