X-ray diffraction study of phase transformation dynamics of Fe and Fe-Si alloys along the shock Hugoniot using an x-ray free electron laser

The x-ray free electron laser (XFEL) enables probing a highly compressed material response at the subnanosecond timescale. We exploit the ultrafast XFEL pulse to combine reflection x-ray diffraction and laser-driven shock compression to perform a study of the phase transformation and stability in Fe and Fe-Si alloys. Our approach enables us to observe that solid-solid phase transformations occur in Fe and Fe-${\mathrm{Si}}_{8.5\phantom{\rule{0.28em}{0ex}}\mathrm{wt}\phantom{\rule{0.16em}{0ex}}%}$ in $\ensuremath{\le}130$ ps at $\ensuremath{\sim}130$ GPa; no transformation is observed in Fe-${\mathrm{Si}}_{16\phantom{\rule{0.28em}{0ex}}\mathrm{wt}\phantom{\rule{0.16em}{0ex}}%}$ up to 110 GPa. Density functional theory calculations predict similar phase relations.