Ultrafast X-ray diffraction measurements of shock-compressed iron and iron alloys

Abstract

The extreme pressures achievable with dynamic compression holds great promise for studying planetary interiors. Phase stability of Fe-Si alloys, which are complex to address, is particularly relevant to understanding telluric planetary cores due to the widely varying properties produced by small changes in Si concentration. Here we report the study of phase stability of pure iron and Fe-Si alloys by x-ray diffraction measurements carried out on shocked samples using an x-ray free electron laser (XFEL). Our setup combined with the brilliance of the XFEL allows us to observe the rapid onset of high-pressure solid-solid phase transformation in Fe and Fe-Si8.5wt%; we observe no such evidence in Fe-Si16wt% up to 110 GPa on the nanosecond timescale. Density Functional Theory calculations provide the conceptual framework to rationalize these observations. Taken together our experiments and calculations support recent dynamic compression measurements and shed light on conflicting static compression results. Our work highlights the need to properly consider the differing intrinsic timescales of the static and dynamic experiments when comparing results, and the complementarity of the techniques in assessing phase diagram and transition mechanisms.

Towards a dynamic compression facility at the ESRF

“…11(b)]. The addition of a small percentage of Si (3.5 wt%) to Fe thus seems to kinetically hinder the complete bcc-hcp phase transformation similar to recent X-ray diffraction measurements under laser compression that have shown no hcp phase transitions for Fe-16 wt%Si (Fischer & Campbell, 2015;Wicks et al, 2018;Edmund et al, 2019;Krygier et al, 2021). It should be noted that the S/N ratio in the FeSi alloy data is lower than in the FeNi alloy.…”