Size-strain characteristics of lead and gold under fast ramp compression

Phase transitions in materials under fast ramp compression are an ongoing research topic, which is part of several global projects like inertial fusion. Currently, X-ray diffraction (XRD) examination of samples under fast ramp compression is limited to the determination of the sample phase state and the unit cell lattice parameters. Here, we propose to extend this examination route by introducing the Williamson-Hall analysis of the XRD data measured in samples under fast ramp rate conditions. To demonstrate the applicability of the method, we performed an analysis for ramp compressed lead $(P = 200 GPa)$ and gold $(P = 1003 GPa)$, which both exhibit a transition from the face-centred cubic (fcc) lattice to the body-centred cubic (bcc) lattice at the studied pressures. The analysis showed that lead under fast ramp compression has a nanocrystalline structure with a crystalline size of $D = (4 \pm 1) nm$ and lattice strain $\varepsilon = 0.006 \pm 0.002$. The effect of extreme hardening of bcc-Pb under fast ramp compression can be explained as the formation of an ultrafine grain structure in this metal. Elemental gold exhibits average crystalline size $D > 12 nm$ and unprecedentedly high, for a pure metallic element, lattice strain $\varepsilon = 0.014 \pm 0.001$ under fast ramp compression.