Structural Stability of Shock Compressed Noble Metals: Role of Microstructural Changes

The structural stability of noble metals to very high pressures under static and shock compression has been widely accepted for a long time, resulting in their use as pressure markers. We present a summary of in situ x-ray diffraction (XRD) results – obtained at the Dynamic Compression Sector located at the Advanced Photon Source – on shock compressed Au, Ag, and Cu that reveal a fcc to bcc transformation with an onset stress between ~150-180 GPa; the fcc-bcc transformation was not observed for Pt shock compressed to over 380 GPa. Furthermore, the XRD results for Au, Ag, and Cu show a copious increase in stacking faults (SFs) before transformation to the bcc structure. In contrast, shock compressed Pt – having much higher SF energy, compared to the other three metals – remains largely free of SFs and retains the fcc structure. These findings suggest that SF formation promotes the fcc-bcc transformation in shock compressed noble metals. Therefore, the role of deformation-induced microstructural changes – an integral aspect of shock compression – needs to be carefully considered in high pressure structural transformations.