Shock, impact, and ballistic loading include complex stress states, pressures of tens of GPa to Mbar, and high strain rate. In situ studies, in which the evolution of microstructure and properties is monitored during loading, are particularly difficult due to the short time scale of these experiments. Here, we use diamond anvil cell (DAC) techniques to compress nanomaterials and lightweight alloys under quasi-hydrostatic and non-hydrostatic stress states of tens of GPa pressures. X-ray diffraction is used to monitor structural changes in-situ, and post-compression TEM is used to directly image these changes.
Using these methods, we study defect activity within sub-10 nm metallic nanocrystals to understand the interaction of crystalline defects with surfaces and in confined volumes. It is found that stacking faults form in 4 nm Au nanocrystals under pressure, and remain in the nanocrystals after pressure is removed. In contrast, twins within 6 nm Au nanocrystals are removed during high-pressure compression, such that single crystalline nanocrystals are formed. These single crystalline nanocrystals were found to be metastable, as the nanocrystals reverted to the twinned state outside of the high-pressure environment.
We find that non-hydrostatic pressure leads to a significant increase in defect density, peak broadening with pressure cycling and the nucleation and growth of precipitates in precipitate hardened Al7075. This results in elevated strength at high pressures. Hydrostatic pressure leads to extensive detwinning in nanotwinned alloys with aligned twins. The mechanisms that result in these microstructural changes are analyzed.