In situx-ray diffraction of fast compressed iron: Analysis of strains and stress under non-hydrostatic pressure

Series of high-pressure x-ray diffraction patterns of iron and its high-pressure polymorphs were collected with \n0.1–0.2-s exposure time utilizing a membrane diamond anvil cell (DAC) for compression at various loading and \nunloading rates to a maximum pressure of 70 GPa. Strain rates of 10−2 s−1 at a maximum pressurization rate \nof 4.1 GPa/s were achieved in non-hydrostatic compression of hcp Fe. Linewidth analysis was used to retrieve \nstrain and uniaxial stress of Fe as a function of pressure upon both compression and decompression. Analysis of \nthe lattice parameters ratio c/a of hcp Fe indicates the presence of complex non-hydrostatic stress states, which \ndeveloped as a function of strain rate, relaxation time, and various levels of hydrostaticity. Our results emphasize \nthe importance of a controlled pressurization in DACs because the experimental loading rate strongly influences \nthe stress state of the sample, particularly on decompression. Our time-resolved x-ray diffraction of the phase \ntransition from bcc Fe to hcp Fe reveals residual grains of bcc Fe capable of surviving to very high pressures \n(>35 GPa) for a few minutes after the transition.