Direct observation of shock-induced phase transformation in polycrystalline iron via in situ x-ray diffraction

Phase transition of polycrystalline iron compressed along the Hugoniot is studied by combining laser-driven shock with in situ x-ray diffraction technique. It is suggested that polycrystalline iron changes from an initial body-centered cubic structure to a hexagonal close-packed structure with increasing pressure (i.e., a phase transition from alpha to epsilon). The relationship between density and pressure for polycrystalline iron obtained from the present experiments is found to be in good agreement with the gas-gun Hugoniot data. Our results show that experiments with samples at lower temperatures under static loading, such as in a diamond anvil cell, lead to higher densities measured than those found under dynamic loading. This means that extrapolating results of static experiments may not predict the dynamic responses of materials accurately. In addition, neither the face-centered cubic structure seen in previous molecular-dynamics simulations or two-phase coexistence are found within our experimental pressure range.