Mechanism of theα−εphase transformation in iron

The $\ensuremath{\alpha}\text{-Fe}\ensuremath{\leftrightarrow}\ensuremath{\epsilon}\text{-Fe}$ pressure-induced transformation under pure hydrostatic static compression has been characterized with in situ x-ray diffraction using $\ensuremath{\alpha}\text{-Fe}$ single crystals as starting samples. The forward transition starts at 14.9 GPa, and the reverse at 12 GPa, with a width of $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\epsilon}$ coexistence domain of the order of 2 GPa. The elastic stress in the sample increases in this domain, and partially relaxes after completion of the transformation. Orientation relations between parent $\ensuremath{\alpha}\text{-Fe}$ and child $\ensuremath{\epsilon}\text{-Fe}$ have been determined, which definitely validates the Burgers path for the direct transition. On the reverse transition, an unexpected variant selection is observed. X-ray diffraction data, complemented with ex situ microstructural observations, suggest that this selection is caused by defects and stresses accumulated during the direct transition.