Strong variant selection observed in the α−ε martensitic transition of iron under quasihydrostatic compression along [111]α

The $\ensuremath{\alpha}$ (bcc) phase of single-crystalline iron was compressed along the ${[111]}_{\ensuremath{\alpha}}$ direction under quasihydrostatic and nonhydrostatic conditions. The emergence of the $\ensuremath{\epsilon}$ (hcp) phase variants via the pressure-induced martensitic transformation was investigated using x-ray diffraction and optical observations of the sample surface. In quasihydrostatic compression, the $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\epsilon}$ transition occurred at approximately 14 GPa accompanied by the sudden appearance of texture structures on the mirror-polished sample surface. The texture structure divided the sample surface into two characteristic regions: distorted regions with undulations and flat regions without undulations. Although martensitic transformation based on the Burgers model allows for the emergence of 12 $\ensuremath{\epsilon}$ variants from one single-crystalline phase, the transition proceeded according to the Burgers model, but selected variants preferably emerged depending on the regions in the texture structure. The flat region consisted of only three variants whose $c$ axes were parallel to the sample surface. The region near the undulation exhibited a large number of variants whose $c$ axes aligned parallel and nonparallel to the sample surface. The $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\epsilon}$ transition under nonhydrostatic compression deviated from the Burgers model, and the variant selection was not observed. This study demonstrates that the quasihydrostatic compression along ${[111]}_{\ensuremath{\alpha}}$ reduces the number of variants via strong variant selection. The strong preferred orientation is useful for investigating the anisotropy of physical properties in the $\ensuremath{\epsilon}$ phase.