Microstructural effects and mechanism of bcc-hcp-bcc transformations in polycrystalline iron

Cycling the alpha <-> epsilon transformation in polycrystalline Fe is investigated using in situ x-ray diffraction under quasihydrostatic conditions. The forward alpha -> epsilon transformation starts at 14 +/- 1 GPa and completes at 18 +/- 1 GPa while the reverse epsilon -> alpha transformation starts at 10.5 +/- 0.5 GPa and completes at 6 +/- 1 GPa. The anomalous evolution of c/a ratios of epsilon-Fe measured at the onset of the alpha -> epsilon transition in earlier studies is not observed. Microstructural features are consistent with a Burgers path for the transformation. The forward alpha -> epsilon transformation is sensitive to the average and intergranular stresses, inducing a strong variant selection with the c axes of the new epsilon phase preferentially aligned perpendicular to compression and concentrated into one single orientation. Partial texture memory is observed as the sample returns to the alpha phase but, as transformation cycles go on, irreversible changes occur in the microstructures that are later fully induced by the cycles of phase transformations.