Microstructural effects and mechanism of bcc-hcp-bcc transformations in polycrystalline iron
Sébastien Merkel, Ainhoa Lincot, Sylvain Petitgirard
Abstract
Cycling the $\ensuremath{\alpha}\ensuremath{\leftrightarrow}\ensuremath{\epsilon}$ transformation in polycrystalline Fe is investigated using in situ x-ray diffraction under quasihydrostatic conditions. The forward $\ensuremath{\alpha}\ensuremath{\rightarrow}\ensuremath{\epsilon}$ transformation starts at $14\ifmmode\pm\else\textpm\fi{}1$ GPa and completes at $18\ifmmode\pm\else\textpm\fi{}1$ GPa while the reverse $\ensuremath{\epsilon}\ensuremath{\rightarrow}\ensuremath{\alpha}$ transformation starts at $10.5\ifmmode\pm\else\textpm\fi{}0.5$ GPa and completes at $6\ifmmode\pm\else\textpm\fi{}1$ GPa. The anomalous evolution of $c/a$ ratios of $\ensuremath{\epsilon}$-Fe measured at the onset of the $\ensuremath{\alpha}\ensuremath{\rightarrow}\ensuremath{\epsilon}$ transition in earlier studies is not observed. Microstructural features are consistent with a Burgers path for the transformation. The forward $\ensuremath{\alpha}\ensuremath{\rightarrow}\ensuremath{\epsilon}$ transformation is sensitive to the average and intergranular stresses, inducing a strong variant selection with the $c$ axes of the new $\ensuremath{\epsilon}$ phase preferentially aligned perpendicular to compression and concentrated into one single orientation. Partial texture memory is observed as the sample returns to the $\ensuremath{\alpha}$ phase but, as transformation cycles go on, irreversible changes occur in the microstructures that are later fully induced by the cycles of phase transformations.