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

Abstract

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

Ultrafast X-ray Diffraction Study of a Shock-Compressed Iron Meteorite above 100 GPa

“…Specifically, the high strain rate and short timescale of these experiments inhibit defect and dislocation motion, such that the interplanar spacing of the (011) planes, which are perpendicular to the shock propagation direction, are completely unchanged during compression. In this case, the [100] of a bcc lattice remains less compressed and larger than expected as it becomes the c-axis in an hcp lattice and makes it possible to get an apparently large c/a ratio [40]. Our experimental data show a distribution in the c/a ratio-some instances with larger than, similar to, or less than the ideal ratio.…”

Formation of Expanded Phases in Ferritic Stainless Steel Nitrided at Low Temperatures

“…To the best of our knowledge, it is the first study that reports the formation of this "expanded" ε′-martensite in low-temperature nitrided ferritic stainless steels. In pure iron, bcc-to-hcp martensitic transformation was reported to occur when high static or dynamic compressive stresses were applied [37][38][39][40][41]. Very high stress, 14-15 GPa, was required to start the transformation at room temperature [37][38][39], while a decrease of the needed pressure up to 4.1 GPa was reported as temperature increased up to 297 • C [39].…”

“…In pure iron, bcc-to-hcp martensitic transformation was reported to occur when high static or dynamic compressive stresses were applied [37][38][39][40][41]. Very high stress, 14-15 GPa, was required to start the transformation at room temperature [37][38][39], while a decrease of the needed pressure up to 4.1 GPa was reported as temperature increased up to 297 • C [39]. A decrease of the transformation pressure was observed also when Fe was alloyed with Cr, suggesting that this element tended to stabilize the hcp phase [42].…”

Strategic sampling with stochastic surface walking for machine learning force fields in iron's bcc–hcp phase transitions

“…Since then, scientists have conducted theoretical and experimental research to better understand the microscopic mechanism involved. [6][7][8][9][10][11][12] Based on the extended X-ray absorption ne structure (EXAFS) technique, Wang and Ingalls 13 initially proposed three possible microscopic mechanisms for the phase transition from bcc to hcp. The rst two mechanisms involve different paths of bcc-to-hcp transition, while the third mechanism involves a path from bcc to an intermediate fcc phase and then to hcp phase.…”

“…Since then, scientists have conducted theoretical and experimental research to better understand the microscopic mechanism involved. 6–12…”