Orientation dependence of the <i>α</i> –ε– <i>α</i> transition cycle in dynamically compressed polycrystalline iron samples

Iron samples with varying initial grain sizes were submitted to dynamical compression using laser ramp loading. Electron backscatter diffraction mappings of crystal orientations were performed in their cross section and evidenced the emergence of reversion variants (i.e., grains not present in the initial microstructure induced by the loading–unloading cycle) whose sizes scale with the initial grain size. The analysis procedure of these variants combines the comparison of their orientations with theoretical predictions from a graph of transformation sequences and the characterization of their inclinations in terms of kinematical compatibility conditions. It appears that reversion variants can be α–Fe twins or α–ε–α′ variants formed either within ε–Fe grains or at their boundaries. The coexistence of two ε–Fe variants is identified as a condition for the formation of α–ε–α′ variants. Stress inhomogeneities induced by α–Fe twinning are also highlighted.