Observations of twinning microstructure in iron ramp-compressed through the α-ε phase transition

In situ x-ray diffraction experiments of single crystal iron ramp-compressed along the [100] axis at 4 GPa/ns through the α-ε phase transformation at 13 GPa were performed at the Dynamic Compression Sector. Unlike shock experiments, in these ramp compression experiments the bcc lattice is observed to be nearly isotropically relaxed before the phase transition. There was a mixed α/ε phase region starting at ~13 GPa which was observed for 0.75 ns until the peak stress of 18 GPa was reached. In situ x-ray diffraction measurements show the formation of new hcp orientations not reported in shock or quasistatic compression experiments. The new hcp orientations appear to be caused by sequential twins that occur during the phase transition. This twinning mechanism relieves the shear strain caused by the bcc-hcp phase transition from the isotrpically relaxed bcc phase while subject to uniaxial compression. These results show that in iron the induced microstructure through a phase transition and the phase transition mechanism depend on the loading history.