Observations of twinning microstructure in iron ramp-compressed through the alpha - epsilon phase transition

Single crystal iron was ramp-compressed along the [100] axis at 4 GPa/ns through the alpha - epsilon phase transfor-mation at 13 GPa. 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 alpha/epsilon phase region starting at similar to 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 {101 (1) over bar}< 10 (1) over bar(2) over bar > -> {(1) over bar 011}<(1) over bar 01 (2) over bar > sequential twins that occur during the phase transition. This twinning mechanism relieves the shear strain caused by the bcc-hcp phase transition from the isotropically 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.