Observations of twinning microstructure in iron ramp-compressed through the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>α</mml:mi><mml:mo>−</mml:mo><mml:mi>ε</mml:mi></mml:mrow></mml:math> phase transition.
J. Hawreliak, Stefan J. Turneaure
Abstract
Single crystal iron was ramp-compressed along the [100] axis at 4 GPa/ns through the $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\epsilon}$ phase transformation 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 $\ensuremath{\alpha}/\ensuremath{\epsilon}$ phase region starting at $\ensuremath{\sim}13$ 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 ${10\overline{1}1}\ensuremath{\langle}10\overline{1}\overline{2}\ensuremath{\rangle}\ensuremath{\rightarrow}{\overline{1}011}\ensuremath{\langle}\overline{1}01\overline{2}\ensuremath{\rangle}$ 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.