High-Pressure α⇄ε Martensitic Transformation in Iron

A study of the high-pressure transformation in iron using an opposed-anvil x-ray diffraction apparatus and high-pressure light metallography has shown that the 130-kbar transformation is martensitic. The bcc and hcp phases were found to coexist over a large pressure range, and there is a large hysteresis between the forward and reverse transformation-start pressures. A room-temperature equilibrium pressure for the bcc and hcp phases of 107 ± 8 kbar is proposed, and discrepancies in the current P-T equilibrium diagram for iron are discussed.