An atomistic model for the fcc→bcc transformation in iron with the Nishiyama–Wasserman orientation relationship: Insight from the Quasi-particle Approach

In this work, the Quasi-particle Approach (QA) is applied to study the displacive fcc ( γ ) → bcc ( α ) transformation in pure iron, in the case of the Nishiyama–Wasserman (NW) orientation relationship with ( 232 ) γ habit plane (HP). For the first time, the Kurdjumov–Sachs (KS) inhomogeneous shuffle shear transformation path is observed for a HP deviated from ( 111 ) γ . It is found that the correct NW OR is produced by the concurrent action of the two twin-variants of the KS mechanism. This composite process manifest itself through a specific interface structure consisting in coherent terraces in the ( 111 ) γ plane, delineated by terrace steps in two different crystallographic directions, and a particular interface propagation mode carried by the kink assisted glide of two sets of screw dislocations bordering terrace steps. These results provide new insights into the atomic-scale processes governing fcc → bcc phase transformations, highlighting the critical role of the transformation pathway on the interface structure and propagation mode.