Origin of phase stability in Fe with long-period stacking order as an intermediate phase in cyclic γ−ε martensitic transformation

Abstract

A class of Fe-Mn-Si--based alloys exhibit a reversible martensitic transformation between the $\ensuremath{\gamma}$ phase with a face-centered cubic (fcc) structure and an $\ensuremath{\epsilon}$ phase with a hexagonal close-packed (hcp) structure. During the deformation-induced $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\epsilon}$ transformation, we identified a phase that is different from the $\ensuremath{\epsilon}$ phase. In this phase, the electron diffraction spots are located at the 1/3 positions that correspond to the ${0002}$ plane of the $\ensuremath{\epsilon}$ (hcp) phase with 2H structure, which suggests long-period stacking order (LPSO). To understand the stacking pattern and explore the possible existence of an LPSO phase as an intermediate between the $\ensuremath{\gamma}$ and $\ensuremath{\epsilon}$ phases, the phase stability of various structural polytypes of iron was examined using first-principles calculations with a spin-polarized form of the generalized gradient approximation in density functional theory. We found that an antiferromagnetic ordered $6{\mathrm{H}}_{2}$ structure is the most stable among the candidate LPSO structures and is energetically closest to the $\ensuremath{\epsilon}$ phase, which suggests that the observed LPSO-like phase adopts the $6{\mathrm{H}}_{2}$ structure. Furthermore, we determined that the phase stability can be attributed to the valley depth in the density of states, close to the Fermi level.