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 γ phase with a face-centered cubic (fcc) structure and an ϵ phase with a hexagonal close-packed (hcp) structure. During the deformation-induced γ-ϵ transformation, we identified a phase that is different from the ϵ phase. In this phase, the electron diffraction spots are located at the 1/3 positions that correspond to the {0002} plane of the ϵ (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 γ and ϵ 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 6H2 structure is the most stable among the candidate LPSO structures and is energetically closest to the ϵ phase, which suggests that the observed LPSO-like phase adopts the 6H2 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.