Origin of phase stability in Fe with long-period stacking order as an intermediate phase in cyclic $\gamma$-$\epsilon$ martensitic transformation

TL;DR

This study investigates the origin of phase stability in Fe alloys with long-period stacking order, revealing an intermediate 6H$_2$ structure that is energetically close to the $ ext{epsilon}$ phase and explaining its stability through electronic density of states.

Contribution

The paper identifies the 6H$_2$ antiferromagnetic structure as a stable LPSO phase in Fe, providing insights into phase stability during martensitic transformation via first-principles calculations.

Findings

The 6H$_2$ structure is the most stable LPSO phase among candidates.

The LPSO phase is energetically close to the $ ext{epsilon}$ phase.

Phase stability is linked to the density of states near the Fermi level.

Abstract

A class of Fe-Mn-Si-based alloys exhibits a reversible martensitic transformation between the $\gamma$ phase with a face-centered cubic~(fcc) structure and an $\epsilon$ phase with a hexagonal close-packed (hcp) structure. During the deformation-induced $\gamma$--$\epsilon$ transformation, we identified a phase that is different from the $\epsilon$ phase. In this new phase, the electron diffraction spots are located at the 1/3 positions that correspond to the $\{$0002$\}$ plane of the $\epsilon$ (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 $\gamma$ and $\epsilon$ 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 6H$_2$ structure is the most stable among the candidate LPSO structures and is energetically close to the $\epsilon$ phase, which suggests that the observed LPSO-like phase adopts the 6H$_2$ 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.