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

A class of Fe-Mn-Si based alloys was found to exhibit a reversible martensitic transformation between the gamma (fcc) and epsilon (hcp) phases. During the deformation-induced gamma-epsilon transformation, a new phase was identified that is different from the epsilon-phase, where the electron diffraction spots are located at the 1/3 position corresponding to the hcp (2H) phase, which suggests long-period stacking order (LPSO). To understand the actual stacking pattern and discuss the possible realization of a LPSO phase as the intermediate phase 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 6H2 structure is the most stable among candidate LPSO structures and is energetically close to the epsilon phase, which suggests that the observed LPSO-like phase adopts the 6H2 structure. We determined that the origin of the phase stability can be attributed to the depth of the deep valley in the density of states close to the Fermi level.