Elucidating deformation pathways and interface characteristic of self-accommodated dual γ/ε phase microstructure in Fe–Mn–Si–Al alloy

Abstract

There has been an intense scientific interest in investigating the phenomenon of deformation-induced ε-ε martensite (hcp) interaction owing to the thermodynamically paradoxical reverse transformation and mechanical twinning. In this study, detailed transmission electron microscopy has been employed to examine the crystallographic orientation relationship, phase stability and boundaries between the intersection phases on the ε-ε intersections in a 10% tensile deformed Fe–30Mn–4Si–2Al alloy. The transformation/twinning scheme is systematically summarized initiating from the austenite matrix (γ) and two deformation-induced ε-martensite variants. It involves diverse intersection reactions: mechanical ε-twins, a 90°-rotating γ-phase from the γ matrix (γR), and 90°-rotate ε-phase re-transformed from the intersection γ. All these phases share a common 〈101〉γ || <21¯1¯0>ε axis that is equivalent to the intersection axis of the crossing {111}<12¯1>γ shears and interrelated with rotational angles with respect to the axis. Electron diffraction, coupled with stereographic analysis, revealed the distinct orientation relationships between γ–ε and γR–ε phases. The boundaries between the intersection γR and neighboring ε-phase are inclined from the corresponding {111}γ||{101¯1}ε planes. By means of the phenomenological theory of martensite crystallography (PTMC), the inclination of the boundary is rationalized by considering the lattice-invariant shear on the double {111}γ plane inside the intersection γ to satisfy the invariant plane condition of the boundary.