Self-Accommodated Dual Γ/Ε Phase Deformation Microstructure Accompanying a Reverse Transformation of Deformation-Induced Ε-Martensite

Abstract

We previously reported on crossing {111} γ shears at the intersection of deformation-induced hexagonal close packed e-martensite causing an abnormal (thermodynamically paradoxical) reverse transformation and mechanical twinning. In the present study, detailed transmission electron microscopy on the e-e intersections in a 10% tensile deformed Fe-30Mn-4Si-2Al alloy has been employed to examine the crystallographic orientation relationship, phase stability and boundaries between the intersection phases. The transformation/twinning scheme is systematically summarized in a tree diagram initiating from the γ matrix and two deformation-induced e-martensite variants, diversifying into mechanical e twins, γ-phase at the intersection which is 90o-rotated from the γ matrix, and 90o-rotated e-phase re-transformed from the intersection γ. All these phases share a common γ || e axis that is equivalent to the intersection axis of the crossing {111} γ shears and interrelated with rotational angles with respect to the axis. The boundaries between the intersection γ and neighboring e are inclined from the corresponding {111}γ || {10-10}e planes. By means of the phenomenological theory of martensite crystallography (PTMC), the inclination of the boundary is rationalized considering the lattice-invariant shear on the double {111}γ plane inside the intersection γ to satisfy the invariant plane condition of the boundary.