The γ→ε→α′ Martensitic Transformations in an Fe–Mn–C Alloy

Close-packed hexagonal ε and body-centered cubic (or tetragonal) α′ martensites in an Fe-12.0%Mn-0.48%C (wt %) alloy have been examined by optical and electron microscopy and electron and X-ray diffractions. The α′ martensites are clearly shown to form in ε martensite bands via the γ→ε→α′ transformation process, having six variants of orientations in one ε martensite. They make three pairs by two variants which contact to each other forming a junction plane. Two surface trace analysis shows that the junction plane is parallel to a {1100}ε (or {112}γ) plane which is perpendicular to the (0001)ε (or (111)γ) habit plane of the related ε martensite bands, and that habit planes of the paired two α′ martensites deviate from the {1100}ε junction plane in opposite ways by about 7°. Both the habit planes and crystallographic axes of the paired two martensites are twin-related to each other martensite with respect to the {211}α′ twin plane and also to the junction plane. (011)[011]α′ planar defects are found by electron microscopy in α′ martensites. The twin-related habit planes and crystallographic axes of paired two α′ martensites are well explained by a phenomenological calculation for the ε to α′ transformation using the (0001)[1100]ε (or (011)[011]α′) lattice invariant shear observed by electron microscopy and the lattice parameters determined by X-ray diffraction.