Observation of the shear mode of the ε → τ phase transformation in a Mn-Al-C single crystal

An ε-phase Mn-Al-C single crystal was grown and annealed to obtain an intermediate stage of the ε → τ transformation. The crystal was analysed by transmission electron microscopy, and direct evidence for the operation of the shear mode was obtained. The transformation occurs in two steps from the hcp structure (ε) over the orthorhombic structure (ε′ phase) to the tetragonal τ phase. The study reveals the morphologies of ε′ and τ, the necessity for reordering during the second transformation step (ε′ → τ) which is not a purely martensitic transformation, and the selection of τ variants which is controlled by stresses through the selection of one ε′ variant.