Geometrical Models and Matrices for Structures at Intersection of Crossing ε-Martensite Variants

Abstract

Deformation-induced martensitic transformation from face-centred-cubic γ-austenite to hexagonal close-packed ε-martensite and body-centred-tetragonal α'-martensite has attracted much attention owing to the transformation-induced plasticity effect in austenitic stainless steels, high- and medium-Mn austenitic steels, ferrous shape memory alloys, and high entropy alloys. For better understanding of the complicated triple γ/ε/α' phase deformation microstructure, various structures at the intersection of different variants of deformation-induced ε-martensite are examined. A type-316 austenitic steel single crystal is compressively deformed along the [0 0 1]_γ axis at a cryogenic temperature (173 K), and a deformation microstructure on the (1 1 0)γ surface is observed by means of scanning electron microscopy equipped with an electron backscattering diffraction analysis system. Depending on the shear angle with respect to the intersection axis, either 90º (Type I) or 30º (Type II), three types of atomic rearrangements of the intersection volume are observed: (1) reverse transformation into the γ-phase, which is rotated by 90º from the parent γ-austenite (Type I); (2) 10-12 twinning of either crossing ε-martensite variants (Type I); (3) secondary martensitic transformation into α'-martensite (Type II). Transformation matrices for the intersection products are built, which are then used to successfully calculate their crystallographic orientations starting from the orientation of the parent γ-phase. In addition, the atomic rearrangements and orientational changes are visualized by the distortion and/or kinking of Thompson's regular tetrahedron representing the initial atomic arrangement and the orientation of the parent γ-phase.

Keywords: Martensitic phase transformations, Deformation twinning, Crystallographic orientation, Electron backscattered diffraction (EBSD), Single crystal