Unveiling the transformation pathways of hierarchical 90-twin - ' triple phase structure formation at - martensite intersection

Deformation-induced martensitic transformation (gamma-austenite -> epsilon-martensite/alpha'-martensite) in austenitic steels has garnered significant interest owing to its transformation-induced plasticity effect. To elucidate the orientation-dependent intricate gamma/epsilon/alpha' phase microstructure at deformation-induced epsilon-epsilon intersection, a single crystal of 316 austenitic stainless steel was compressed along the [001]gamma axis at a cryogenic temperature (173 K). Electron backscattered diffraction analysis was employed to reveal the deformed microstructure on the (110)gamma surface. A hierarchical triple phase structure was discovered at epsilon-epsilon intersection, where the gamma rotated 90 degrees from the matrix (gamma 90), {1012} epsilon-twin, and alpha' phases coexist. Depending on the operative shear angle with a common intersection axis, either 90 degrees (Type I) or 30 degrees (Type II), three distinct atomic rearrangements of the intersection volume were observed: gamma 90 was present at Type I intersection, and alpha'-phase was developed at Type II intersection, respectively. {1012} epsilon-twin also occurred at Type I intersection, serving as an accommodation mechanism alongside the intersection gamma 90. EBSD results confirm the Shoji-Nishiyama (S-N), Kurdjumov-Sachs (K-S), and Burgers (B) orientation relationships within the complex gamma - epsilon - alpha' triple phase structures at epsilon-epsilon intersection. Transformation paths for three intersection products were visualized by the unified tetrahedron model, considering T/2 or T/3 gamma-twinning shear as an intermediate state. Moreover, a novel scheme is proposed to index the alpha'-martensite crystallographic variants at epsilon-epsilon intersections by establishing a correlation between the Bain distortion and double shear process.