The mechanisms of γ (fcc) → ε (hcp) → α′ (bcc) and direct γ (fcc) → α′ (bcc) martensitic transformation in a gradient austenitic stainless steel

Strain-induced martensitic transformation (SIMT) of face-centered cubic austenite (γ-fcc) to body-centered cubic structured martensite (α′-bcc) plays a crucial role in the controlling of the microstructure and properties of steels. So far, the SIMT is reported to be accomplished via the intermediate phase of hexagonal closed packed (hcp) ε-martensite that is in the sequence of the γ (fcc) → ε (hcp) → α′ (bcc), which followed the two-shearing mechanism proposed by the Bogers–Burgers–Olson–Cohen. Here, we reported the strain-dependent direct transformation of γ (fcc) → α′ (bcc) in addition to the γ (fcc) → ε (hcp) → α′ (bcc) sequence in a gradient austenitic 304 stainless steel. And we proposed the new mechanisms involved in the two transformation sequences. The atomic-scale observation via high-resolution transmission electron microscopy (HRTEM) found that the γ (fcc) → ε (hcp) transition is considered as the gliding of Shockley partial dislocations on every second (111) γ plane. And the ε (hcp) → α′ (bcc) transformation is executed by continuous lattice distortion at the single ε-plate, while by two-shearing at the intersection of two ε-plates. Moreover, the γ (fcc) → α′ (bcc) direct transformation was accomplished by the single-shearing on every (111) γ planes. The results obtained here may enhance the insight understanding of martensitic transformation at atomic scale.