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 (gamma-fcc) to body-centered cubic structured martensite (alpha '-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) epsilon-martensite that is in the sequence of the gamma (fcc) -> epsilon (hcp) -> alpha ' (bcc), which followed the two-shearing mechanism proposed by the Bogers-Burgers-Olson-Cohen. Here, we reported the strain-dependent direct transformation of gamma (fcc) -> alpha ' (bcc) in addition to the gamma (fcc) -> epsilon (hcp) -> alpha ' (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 gamma (fcc) -> epsilon (hcp) transition is considered as the gliding of Shockley partial dislocations on every second (111)(gamma) plane. And the epsilon (hcp) -> alpha ' (bcc) transformation is executed by continuous lattice distortion at the single epsilon-plate, while by two-shearing at the intersection of two epsilon-plates. Moreover, the gamma (fcc) -> alpha ' (bcc) direct transformation was accomplished by the single-shearing on every (111)(gamma) planes. The results obtained here may enhance the insight understanding of martensitic transformation at atomic scale.