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

• Published in: Journal of materials science 2022-02, Vol.57 (8), p.5230-5240
• Main Authors: He, Yinsheng, Gao, Jianbing, He, Yizhu, Shin, Keesam
• Format: Article
• Language: English
• Subjects: Austenitic stainless steels; Characterization and Evaluation of Materials; Chemistry and Materials Science; Classical Mechanics; Crystallography and Scattering Methods; Dislocations; High resolution electron microscopy; Martensite; Martensitic stainless steels; Martensitic transformations; Materials Science; Metals & Corrosion; Polymer Sciences; Shearing; Solid Mechanics; Stainless steel
• ISSN: 0022-2461; 1573-4803
• DOI: 10.1007/s10853-022-06936-z

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.