Enhanced strength-ductility synergy and deformation mechanisms of heterostructural 316 LN austenitic stainless steel

Austenitic stainless steels (ASSs) are extensively employed in civil and defense industries due to their excellent corrosion resistance and formability, yet the relatively low yield strength limits applications in cryogenic environments. This study investigates the strength-ductility trade-off through rolling at liquid nitrogen temperature (LNT) followed by annealing to produce three characteristic microstructures: nanograined (NG), heterogeneous structure (HS), and coarse-grained (CG). The HS structure, characterized by a bimodal grain size distribution, demonstrates superior mechanical performance at both room and cryogenic temperatures, achieving a yield strength of 1226 MPa with 23.2% elongation under LNT. This performance arises from synergistic back-stress strengthening, grain refinement, and the activation of multiple deformation mechanisms including dislocation slip, stacking faults, twinning, and strain-induced martensitic transformation. Furthermore, stacking fault energy (SFE) critically governs the dominant deformation mechanisms, decreasing with lower temperature and larger grain size, thereby promoting extensive ε- and α'- martensitic transformation under cryogenic condition while suppressing it at room temperature. The bimodal grain distribution introduces abundant heterogeneous interfaces and nucleation sites, enabling 96.3% martensitic transformation, which directly enhances work hardening, promotes effective strain partitioning, and locally stabilizes the retained FCC austenite.