Martensitic-transformation-driven strain-softening in the cryogenic deformation of austenitic stainless steel

Austenitic stainless steel, which exhibits superior cryogenic mechanical properties, is considered the most promising material for hydrogen transport applications. However, the cryogenic deformation of austenitic stainless steel often leads to Lüders-type yielding, which can induce formability issues in materials. This study compared the temperature-dependent deformation mechanisms and mechanical properties of 304 austenitic stainless steel, focusing on the relationship between the deformation mechanisms and Lüders-type yielding. While the 293 K tensile curve exhibited a smooth elastic-plastic transition and continuous work hardening, the 123 K tensile curve showed pronounced yield point elongation with strain softening, followed by abrupt strain hardening and fracture. Tensile deformation at both temperatures exhibited a γ-α′ deformation-induced martensitic transformation (DIMT), with a much higher phase transformation rate at 123 K than at 293 K. In the Lüders-type yield range, dislocation plasticity and DIMT were the main deformation mechanisms at 293 K and 123 K, respectively. At 123 K, stacking faults and ε plates are formed under low stress levels, with α' martensite nucleated at the intersection of the multi-variant plate-type defects. The propagation of the Lüders band could accommodate plastic deformation by DIMT and result in strain softening of the material, indicating that the transformation-softening effect was larger than the hardening effect of the hard martensite. Enhancing γ-phase stability and suppressing or delaying DIMT could alleviate the occurrence of Lüders-type yielding, and future studies should address this issue.