Deformation microstructures and martensitic transformation pathways in cryogenically deformed 316L stainless steel

Abstract Liquefied gas storage and transportation, as well as space propulsion, are driving increasing interest in the cryogenic temperature deformation behaviour of 316L stainless steels. This was investigated here during tensile deformation at 15, 50 and 173 K. Complex transformation pathways, including one-step γ-austenite → α′-martensite, two steps γ → ε-martensite → α′ transformation as well as twinning and stacking fault-assisted γ → α′ transformation, are observed. γ grains with a (111) plane normal direction aligned 50–65°from the loading direction appear more likely to form the ε phase. Further, high-resolution transmission Kikuchi diffraction mapping revealed that the nucleation process of α′ can be assisted by ε and stacking faults at all cryogenic temperatures, whereas twins can also serve as sites for α′ nucleation when deformed at 173 K. For two-step transformation, separate lenticular α′ nucleate following Kurdjumov–Sachs orientation relationship (OR) within the shear band, once grown out of the shear band, Pitsch OR is preferable. As for one-step transformation, irregular oval α′ nucleates directly at γ grain boundaries with Nishiyama–Wassermann OR. These findings provide new insights into the correlation between the various transformation pathways and deformation mechanisms, as well as their improved performance at low temperatures.