Grain-orientation-dependent phase transformation kinetics in austenitic stainless steel under low-temperature uniaxial loading

Deformation-induced martensites generally follow the principle on the selection of their variants in intrinsic crystallographic orientations with regard to the parent grains, which should be significantly affected by the cooperative rotation of the matrix. In this paper, the microstructural changes related to the deformation-induced transformation from metastable γ austenite to ε and α′ martensites in 304 austenitic stainless steel upon uniaxial tensile loading at 180 K was investigated by employing in-situ synchrotron-based high-energy X-ray diffraction technique. The detailed information on low-temperature phase transformation kinetics was analyzed in terms of the grain rotation and the change in phase volume, stress partitioning, and dislocation density, which were further compared with experimental observations for the room temperature deformed specimen almost without stress-induced martensite. The elastic strain measured in the newly formed α′ martensite was quite low (only ~200 μɛ) upon tensile loading due to stress relaxation, evidencing the role of α′ martensite nucleation in strain accommodation. The minor statistical evolution of texture for all constituent phases, in combination with the martensitic variant selection principle, enables us to reveal the complex interactions of deformation and transformation, finding that the ε martensite firstly originated in the [0 0 1]//LD-oriented grains of γ matrix, while α′ martensite was initially formed in the [1 1 1]//LD-oriented γ grains. Furthermore, the interplay of phases enhanced the grain rotation toward [0 0 1] during deformation at 180 K, which could be elucidated by the influence of transformed martensites on the specific selection of slip systems.