Investigation of microstructure evolution and martensite transformation developed in austenitic stainless steel subjected to a plastic strain gradient: A combination study of Mirco-XRD, EBSD, and ECCI techniques.

The evolution of the microstructure and deformation mechanism at different levels of plastic strain are investigated for 304 L austenitic steel with a combination of micro X-ray diffraction (XRD), electron backscattered diffraction (EBSD) and electron channeling contrast imaging (ECCI). A plastic strain gradient is developed along the longitudinal rolling direction in a wedge-shaped 304 L austenitic steel sample. The graded deformed microstructure includes various amounts of deformation bands, ε- and α'-martensites, and their intersections form during the plastic deformation. ECCI observations reveal several deformation mechanisms of the formation of partial dislocations, dislocations, and interactions. This study suggests geometrically necessary deformation bands (GNDBs) are introduced and stored instead of geometrically necessary dislocations (GNDs) in low stacking fault energy (SFE) materials such as austenite steels. Consequently, increasing the strain gradient leads to an increase in the geometrically necessary martensitic transformation (GNMT); this is the result of the deformation-induced martensite in these materials. In addition to the statistically stored dislocations (SSDs), GNDs are generated at the grain boundaries of the fragmented grains to preserve the continuity of the grains. Accordingly, the strain hardening of the austenite steel includes multiple interactions of the deformation bands, SSDs, GNDs, GNDBs, and GNMT. From the viewpoint of microstructure design, our study provides quantitative information about the relationship between the amount of plastic deformation and the extent of microstructure evolution, in a continues design space.