Identical Area Observations of Deformation-Induced Martensitic Transformation in SUS304 Austenitic Stainless Steel

In this study, an identical area of a SUS304 austenitic stainless steel specimen was observed by electron backscattering diffraction measurements at different strains in tensile test at ambient temperature, in order to investigate the details of deformation-induced martensitic transformation. Firstly, a number of thin deformation twins were formed in austenite grains. Most of the martensite crystals were observed either near grain boundary triple junctions or inside the deformation twins. Secondly, it was found that martensite crystals preferentially appeared in the austenite grains whose < 001 > crystal directions were close to the tensile direction. Furthermore, when austenite grains had several martensite crystals inside, only one or two variants were observed among 24 variants possible under Kurdjumov-Sachs orientation relationship, which indicated the existence of variant selection rules. Patel and Cohen model and Bogers-Burgers model were examined to understand the variant selection, but both models could not explain the variant selections. The result suggests that complicated stress states govern the deformation-induced martensitic transformation in polycrystalline austenite.