Strain-induced martensite formation in austenitic stainless steel

In situ measurements of the strain-induced martensitic transformation (SMTs) of SUS304 stainless steel that takes place during tensile loading at room temperature were performed around the notch of a dumbbell-shaped specimen where high stress concentration occurs. Even in the low plastic strain regime, with loading to 0.2 % proof stress (sigma (0.2)), some SMTs occurred. However, the area fraction of the Fe-alpha'-martensite phase did not increase significantly even when the sample was loaded to the ultimate tensile strength (sigma (UTS)). After the sigma (UTS) point, the total fraction of the Fe-alpha' phase increased dramatically to the fracture point (sigma (f)). The phase textures of Fe-alpha' and Fe-gamma were almost equal at (sigma (UTS) - sigma (f))/2, and the Fe-alpha' phase was observed over almost the entire measurement area around the notch at the sigma (f) point. However, the area fraction of the Fe-alpha' phase at the sigma (f) point decreased far away from the fracture surface, to an extent that the total fraction of the Fe-alpha' phase was almost the same as that of the Fe-gamma phase in an area about 1.7 mm from the fracture face. Different martensite characteristics were detected in the stainless steel, depending on the applied load level. This was attributed to the severity of deformation. In particular, deformation twinning, created around sigma (UTS), and severe plastic deformation before fracture make a strong Fe-alpha' phase. Details of this phenomenon are interpreted using various approaches, including electron backscatter diffraction analysis and finite element analysis.