KINETICS OF α'-MARTENSITE FORMATION DURING FATIGUE DEFORMATION IN METASTABLE AUSTENITIC STAINLESS STEEL

Abstract
The effect of applied total strain range on the critical condition necessary for the onset of α'-martensitic transformation kinetics during the fatigue deformation was studied in AISI type 304 metastable austenitic stainless steel at room temperature. In the case of fatigue deformation, the α'-martensite formation was observed even in the condition that the saturated stress amplitude of austenite phase is smaller than the critical applied stress for the onset of α'-martensite formation for the monotonic tensile deformation. Furthermore, the amount of α'-martensite increased with the number of cycles even in the saturated stage of austenite phase in which the stress amplitude of austenite phase is almost constant during the fatigue deformation. The α'-martensite formation was enhanced as the applied total strain range was increased. Such an enhancement of transformation was closely related with the increase in saturated stress amplitude of austenite phase due to the increase in applied total strain range. It was emphasized that the α'-martensite was preferentially induced in the near-surface layer of the specimen by the increase in local concentrated stress due to the increase in number of piled-up dislocations with the number of cycles even in the saturated stage of austenite phase.