Effect of austenite stability on the low cycle fatigue behavior and microstructure of high alloyed metastable austenitic cast TRIP-steels

Room-temperature total strain controlled low-cycle fatigue tests were carried out on two types of high alloyed metastable austenitic cast TRIP steels with different chemical compositions resulting in different austenite stabilities. The cyclic stress response revealed combinations of cyclic hardening, saturation and cyclic softening, depending on the applied total strain amplitude. In the case of the more metastable TRIP steel a considerable amount of deformation-induced alpha'-martensite is responsible for a high degree of cyclic hardening. Conversely, the more stable TRIP steel shows also a high degree of hardening without a significant transformation of austenite into alpha'-martensite, however at significantly higher strain amplitudes. The deformation-induced alpha'-martensite was detected in situ with a feritscope sensor. EBSD measurements were performed to investigate the locations where the phase transformation from austenite to martensite takes place. It was observed that the martensitic transformation occurs preferentially inside deformation bands. The deformation microstructures were characterized by scanning electron microscopy. Dislocation structures were studied by ECCI (electron channelling contrast imaging). Planar and well developed cell/wall structures were observed, depending on the applied total strain amplitude. (C) 2010 Published by Elsevier Ltd.