Microstructure and martensitic transformation in 316L austenitic steel during multiaxial low cycle fatigue at room temperature

Austenitic stainless 316L steel in cold worked condition was subjected to torsional cyclic loading with equivalent strain amplitudes of 0.3% and 0.4% and multiaxial (in-phase and out-of-phase) cyclic loading with equivalent strain amplitude of 0.35% at room temperature. Cyclic plastic response has been measured and analyzed. Internal structure in the as-received state was compared with the deformation microstructures that developed during cyclic straining with various strain histories. In all three investigated cases the volumes with dislocation structures corresponding to cyclic strain localization (persistent slip bands, irregular wall and channel structures) were found. In addition, also islands of deformation induced alpha'-martensite were observed. Transmission electron microscopy, selected area electron diffraction and ferritscope measurements allowed to estimate the fraction and distribution of alpha'-martensite islands. The shape of fatigue hardening/softening curves has been discussed in relation with the localization of cyclic plastic strain in the low energy dislocation structures and its blockade by the islands of alpha'-martensite.