Deformation response and microstructure evolution in 304LN stainless steel subjected to multiaxial fatigue loading under different strain paths

Axial-torsion low cycle fatigue (LCF) experiments were conducted on 304LN stainless steel under in-phase triangular (IPT), 90° out-of-phase triangular (OPT), 90° out-of-phase sinusoidal (OPS), and 90° out-of-phase trapezoidal (OPZ) loading paths with same applied axial strain ($\epsilon$) and equivalent shear strain amplitude ($\frac{\gamma }{\sqrt{3}}$), to study the material's cyclic stress response (CSR) under each strain path and to further correlate the responses with dislocation substructure and martensite formation. The CSRs exhibited primary hardening followed by softening and a secondary hardening. Both primary and secondary hardening were found to increase, and the softening was found to decrease as per the following sequence IPT < OPT < OPS < OPZ. Electron backscattered diffraction (EBSD) revealed that both deformation-induced Martensite (DIM) fraction and Kernel average misorientation (KAM) increases in the same sequence for different paths, indicating the tendency of formation of DIM increases and the propensity of recovery decreases in the same order. The cyclic stress response and DIM formation under non-proportional loadings are rationalized by (i) higher non-proportionality of strain path for OPS than OPT for same equivalent strain amplitude, and (ii) greater equivalent strain amplitude in OPZ than OPS. A greater fraction of planes subjected to combined resolved shear and tensile stress under OPS path than OPT path leads to a greater DIM formation in the OPS path than OPT path, reflecting the effect of non-proportionality factor. A greater magnitude of resolved shear and normal strain, and high tensile normal stress causes higher DIM formation and hardening in the OPZ path as compared to OPS path, reflecting the effect of equivalent strain amplitude.