Neutron Diffraction Study of Low-Cycle Fatigue Behavior in an Austenitic–Ferritic Stainless Steel

By performing in situ neutron diffraction experiments on an austenitic–ferritic stainless steel subjected to low-cycle fatigue loading, the deformation heterogeneity of the material at microscopic level has been revealed. Based on the in situ neutron diffraction data collected from a single specimen together with the mechanical properties learned from the ex situ micro-hardness, a correlation has been found. The performance versus diffraction-profile correlation agrees with the cyclic-deformation-induced dislocation evolution characterized by ex situ TEM observation. Moreover, based on the refined neutron diffraction-profile data, evident strain anisotropy is found in the austenite. The high anisotropy in this phase is induced by the increase in dislocation density and hence contributes to the hardening of the steel at the first 10 cycles. Beyond 10 fatigue cycles, the annihilation and the rearrangement of the dislocations in both austenitic and ferritic phases softens the plastically deformed specimen. The study suggests that the evolution of strain anisotropy among the differently oriented grains and micro-strain induced by lattice distortion in the respective phases mostly affect the cyclic-deformation-induced mechanical behavior of the steel at different stages of fatigue cycles. The stress discrepancy between phases is not the dominant mechanism for the deformation of the steel.