Cyclic deformation and fatigue behavior of a fully austenitic stainless steel with a gradient nanostructure

Deformation-induced phase transformation significantly enhances the fatigue properties of gradient nanostructured (GNS) austenitic stainless steels, making it essential to understand their cyclic deformation and fatigue behavior when this transformation is suppressed. Here, the steady-state austenitic stainless steel, Fe-25Cr-20Ni, was used to fabricate GNS specimen by surface mechanical rolling treatment. Fully reversed strain-controlled tensioncompression fatigue experiment and uniaxial tension experiment were conducted on GNS and coarse-grained specimens at room temperature. Mechanical experimental results show that the GNS layer with a thickness of 1.4 mm increases the yield strength and tensile strength by 96.5% and 19.5%, respectively, and the hardness of the outermost layer is increased by 2.75 times compared to the CG specimen. The fatigue results show that the GNS specimen exhibits a ' hardening-softening ' feature and mean stress relaxation behavior, and the pseudo fatigue limit of GNS specimen increases by 58.3% compared to CG specimen. The enhanced fatigue strength in the low cycle fatigue regime results from the thicker GNS layer, while the poor fatigue ductility of the surface nanograins contributes to the deterioration of fatigue life. In the high cycle fatigue (HCF) regime, GNS specimens achieves the synergy of fatigue ductility and fatigue strength due to synergistic strengthening of multiple factors. Fractography show that the strain amplitude of 0.45% is a kink point at which cracks initiate from the surface and core. The GNS layer not only inhibits surface crack initiation in HCF regime, and alters the fatigue crack initiation location to delay crack propagation.