Fatigue behaviors of 2205 duplex stainless steel with gradient nanostructured surface layer

Fatigue behaviors of 2205 duplex stainless steel (initially composed of austenite and ferrite phase) with a gradient nanostructured (GNS) surface layer induced by the ultrasonic rolling process (USRP), are investigated in both strain-controlled high-cycle fatigue (HCF) and low-cycle fatigue (LCF) tests. Results showed that the fatigue life is improved in HCF (-5 times longer under 0.3% strain) but decreased in LCF (-53% under 0.8% strain). This is different from the simultaneous enhancement of fatigue behaviors in both HCF and LCF in other materials with GNS. The effect of martensite phase transformation (MPT), residual compressive stress as well as the GNS layers have been investigated to clarify the mechanisms of the fatigue behavior of the USRP samples. Besides the suppression of the surface crack initiation and propagation by residual compressive stress and GNS layers, the MPT during the USRP and cyclic loading processes greatly affects the fatigue behaviors. For HCF tests, not only the hard-brittle martensite phase induced by the USRP process improved the material strength but also the MPT process during cyclic loading absorbed the strain energy released from the crack growth, which increased the HCF life. However, for LCF tests, the hard-brittle martensite phase decreased the ductility of the GNS layer and accelerated the crack growth rate under higher strain amplitude, resulting in the decrement of LCF life. This work presents the first investigation on fatigue behaviors of duplex stainless steel with GNS structure.