Low-cycle fatigue-induced martensitic transformation in SAF 2205 duplex stainless steel

The low-cycle fatigue (LCF) behavior of SAF 2205 duplex stainless steel at the strain amplitudes of 0.9% and 1.5%, combined with strain ratios R = −1.0 and −0.2, exhibits a mixed mode of cyclic hardening and cyclic softening as the cycle life increases until failure. The microstructure of the as-received metal is composed of ferrite (α) and austenite (γ) phases with 51α/49γ, vol.%. The evolution of the α/γ phase in the samples treated by all LCF tests has been revealed under the high-resolution transmission electron microscope. Dislocation cell structures and persistent slip bands (PSB) can be observed in the α phase in the samples of all LCF tests. The tangled dislocations accumulated at the stacking faults in the γ phase can be seen in the sample of strain amplitude of 0.9% with R = −1. At the same strain amplitude of 0.9% but with R = −0.2, the microstructure of the γ phase transforms to the distinct ɛ-martensite intersected mutually, which is promoted by a higher 0.6% tensile mean strain than the previous strain ratio, R = −1, having 0% mean strain. Further, at a relatively higher strain amplitude of 1.5% with R = −1, γ-austenite has transformed to the strain-induced thin lath-like α′-martensite sheaths. At the same strain amplitude with R = −0.2, the thin lath-like α′-martensite sheaths grows into thicker and longer strain-induced martensite bundles as the tensile mean strain increases from 0% to 1.0%. The strain-induced martensite present at the failure region correlates on intimate terms with the maximum microhardness distribution at the γ-austenite grains of the failure region at strain amplitude of 1.5% with R = −0.2. It suggests that the localized transformation of retained austenite into martensite at a tip of a fatigue crack improves the fatigue resistance by either hindering the crack propagation or reducing the crack growth rate.