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 (alpha) and austenite (gamma) phases with 51 alpha/49 gamma, vol.%. The evolution of the alpha/gamma 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 a phase in the samples of all LCF tests. The tangled dislocations accumulated at the stacking faults in the gamma 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 gamma phase transforms to the distinct epsilon-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, gamma-austenite has transformed to the strain-induced thin lath-like alpha'-martensite sheaths. At the same strain amplitude with R = -0.2, the thin lath-like alpha'-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 gamma-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. (c) 2005 Elsevier B.V. All rights reserved.