Cyclic deformation behavior and failure mechanism of S32205 duplex stainless steel under torsional fatigue loadings

Torsional low cycle fatigue tests of ferrite-austenite duplex stainless steel S32205 at various shear strain amplitudes were conducted. The relationship between the evolution of dislocation structures in each constitutive phase and cyclic stress response and cyclic stress-strain curves was thoroughly studied. The surface damage evolution and short cracks nucleation behavior were also discussed. Results show that the cyclic softening occurred at all the strain amplitudes from 0.3% to 1.2% is due to the redistribution and localization of plastic strain and the development of strain transfer between phases, while the formation of regular dislocation cells contributes to achievement of cyclic saturation when the strain amplitude reaches 0.7%. The cyclic stress-strain curves show two-regimes with different cyclic strain hardening rates. As the plastic strain amplitude is smaller than 0.32%, the ferrite and austenite jointly accommodate the plastic strain within material matrix with the coexistence of representative planar slip and incipient wavy slip dislocation configurations in both phases, leading to a low cyclic strain hardening rate. By contrast, the ferrite dominates the plastic deformation with the formation of well-developed low-energy dislocation structures as the plastic strain amplitude is over than 0.32%, resulting in a high cyclic strain hardening rate. Moreover, microstructure observations reveal that the pronounced persistent slip markings in ferrite grains and phase boundaries are the preferential sites for microcrack nucleation and growth.