Low-cycle fatigue behavior of Mn-N bearing duplex stainless steel under consideration of deformation-induced martensitic transformation

In this work, the cyclic mechanical response and fatigue life a duplex stainless steel with metastable austenite was studied at 0.3% <= ca <= 2.0%. The martensitic transformation characteristics and kinetics rules were analyzed by EBSD, XRD and Feritscope. The fatigue damage and fractographic features were well elucidated through sufficient EBSD and SEM observations. The results showed that a critical plastic strain amplitude (ccri approximate to 0.59%) and an accumulated plastic strain threshold (depends on plastic strain amplitude) are necessary for alpha ' bcc-martensitic transformation in the bulk. This leads to abnormal fatigue performance at 1.2% <= ca <= 2.0% compared that at 0.3% <= ca <= 1.0%. In particular, the fatigue micro-cracks, which nucleate mainly at the gamma/alpha interfaces at 0.3% <= ca <= 1.0%, tend to nucleate inside original austenite (gamma+alpha ') at 1.2% <= ca <= 2.0%. And on the fatigue fracture surface, micro-voids and secondary cracks prevail in original austenite at 1.2% <= ca <= 2.0%, which are mainly present along the interfaces between ferrite and original austenite, respectively, at 0.3% <= ca <= 1.0%. Besides, alpha ' bcc-martensitic transformation can be largely induced ahead of the main crack tip as it advances for all strain amplitudes studied, which also plays a crucial role in short crack propagation. As a result, two distinguished stages with a close transition point of ca approximate to 1.0 % are shown in the fatigue-life relations.