Cyclic plastic response of MnN bearing duplex stainless steel: The role of martensitic transformation

In this work, the cyclic stress/strain behavior of a MnN bearing duplex stainless steel with metastable austenite was studied within a strain amplitude range of 0.15∼1.5%. The microstructural mechanisms of constituent phases during cyclic deformation were well clarified through TEM and EBSD observations. Based on the mechanical and microstructural characteristics, the deformation coordination between austenite and ferrite was further discussed by analyzing the variation of constituent phases-related “characteristic peaks” on the second derivative curve of hysteresis loop. The results showed that during the first few cycles, the evolution of planar substructures is activated in both phases, leading to an initial cyclic hardening. After that dislocation rearrangement in the order of “lines→patches→walls→cells” and martensitic transformation with the sequence of γ(SFs) → ε → α’ occur in ferrite and austenite, respectively. The development of the two micro-mechanisms with opposite stress responses leads to complex stress responses involving softening, saturation and/or secondary hardening during cyclic deformation. In particular, the formation of α'bcc-martensite leads to a terminated cyclic softening of the present DSS, characterized by a cyclic saturation at 1.0%≤εa≤1.2% and a secondary cyclic hardening at εa = 1.5%. Besides, a variation of cyclic strain hardening characteristic from a single stage to three stages is shown as cyclic deformation proceeds, which is closely related to the variation of deformation coordination and dominant mechanisms with strain amplitude and cycle number. In this regard, α'bcc-martensite plays a crucial role in the deformation coordination through repartitioning the plastic strain between austenite and ferrite.