Microstructural evolution and cyclic softening / hardening response of a TRIP-assisted duplex stainless steel

Highlights: • Microstructural evolution and cyclic mechanical response are sensitive to strain amplitude. • Dislocation rearrangement and martensitic transformation occur in ferrite and austenite, respectively. • Dislocation rearranges in the order of patch, wall and poor-formed cell with increasing strain amplitude. • Martensitic transformation follows the sequence of γ.→stacking faults (SFs)→ε→α′ • Detailed statistical analysis of the hysteresis loop allows to obtain the cyclic mechanical response of individual phases. In this work, the cyclic softening/hardening characteristics of a TRIP-assisted duplex stainless steel (DSS) was studied within the strain amplitude range of $0.2\text{~}1.2\%$, and the evolution of dislocation structures was characterized by TEM to clarify the cyclic deformation mechanism of both phases. At various strain amplitudes (ε_a), the test DSS showed different levels of cyclic softening after an initial cyclic hardening except for the case of ε_a=0.2%, where a direct cyclic saturation was present. Cyclic softening rate ($\delta$) was sensitive to ε_a, and particularly, it showed a continuous decrease at ε_ap>0.25% (ε_a>0.5%). In the samples approximately cycled to failure, the dislocation structure in ferrite mainly developed in the order of patch, wall and/or poor-formed cell with the increasing ε_a, which corresponded to the progressively enhanced rearrangement of dislocation, while the strain-induced martensitic transformation (SIMT) with a sequence of γ→stacking faults (SFs)→ε→α′ was activated with increasing ε_a, and it was considered to be a main cause for the decreasing $\delta$. Moreover, the cyclic response of the individual phases was further discussed by analyzing the "characteristic peak" on the second derivative curve of cyclic hysteresis loop in relation to the evolution of substructures in the two phases.