The effect of strain-induced martensite transformation on strain partitioning and damage evolution in a duplex stainless steel with metastable austenite

Transformation induced plasticity (TRIP) assisted lean duplex stainless steels (LDSS) possess a multi-phase microstructure during deformation due to strain-induced martensite transformation (SIMT). The effect of SIMT on the strain partitioning into constituent phases, which in turn affects the SIMT kinetics is highlighted. The individual stress-strain relationships of each phase are obtained via a microstructure-based model and the gradual fragmentation of austenite grain resulted from SIMT is especially considered. A modified model of partitioned strain in the austenite considering the contribution of martensite is proposed, whereby the actual SIMT kinetics in the parent austenite phase can be quantitatively determined. Besides, the contributed strain of each constituent phase during the progress of deformation can be evaluated directly. Furthermore, EBSD and in-situ tensile tests were carried out to characterize the influences of SIMT and resulting high strain localization on the damage evolution. The cracking nucleation initiates at the α/α′ interface as well as the inside of ferrite near the interface. Subsequently, they grow along the α/α' interface with further straining and, finally, merge into one interfacial crack. The combined crack forks off at the interface with small curvature, then the secondary cracks penetrate into austenite/martensite layer and ferrite layer, respectively, resulting in the final failure. The overall fracture of LDSS shows the ductile feature, although the ferrite and the martensite in the local region of fracture surface show the trace of quasi-cleavage fracture, the remained austenite exhibits dimple-typed fracture.