Position-dependent shear-induced austenite-martensite transformation in double-notched TRIP and dual-phase steel samples

While earlier studies on transformation-induced-plasticity (TRIP) steels focused on the determination of the austenite-to-martensite decomposition in uniform deformation or thermal fields, the current research focuses on the determination of the local retained austenite-to-martensite transformation behaviour in an inhomogeneous yet carefully controlled shear-loaded region of double-notched TRIP and dual-phase (DP) steel samples. A detailed powder analysis has been performed to simultaneously monitor the evolution of the phase fraction and the changes in average carbon concentration of metastable austenite together with the local strain components in the constituent phases as a function of the macroscopic stress and location with respect to the shear band. The metastable retained austenite shows a mechanically induced martensitic transformation in the localized shear zone, which is accompanied by an increase in average carbon concentration of the remaining austenite due to a preferred transformation of the austenite grains with the lowest carbon concentration. At the later deformation stages the geometry of the shear test samples results in the development of an additional tensile component. The experimental strain field within the probed sample area is in good agreement with finite element calculations. The strain development observed in the low-alloyed TRIP steel with metastable austenite is compared with that of steels with the same chemical composition containing either no austenite (a DP grade) or stable retained austenite (a TRIP grade produced at a long bainitic holding time). The transformation of metastable austenite under shear is a complex interplay between the local microstructure and the evolving strain fields.