The fracture toughness of TRIP-assisted multiphase steels

The relationship between microstructure and fracture resistance of TRIP-assisted multiphase steels has been investigated by processing and characterizing eight specifically designed microstructures differing in terms of the volume fractions of the constitutive phases, i.e. ferrite, bainite, retained austenite and martensite, by the stability of the retained austenite, and by the connectivity of the phases. Fracture resistance has been quantified by the fracture strain in uniaxial tension, ε¯f, the fracture toughness at cracking initiation, JIc, and by the essential work of fracture, we. The fracture mechanisms were characterized by microscopic observations and by profilometric measurements of the crack tip necking process typical in thin sheet fracture. The fracture toughness at cracking initiation of TRIP-aided steels corresponds to an equivalent Dual Phase steel in which martensite replaces retained austenite and is thus not affected by the retained austenite stability. The ductile tearing resistance of TRIP-aided steels, quantified by we, increases significantly with the retained austenite stability. This improved tearing resistance is explained by the effect of the additional work-hardening induced by the late transformation of retained austenite within the crack tip necking zone.