Effect of Martensite Volume Fraction on Void Formation Leading to Ductile Fracture in Dual Phase Steels

This study examines the ductile fracture of three dual-phase steels that contain low volume fractions of martensite (3.2%, 6.2% and 10%) and are composed of nearly the same quality of ferrite and martensite. The strains to rupture of the DP steel with 3.2% martensite are considerably higher than those of the DP steels with 6.2% and 10% martensite. The evolution of voids with respect to equivalent plastic strain, which is transformed from the thickness reduction of the specimen, was analyzed by SEM observation in fractured tensile specimens. The observation clarified the fact that the evolution of void size saturates at the specific strain in the DP steel with 3.2% martensite, whereas the void size in the DP steels with 6.2% and 10% martensite grows exponentially with strain. This significant favorable behavior of the DP steel with 3.2% martensite is probably caused by the relatively low heterogeneity of the stress and strain partitioning between the two phases, which allows attaining a significant higher ductility of this steel.