Effect of microstructural factors on void formation by ferrite/martensite interface decohesion in DP980 steel under uniaxial tension

The effect of microstructural factors on ferrite/martensite (F/M) interface decohesion in DP980 steel under uniaxial tension was investigated using ex-situ tensile testing and the crystal plasticity finite element method (CPFEM). Ex-situ tensile testing of a miniaturized specimen with a 100 μm-thick grooved gauge region revealed that the main ductile failure mechanism in DP980 steel was F/M interface decohesion. First, the effect that the crystallographic orientation of ferrite exerted on the ductile fracture by F/M interface decohesion was investigated using the representative volume elements (RVEs) of single-crystalline ferrite matrix containing a hard martensite particle. Then, the effects of martensite morphology and grain boundary alignment on the heterogeneity of strain-stress partitioning, in-grain orientation gradient for the ferrite matrix, and the corresponding void formation by F/M interface decohesion were investigated using three types of RVEs with a bi-crystalline ferrite matrix that contained one of the followings: (i) a circular martensite particle, (ii) an elliptical martensite particle, and (iii) two adjacent elliptical martensite particles at the F/F grain boundary. To capture the void formation by F/M interface decohesion, a cohesive zone model based on traction-separation law was introduced along the interfaces between the ferrite and martensite phases. The simulation revealed that the crystallographic orientation for ferrite, the martensite morphology, and the grain boundary alignment all significantly affected the heterogeneity of strain-stress partitioning, the in-grain orientation gradient for the ferrite matrix, and the void formation by F/M interface decohesion in DP980 steel under uniaxial tension.