In-situ experiment and numerical modelling of the intragranular and intergranular damage and fracture in plastic deformation of ductile alloys

In this research, a unified damage indication considering the intragranular and intergranular damage initiation and evolution was developed for studying the damage and fracture behaviours of the excellent ductile alloys represented by TWIP steels, and a cohesive zone model-crystal plasticity finite element method (CZM-CPFEM) approach was developed, where the crystal plasticity with coupled slip and twinning and the strain energy-based damage criterion was employed to reveal the plastic deformation and damage in grain interior (GI), while the quadratic nominal stress (QUADS) and the power law of the CZM were selected to describe the damage and cracking at the grain boundaries (GBs). The stress-strain responses, twin evolutions, damage nucleation and cracking of fine-grained (FG) and fine-/ultrafine-grained (F/UFG) TWIP steels were validated by in-situ SEM/EBSD tensile experiments. The effects of grain size, misorientation angle, grain orientation and initial microvoids on the GI and GB damage and fracture were studied and analysed by combining micromechanical tests and the CZM-CPFEM approach. The results demonstrated that the interaction of deformation mechanisms promoted the preferential initiation of microcracks at GBs and their junctions, while slip bands and twin bundles in GI induced the rapid growth and extension of the localized microcracks, eventually resulting in the mixed fracture mode of intergranular and intragranular cracks. In addition, GB damage was dominant for F/UFG TWIP steels. Increasing grain size can effectively suppress GB damage and increase the proportion of GI damage. Larger misorientation angles can weaken GB properties, while smaller misorientation angles effectively promote strain/stress coordination and delay GI and GB damage. Larger Schmid factors for slip and twinning are favourable for activating dislocations and twins, promoting strain/stress coordination to retard microcrack initiation and improving uniform elongation. Moreover, both initial microvoids can effectively reduce uniform tensile strength (UTS) and fracture strain. Specifically, the microvoids located at GBs and their junctions increase the percentage of GB damage and the possibility of intergranular cracking, especially at the quadruple junctions of GBs.