Crystal plasticity modelling of carbide network effects on microstructural strain localization and fracture behaviour in bainitic steels

Various steel microstructures contain carbides that are designed to enhance the strength of the material. In Reactor Pressure Vessel (RPV) steel, the carbides are an inherent part of the microstructure, finely distributed throughout the grains and grain boundaries. This work focuses on the investigation of the effect of carbides ranging from 80 nm to 1μm on strain and stress localization when the carbides are explicitly introduced in polycrystalline models. Two size dependent crystal plasticity finite element models are used in the investigation to evaluate their feasibility to address localization phenomena with carbide strengthened microstructures. We analysed the effects of carbides on quasi-2D Scanning Electron Microscopy (SEM) based microstructures with realistic carbide mapping, and utilized synthetic 3D computational grain-carbide microstructures to investigate spatial and shape effect of carbides. Microscale digital image correlation (uDIC) measurements show that strain localization is influenced significantly by carbide networks and carbides can promote slip in grains with low Schmid's factor. Lastly, we demonstrated the effect of large carbides on fracture predictions using a newly developed microstructurally informed brittle fracture model, which represents a step forward compared with existing Beremin-type approaches. It was observed that carbide induced stress/strain heterogeneity alters fracture probability predictions notably.