Investigation on the size and distribution effects of O phase on fracture properties of Ti2AlNb superalloy by using image-based crystal plasticity modeling

Ti2AlNb superalloy consisting of O-phase and B-phase often exhibits deformation heterogeneity and incompatibility due to the mechanical characteristics of phases. To study the size and distribution effects of the O phase on the fracture properties of Ti2AlNb superalloy, three types of material with different O-phase sizes and distributions were prepared through heat treatments, and the tensile properties of the heat-treatment samples were investigated. An image-based crystal plasticity model was built based on the backscattered electron images of the superalloy to probe the effects of O-phase size and distribution on stress, strain partitioning, and fracture. The strain hardening and fracture behaviors were discussed in terms of the strain partitioning between O-phase and B-phase, and the formation and propagation of micro-cracks, respectively, by combining the simulation and experimental results. Simulation results show the stress and strain partitioning between O/B phase interfaces increases with the increase of O-phase size, and micro-cracks are formed when the local stress concentration exceeds the critical value. The presence of micro-cracks imposes high shear stress on the neighboring B-phase matrix and promotes fast crack propagation through the B-phase matrix, resulting in cleavage fracture. Inversely, micro-cracks tips can be alleviated when the stress concentration at O/B phase interfaces can be relaxed by the plastic deformation of the B-phase matrix. The variation of stress and strain partitioning with deformation during plastic deformation shows that strain partitioning isn't related to O-phase size, but stress partitioning depends on it. Thus, the effects of O-phase size on fracture mainly depend on stress partitioning. This paper presents a meaningful method to model the phase distribution, and the effects of O-phase size and distribution on stress, strain partitioning, and fracture behavior are studied.