High-temperature behaviors of grain boundary in titanium alloy: Modeling and application to microcrack prediction

Grain boundary (GB) deformation usually occurs during the high-temperature deformation process of titanium alloy, which may result in microcrack initiation and propagation. GB exhibits the characteristics of viscosity softening and rate sensitivity in its deformation, which lead to flow softening, enhanced plasticity and rate sensitive damage of the alloy. In view of the fact that it is extremely hard to directly observe GB deformation in experiments, a cohesive zone model (CZM) of GB has been proposed by taking into account the aforementioned characteristics. The CZM model is then combined with an actual-microstructure-based crystal plasticity finite element model (CPFEM) to include the effect of grain morphology, grain orientation and α and β phases of titanium alloy. The model is applied to study the microcrack initiation and propagation of a near-α titanium alloy. The predicted microcrack initiation and propagation were verified by experimental observations, which proves the reliability of the proposed model. Then, the model is applied to study the fracture rule of GBs, the effects of grain morphology, misorientation and property on the fracture of GBs. The results show that (a) the α-α GBs tend to be the most difficult one to deform, while the β-β GBs are the easiest one; (b) grain morphology affects microcrack initiation and propagation most. The α-β GBs decohesion at triple junctions is the dominant type for crack initiation, then the crack propagates mainly along the straight GBs vertical to tensile axis; (c) the critical strength of GB reaches the minimum at the case that the grains misorientation falls into 45–60°.