Fracture toughness of Ti₂AlNb alloy with different Al content: Intrinsic mechanism, extrinsic mechanism and prediction model

In this research, two Ti₂AlNb based alloys were investigated to reveal the effect of Al content on fracture toughness, and the fracture mechanism of both alloys was discussed from intrinsic and extrinsic aspects. In addition, the crack tip plastic zone model and the prediction model of fracture toughness were established based on tensile properties. The results show that the increase of Al content leads to more O phase content, finer acicular O phase and higher Al content in three constituent phases, thus improving the strength and reducing the ductility and fracture toughness. Meanwhile, the cleavage morphology with tearing ridges on the fracture surface indicates that the fracture mechanism is quasi cleavage fracture. The brittle nature of acicular O phase and equiaxed particles provides nucleation sites of microcrack and promotes crack propagation. As another aspect, the intrinsic mechanism of crack tip bluntness caused by B2 phase and the extrinsic mechanism of crack deflection at grain boundary strengthen the resistance of crack propagation. The increase of Al content brings out the reduction of crack tip plastic zone and crack path tortuosity, which makes a significant decrease in fracture toughness. Moreover, the fracture toughness predicted based on tensile properties is in agreement with that obtained by experiment, and the fracture toughness of two alloys is dominated by intrinsic contribution.