The analysis of fracture toughness and fracture mechanism of Ti60 alloy under different temperatures

Fracture toughness and fracture mechanism of Ti60 alloy with duplex microstructure under different temperature conditions were investigated in the present work. The experimental result shows that the fracture toughness of CT samples increases from the room temperature (40 MPa m^1/2) to 400 °C (71.45 MPa m^1/2) but declines at 600 °C (62.55 MPa m^1/2). It is observed from fracture surface through SEM that the predominant fracture mechanism has changed from quasi-cleavage fracture at room temperature to ductile fracture at higher temperatures. In addition, the tortuosity of crack propagation path has a limited impact on the fracture toughness. Path selections for crack propagation are obtained through SEM observation which can be summarized as: cut through lamellar α, parallel to lamellar α, bypass the equiaxed α_p and cut through the equiaxed α_p. Moreover, it is found that the intrinsic contribution is the primary reason leading to the change of the fracture toughness of Ti60 alloy under different temperatures. Meanwhile, it is noteworthy that the area of the crack tip plastic zone increases from RT to 400 °C but decreases at 600 °C, which is seen as the main impact of temperature on fracture toughness. To be exact, the CT sample with a larger area of the plastic zone could provide a higher K_1C value. Furthermore, a prediction model of K_1C based on tensile properties is established, which has a good accuracy with experimental results. The model is useful in predicting the fracture toughness of Ti60 alloy at different test temperatures.