Studies of High-Temperature Notch Fatigue Property and Fracture of a Directional Solidified Co-Base Superalloy

The notch fatigue behaviours of a directionally solidified Co-base superalloy at 700°C and 900°C in air have been investigated and the fracture surfaces have been observed using SEM. The results show that the alloy possesses a high notch fatigue resistance, and that the notch fatigue limits at 700°C and 900°C are 285 MPa and 203 MPa, respectively. The alloy exhibits low notch sensitivity (about 0. 15). At 700°C, a fatigue crack propagates along the {111} slip plane, while at 900°C, under the effect of a cyclic stress and environmental oxidization, the crack grows initially along the {001} cleavage plane and then propagates with a striate mode. The high notch fatigue resistance of the alloy is attributed to its peculiar microstructure and excellent static mechanical properties, especially high ductility.