High cycle fatigue behavior of a directionally solidified Ni-base superalloy DZ951

Smooth and notched specimens of a directionally solidified Ni-base superalloy DZ951 alloy were subjected to high cycle fatigue (HCF) loading at temperatures of 700 and 900 °C in ambient atmosphere. The results show that the fatigue strength at 700 °C is higher than that at 900 °C while the notch sensitivity at 900 °C is lower than that at 700 °C. As for the smooth specimens, the fracture mode exhibits strong temperature dependence. At 700 °C, only stage I crack propagation can be observed while both stage I and stage II crack propagation can be found at 900 °C. A correlation exists between the crack propagation mode and deformation mode. The fracture mode observed at 700 °C is associated with single planar slip while that at 900 °C is associated with the combination action of planar slip, cross-slip and climb of dislocations. The notch effect plays a significant role in crack initiation and propagation process. The multiple primary cracks initiate at the surface for the notched specimens while single primary crack initiates at the carbide/matrix interface for the smooth specimens.