Fatigue properties of Ni-base superalloy in long life region at elevated temperature

The fatigue properties of a nickel-base superalloy, Inconel 718, in long-life region up to 10⁸ cycles were investigated under rotating bending at room temperature and elevated temperature of 300°C, 500°C and 600°C. S-N curves at 500°C and 600°C showed the stepwise shapes, though those in cases of room temperature and 300°C were the conventional one-step ones. All the fractures originated from the specimen surface at room temperature and 300°C, while the internal fractures occurred in long-life region beyond 10⁷ cycles and the surface fractures at the stress levels over the horizontal line of S-N curves corresponding to the fatigue limit for the surface fracture at 500°C and 600°C. Intergranular cracking were observed at the origin of internal fracture.