Fracture Mechanism in Fatigue of Nickel-Based Superalloy Inconel 718 at Elevated Temperatures

In order to investigate the fracture mechanism in fatigue of nickel-based superalloy at elevated temperature, rotating bending fatigue tests were carried out for Inconel 718 at 500°C and 600°C up to 108 cycles. At both temperatures, fatigue fracture initiated from the subsurface of specimens in the long life region, though the origin of fracture was on the surface of specimens in the short life region. As a result, S-N curves showed a two-step shape. Although surface cracking was observed even in the long life region at elevated temperatures, similar results as observed at room temperature, surface cracks ceased propagating after extending to approximately 20-30μm at elevated temperatures. Meanwhile, intergranular cracking was observed at the origins of subsurface fracture, which was found to be nucleated in the early stage of fatigue.