Oxygen-induced intergranular fracture of the nickel-base alloy IN718 during mechanical loading at high temperatures

Abstract

There is a transition in the mechanical-failure behavior of nickel-base superalloys from duc- tile transgranular crack propagation to time-dependent intergranular fracture when the tempera- ture exceeds about 600 °C. This transition is due to oxygen diffusion into the stress field ahead of the crack tip sufficient to cause brittle decohesion of the grain boundaries. Since very high crack- ing rates were observed during fixed-displacement loading of IN718, it is not very likely that grain boundary oxidation governs the grain-boundary-separation process, as has been proposed in sev- eral studies on the fatigue-damage behavior of the nickel-base superalloy IN718. Further studies on bicrystal and thermomechanically processed specimens of IN718 have shown that this kind of brittle fracture, which has been termed “dynamic embrittlement”, depends strongly on the struc- ture of the grain boundaries.