Oxidation Assisted Intergranular Cracking in Alloy 718: Effects of Strain Rate and Temperature

Alloy 718 is the most widely used superalloy in industry due to its excellent mechanical properties, as well as its oxidation and corrosion resistance over a wide range of temperatures and solicitation modes. Nevertheless, it is a well-known fact that this alloy is sensitive to oxidation assisted intergranular cracking under loading in the temperature range encountered in service. The mechanisms resulting in such degradation are not well-understood, but it has been well established that a relation exists between a change in fracture mode and the apparition of plastic instability phenomena over a wide range of temperatures. Quantification and characterization of the damaging process provide important information leading to a better understanding of the degradation mechanisms involved in the oxidation assisted intergranular cracking of this alloy. These observations allow various domains to be defined in the strain rate -  temperature plane, where the damaging process characteristics are different: a high strain rate / low temperature domain in which instabilities occur and where the fracture mode is systematically transgranular ductile, an intermediary domain where numerous intergranular crack initiations can be observed, and a slow strain rate / high temperature domain where crack propagation is enhanced. These results lead to the proposal of consistent scenarii to explain grain boundary opening due to applied intergranular normal stress and critical decohesion stress changes.