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

Abstract

Alloy 718 is the superalloy the most widely used in industry due to its excellent mechanical properties as well as oxidation and corrosion resistance in 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 range of temperature met in service. Mechanisms resulting in such degradation are not well-understood, but it is well established that a relation exists between a change in fracture mode and the apparition of plastic instabilities phenomenon in 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 oxidation assisted intergranular cracking of this alloy. These observations allow to define different domains in the strain rates - temperature plane where the characteristics of damaging process are different: a domain of high strain rates / low temperature in which instabilities occur and where fracture mode is systematically transgranular ductile, an intermediary domain where numerous intergranular crack initiations can be observed, and a domain of slow strain rates / high temperature where crack propagation is enhanced. The present results lead to propose consistent scenarii to explain grain boundary opening due to the evolutions of applied intergranular normal stress and critical decohesion stress.