On the mechanism of oxidation-fatigue damage at intermediate temperatures in a single crystal Ni-based superalloy

Abstract The combined effects of environment (oxidation) and mechanical load (fatigue) that control crack propagation in a single crystal Ni-based superalloy have been investigated with particular focus on the intermediate service temperature range. Fatigue tests have been carried out at different frequencies, hold times and environments, to study the parameters influencing crack propagation at 550 °C. The direct current potential drop method was used to monitor the crack growth while STEM-EDS were used to analyse the fracture mode and crack tip regions. It was found that the micro-mechanism of fatigue crack propagation at intermediate temperatures is a complex process with several competing mechanisms acting on the crack tip simultaneously. Crystallographic slip processes by γ′ shearing are active at these temperatures while at the same time thermally activated processes that promote crack propagation through the γ channels also take place. In addition, the effects of oxidation were found to be two-fold. It was demonstrated that these temperatures are not high enough to cause macroscopic embrittlement of the crack tip but finger-like protrusions were found to penetrate the material ahead of the crack tip at the nano-scale. The kinetics of such a mechanism were accentuated by the plastic strains at the crack tip, which given enough time, can promote cleavage fracture at the γ/γ′ interface. At the same time, given that the crack driving force is lower than a transition value, oxide formation on the crack tip surfaces can bridge the opening of the crack tip and reduce the effective driving force.