Oxidation assisted crack propagation of alloy 718

The purpose of the study is to get a better understanding of the detrimental effect of environment encountered in alloy 718 at high temperature in order to design microstructures with an improved resistance to oxidation assisted cracking. Creep-fatigue tests under various oxygen partial pressures have shown the existence of a transition pressure corresponding to a brutal increase in the crack growth rate. This transition pressure corresponds to a transition in the oxidation mechanism and kinetic of alloy 718. This experimental fact has been used to modify local interaction conditions between oxidation process and mechanical behaviour during cracking process. Specific tests have then been performed, superimposing a square wave pressure on either side of the transition pressure to different mechanical cycles. The effect of an overageing treatment on the crack growth rate is examined, in relation with microstructural changes. An intergranular embrittlement mechanism is proposed which rely on the interaction between the nickel oxide nucleation and growth and the local mechanical behaviour of the alloy at the crack tip.