Oxidation effects on the fatigue crack growth behaviour of alloy 718 at high temperature

The purpose of this study was to investigate oxidation assisted crack growth phenomena encountered in nickel-based alloys at high temperatures. Fatigue crack growth tests conducted at 650 degrees C and under a range of oxygen partial pressures revealed the existence of a transition pressure. This pressure is in no way correlated to the loading conditions, but rather it Varies with the chromium content in the alloy, and is furthermore directly linked to the oxidation mechanisms which were identified by using analytical TEM. By means of specific mechanical tests, superimposing a square wave oxygen pressure cycle to a fatigue or creep-fatigue mechanical cycle, various fundamental aspects of the local interaction between oxidation and deformation at the crack tip were investigated. Embrittlement is due partly to the nickel oxide nucleation and partly to the stress relaxation ability of the material. Chemical and microstructural modifications are recommended in order to improve the cracking resistance. Copyright (C) 1997 Acta Metallurgica Inc.