Nickel Based Alloy Intergranular Embrittlement in Relation with Oxidation Mechanisms

The purpose of this study is to specify the parameters which are responsible for the oxidation assisted intergranular embrittlement of nickel-based alloys. Creep-fatigue tests have been performed under various oxygen partial pressures on Ni-Cr alloys with different chromium contents and a nickel-based superalloy, Alloy 718. These tests give evidence of a sharp increase in the fatigue crack growth rate beyond a transition pressure. The transition pressure is independent of mechanical parameters such as the loading amplitude and the frequency and depends especially on the chromium content. Other tests have been explored, superimposing a square wave pressure on either side of the transition pressure to mechanical cycles. These experiments lead us to conclude that the embrittling process is a very local phenomenon, linked to nickel oxide formation and the stress relaxation ability of the material. (Author)