Influence of local mechanical loadings paths on the oxidation assisted crack initiation of alloy 718

Abstract

At high temperatures, alloy 718, like many other nickel-based superalloys, is sensitive to an oxidation-assisted intergranular crack (OAIC) growth mechanism. Former studies have pointed out that even if intergranular oxidation still occurred, intergranular crack initiation was inhibited due to specific mechanical loadings which were identified as Portevin-Le Châtelier (PLC) plastic instabilities. In the present work, key parameters triggering crack initiation or PLC instabilities in the [550–700°C] temperature range were determined for the studied grade by means of tensile tests on smooth specimens. Then, in order to assess the applicability of such a finding at a scale compatible with the material microstructure, a dedicated tensile V-shaped specimen was designed to generate different surface and sub-surface strain and strain rate histories. Thanks to a dual approach based on the observation of crack initiation location on this specific experimental specimen together with associated FE calculations, the critical mechanical loading paths inducing OAIC initiation have been specified. Thus, assuming that the metallurgical state is homogenous at the structure scale, a mapping of intergranular crack initiation is then obtainable.