718 alloy is the most used Ni base superalloy in industry due to its high manufacturability and high resistance in extreme conditions such as in nuclear fuel assembly. However, when subjected to very high stress, it is reported that this alloy can suffer from embrittlement by a Stress Assisted Grain Boundary Oxidation (SAGBO) mechanism at high temperature or by Stress Corrosion Cracking (SCC) for aqueous media.

Most of the literature on such embrittlement mechanisms focus their interest on the initiation or the propagation stages. However, it was demonstrated that the kinetics of those damage processes is dependent on an intermediate stage: the one of crack coalescence.

This stage consists in the evolution/interaction of crack colony until the formation of a macroscopic crack. The governing and rate limiting mechanisms of this coalescence stage are still poorly understood and there is no commonly accepted criterion to describe its kinetic. Moreover, the sensitivity to microstructural parameters (such grain size, texture and precipitate distribution…) and loading conditions is not documented at all.

The present project aims at improving the knowledge of the coalescence stage thanks to advanced, multimodal X-ray characterization of SCC (ex-situ) and SAGBO (in-situ) crack colony observations. These characterizations, complemented by quantitative image analysis like Digital Volume Correlation, will be performed at the scale of the polycrystalline aggregate were grain boundary type and local strain/stress state of the material will be analysed and correlated to the crack velocity.

___________________________________________________________________

PROJECT PARTNERS:

                   CNRS