15 Novembre – Thesis defense - Léna Verchère

10 h30 Amphi 2 - building A9 (Talence campus)

Interactions between microstructure, mechanics and local electrochemistry. Application to nickel-based alloy A600.

Stress corrosion cracking (SCC) is a local damaging phenomenon, which results from the synergy of microstructural, mechanical and environmental parameters. Understanding the interactions between those parameters allows manufacturers to continuously improve their reliability models for components and thus to improve facility safety. The present study focuses on Alloy 600, a nickel-based alloy which is used in pressurized water reactors and subject to SCC.
Understanding and modelling interactions between the material, mechanics and environment requires decreasing the scale down to heterogeneities of the microstructure, mechanical fields and electrochemical properties, in order to avoid averaging effects. In this study, this scaling was achieved thanks to the use of experimental and numerical local techniques.
A scanning electrochemical microscope (SECM) made it possible to measure currents at the grain scale. The effect of grain crystallographic orientation on the electrochemical properties of the passive film was quantified by means of a coupling of SECM with EBSD. The effect of the grain boundary type was also investigated. Thereafter, same measurements were conducted on a specimen under mechanical load thanks to the development of an experimental set-up composed of an in situ tensile machine coupled to SECM. The microstructure which was obtained by EBSD was then used to recreate a virtual microstructure on which local mechanical fields were determined through a crystal plasticity computation. The influence of cold-work and that of a tensile load on surface reactivity were then demonstrated at the microstructural scale, attesting that SCC models must integrate local heterogeneities.

Event localization