Processus diffusionnels à l'origine de l'évolution de la composition d’un alliage au cours de l'oxydation sélective en pointe de fissures intergranulaires. Application à la CSC de l'Alliage 600 en milieu primaire des REP

Abstract : Stress Corrosion Cracking (SCC) of nickel base alloys is one of the major degradation phenomena in the primary circuit of Pressurized Water Reactors (PWR). Understanding the SCC mechanism is a key issue for the extension of reactor lifetime.A SCC model based on a selective and asymmetrical oxidation of the grain boundary ahead of the crack tip has been proposed in previous studies. Adjacent to this oxide, a chromium-depleted area is observed exclusively in one of the two grains adjacent to the grain boundary. As oxygen transport is found to be faster than chromium diffusion in the alloy, the latter is assumed to be the rate-limiting step of crack propagation. Nevertheless, the mechanism responsible for chromium depletion is still under debate. Indeed, the lattice and the grain boundary diffusion coefficients of chromium in nickel-based alloys at 350°C are not high enough to explain the chromium depletion magnitudes measured in the literature. Accordingly, factors accelerating chromium diffusion in the alloy ahead of the SCC crack tip should exist. Thus, two assumptions have been proposed in this work: plasticity-enhanced chromium diffusion and diffusion-induced grain boundary migration (DIGM).The aim of this study is to confront these two assumptions by combining both experiments and modeling in order to explain chromium depleted areas observed at the SCC crack tip.Thus, diffusion tests under loading were performed in order to study the effect of plastic deformation on chromium diffusion. Plasticity-enhanced diffusion is evidenced. A relationship between the diffusion coefficient and strain rate has been established leading to a 106-fold increase of the diffusion coefficient at 350°C. In addition, thermal treatments and oxidation tests have shown that diffusion-induced grain boundary migration occurs in Ni-Cr alloys. DIGM leads to dissymmetric Cr-depleted areas, observed in the wake of the moving grain boundary.
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Josiane Nguejio Nguimatsia. Processus diffusionnels à l'origine de l'évolution de la composition d’un alliage au cours de l'oxydation sélective en pointe de fissures intergranulaires. Application à la CSC de l'Alliage 600 en milieu primaire des REP. Matériaux. PSL Research University, 2016. Français. ⟨NNT : 2016PSLEM054⟩. ⟨tel-01668572⟩

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