Diffusion processes as possible mechanisms for Cr depletion at SCC crack tip

J. Nguejio (1, 2) , C. Guerre (1) , F. Gaslain (2) , C. Duhamel (2) , J. Crepin (2) , F. Jomard (3) , Marc Maisonneuve (1)

1 CEA-DES (ex-DEN) - CEA-Direction des Energies (ex-Direction de l'Energie Nucléaire)
2 CDM - Centre des Matériaux
3 GEMAC - Groupe d'Etude de la Matière Condensée

Two mechanisms are studied to explain the asymmetrical chromiumdepletions observed ahead of SCC crack tips in nickel-base alloys:diffusion-induced grain boundary migration (DIGM) and plasticity-enhanced diffusion.On the one hand, DIGM is evidenced in a model Alloy 600 by focused ionbeam (FIB) coupled with scanning electron microscopy (SEM) cross-sectionimaging and analytical transmission electron microscopy (TEM) after annealing at500 °C under vacuum and at 340 °C after exposure to primary water. The occurrenceof grain boundary migration depends on the grain boundary character andmisorientation. On the other hand, the effect of plasticity on chromium diffusion innickel single-crystals is investigated by performing diffusion tests during creep testsat 500 and 350 °C. An enhancement of Cr diffusion is observed and a linearrelationship between the diffusion coefficient and strain rate is evidenced. At last, in an attempt to discriminate the two mechanisms, an analytical modeling of theCr-depleted areas observed at propagating SCC crack tips is proposed.

Keywords: Diffusion; Alloy 600; Chromium depletion; DIGM; Plasticity

Domains: Nuclear Experiment [nucl-ex]; Nuclear Theory [nucl-th]

J. Nguejio, C. Guerre, F. Gaslain, C. Duhamel, J. Crepin, et al.. Diffusion processes as possible mechanisms for Cr depletion at SCC crack tip. 18th International Conference on Environmental Degradation of Materials in Nuclear Power Systems - Water Reactors, Aug 2017, Portland, United States. pp.337-357, ⟨10.1007/978-3-319-67244-1_22⟩. ⟨cea-02435083⟩