Diffusion Processes as Possible Mechanisms for Cr Depletion at SCC Crack Tip
Two mechanisms are studied to explain the asymmetrical chromium depletions 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 ion beam (FIB) coupled with scanning electron microscopy (SEM) cross-section imaging and analytical transmission electron microscopy (TEM) after annealing at 500 °C under vacuum and at 340 °C after exposure to primary water. The occurrence of grain boundary migration depends on the grain boundary character and misorientation. On the other hand, the effect of plasticity on chromium diffusion in nickel single-crystals is investigated by performing diffusion tests during creep tests at 500 and 350 °C. An enhancement of Cr diffusion is observed and a linear relationship between the diffusion coefficient and strain rate is evidenced. At last, in an attempt to discriminate the two mechanisms, an analytical modeling of the Cr-depleted areas observed at propagating SCC crack tips is proposed.
KeywordsDiffusion Alloy 600 Chromium depletion DIGM Plasticity
EDF is gratefully acknowledged for financial support. The authors want to thank SERMA Technologies (Grenoble, France) for FIB lamellae preparation for TEM observations. This work was carried out within the MATMECA consortium and supported by the ANR under contract number ANR-10-EQUIPEX-37. It has benefited from the facilities of the Laboratory MSSMat (UMR CNRS 8579), CentraleSupélec, France.
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