The effect of diffusion-induced grain boundary migration ahead of SCC crack tips on propagation

摘要

Grain boundary migration (GBM) has beenfrequently reported associated to stress corrosioncracking (SCC) in Fe- and Ni-based alloys since 2013.The GBM zone observed was always depleted in Cr andFe and enriched in Ni, which is very similar to the resultsreported by Balluffi and Cahn where the process of GBMwas driven by the diffusion of solute elements. Due tothe similarities between the GBM reported by the SCCcommunity and Balluffi and Cahn’s observations, it hasalso been termed as diffusion-induced GBM (DIGM).Although a lot of work has been conducted to understandthe potential roles of DIGM in SCC, it was mainlyfocussed on how it affects intergranular oxidation rate.However, a higher intergranular oxidation rate might notnecessarily lead to a higher SCC crack growth rate(CGR), which is a more important parameter to themechanistic understanding of SCC and to the industry. Inthis study, 4 austenitic alloys within different Ni content(Fe-16%Cr-x%Ni) were SCC tested in simulatedpressurized water reactor (PWR) primary water at 320-360°C. DIGM was observed ahead of all examinedpotentially active crack tips. A method is proposed toquantify the extent of DIGM ahead of SCC crack tips andboth the temperature and Ni content in the alloys areshown to have significant effects on the extent of DIGM.The underlying mechanisms controlling the differenteffects of temperature and Ni content on DIGM arediscussed. After this study, a new understanding of theeffect of DIGM on SCC crack growth has beendeveloped.