The roles of thermal mechanical treatment and δ phase in the stress corrosion cracking of alloy 718 in primary water

Abstract

Eleven variants of the precipitation-hardened alloy 718 were tested for stress corrosion cracking (SCC) resistance in a simulated pressurized water reactor primary water environment. The optimized grade had the lowest SCC susceptibility, while more traditional thermal-mechanical treatment exhibited the highest. The high susceptibility was attributed primarily to δ phase formation at grain boundaries (GBs), with localized deformation playing a secondary role. Intergranular cracks initiated most often at GBs decorated with either the high temperature δ phase, which aligned with respect to the slip planes intersecting the GB, or the low temperature δ phase, which is a film on the GB.