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

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 delta 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 delta phase, which aligned with respect to the slip planes intersecting the GB, or the low temperature delta phase, which is a film on the GB.