The Role of Delta Phase in the Stress Corrosion Cracking (SCC) of Alloy 718 in Simulated Pressurized Water Reactor (PWR) Primary Water (PW) Environment

Abstract

Alloy 718 is an age hardenable, nickel-base alloy used in fuel assemblies of PWRs by virtue of its high strength and resistance to stress corrosion cracking (SCC). Recent results have shown that its SCC susceptibility is enhanced by the niobium-enriched δ phase (Ni₃Nb) formed during thermal mechanical treatments, either at high temperature (HT-δ) or low temperature (LT-δ). Here, a series of experiments was performed to further investigate the role of δ phase in SCC initiation behavior. The tests included constant extension rate tensile (CERT) tests in an inert environment at 320°C and oxidation exposure tests followed by incremental straining CERT tests (to 1.4% and 4.6% plastic strain) in primary water (320°C, 1000 ppm B, 2 ppm Li, 35 cc/kg H₂). The high SCC susceptibility was attributed primarily to HT-δ formation at grain boundaries (GBs). Intergranular cracks initiated most often at GBs decorated with either HT-δ, or in the LT-δ, which is a film at GBs. No impact of the HT-δ phase was observed in SCC behavior in the absence of oxidation. This brittle oxide developing from the presence of δ phase may be responsible for the high SCC susceptibility of Alloy 718 in PW.