The role of secondary phases on SCC initiation of Alloy 718 in simulated PWR primary water

Stress corrosion cracking (SCC) initiation behavior of Alloy 718 with different precipitates was studied by slow strain rate tensile testing (SSRT) in a simulated pressurized water reactor (PWR) primary water environment. It was found that both nano-precipitates (γ', γ'' phases) and δ phases enhanced the SCC susceptibility . Nano-precipitates significantly suppressed the interior slip and increased the local stress concentration at grain boundaries (GBs), thus promoting the initiation of intergranular (IG) cracks. GBs oriented perpendicular to the loading direction favored IG crack propagation . Preferential oxidation of the δ phases and the δ/γ interface were precursors to crack initiation, thereby increasing the SCC susceptibility. The shape of δ phases affected the oxidation degree, and more severe oxidation was observed on needle-like δ phases. • Both nano-precipitates (γ', γ'' phases) and δ phase increase the SCC susceptibility of Alloy 718. • Nano-precipitates suppress interior slip and increase local stress concentration at grain boundaries. • Preferential oxidation of δ phase and δ/γ phase boundaries serve as a precursor to SCC initiation.