Time- and Fluence-to-fracture Studies of Alloy 718 in Reactor

Abstract

A series of Alloy 718 specimens were irradiated in the Halden Reactor under mechanical tensile stresses and in a chemical environment and temperature representative of Pressurized Water Reactor service conditions. The specimens were miniature pin-loaded dogbones, heat treated using either a direct aging cycle or the same aging heat treatment preceded by a solution anneal. Applied stresses ranged between 920 and 1200 MPa. Fracture surfaces examined by SEM displayed a mixture of intergranular regions perpendicular to the applied stress and smoother regions at various angles to the applied stress. It is concluded that intergranular cracking proceeded until the stress on the remaining ligament was sufficient to cause prompt ductile fracture. Fluence at fracture occurred over a range of seven orders of magnitude, with no correlation to applied stress. Time at fracture spanned a much smaller range and was broadly, though weakly, inversely correlated with stress. It appears that time in the environment is a better predictor of failure than is fluence.