Characterization of Materials Properties and Crack Propagation Mechanisms in Damaged Alloy 718 Leaf Springs Following Commercial Reactor Exposure

摘要

In this work, damaged leaf spring arms were discovered during refuelling outages in a pressurized water reactor following operations to ＞45 MWd/mtU burn-up. Two LS assemblies were transferred to Oak Ridge National Laboratory for inspection and detailed characterization of their mechanical and microstructural properties. The irradiated material is evaluated and compared to archival samples. Post irradiation microstructural examination includes changes to precipitate phases, solute redistribution following in-service exposure, and the characterization of the cracked regions of the leaf spring materials. For irradiated samples, crack propagation routes are examined through detailed microscopy analysis (TEM, SEM-EBSD), including focused ion beam milling of sections near the crack tip locations for transmission electron microscopy. Statistics on cracked/non-cracked grain boundary types (twins, random low- and high-angle) is discussed. The role of grain boundary precipitation of the delta-Ni_3Nb phase on the material performance is also discussed.