Characterization of Stress Corrosion Cracking Initiation Precursors in Cold-Worked Alloy 690 Using Advanced High-Resolution Microscopy

Although highly cold-worked UNS N06690 has been shown to be susceptible to stress corrosion crack growth in pressurized water reactor primary water, it is unclear whether cold-work promotes stress corrosion crack initiation under constant load test conditions. To evaluate the stress corrosion cracking initiation response, constant load tensile and blunt notch compact tension testing were performed on 21% and 31% cold-worked samples from two thermally treated UNS N06690 materials in 360°C simulated pressurized water reactor primary water. High-resolution examinations were conducted mid- and post-test to record the evolution of grain boundary precursor damage and intergranular crack nucleation by focused ion beam milling combined with scanning electron microscopy and by transmission electron microscopy. The surface and subsurface morphology of intergranular cavities, local corrosion, and shallow cracks were documented. Nanometer-sized cavities were observed associated with grain boundary carbides in all specimens. The 31% cold-work specimens consistently exhibited shallow intergranular cracks with oxidized crack flanks and a high density of cavities ahead of the oxide front. Three-dimensional distribution of carbides and cavities ahead of the cracked grain boundaries was analyzed to gain mechanistic insights into the processes that lead to crack initiation in highly cold-worked UNS N06690.