Effects of boundary migration and pinning particles on intergranular oxidation revealed by 2D and 3D analytical electron microscopy

To evaluate the beneficial effects of intergranular carbides on inhibiting stress corrosion cracking (SCC), Alloy 600 samples in the thermally-treated (TT) and solution-annealed (SA) conditions were analyzed after exposure to 480 °C hydrogenated steam. Intergranular oxidation was observed in both samples, along with diffusion-induced grain boundary migration (DIGM). Compositional mapping revealed DIGM to be more severe in 600SA, while conventional intergranular solute diffusion involving static boundaries appeared dominant in 600TT. 3D serial sectioning of the boundaries revealed strong variations in the intergranular oxide for 600TT, particularly in the depth of oxide penetration. This was attributed to Cr carbide precipitates, present due to the thermal treatment, pinning against DIGM. Immobilizing the boundary via carbide pinning reduces oxide growth by effectively starving it of the ready solute supply otherwise available to a migrating boundary. Because of this interaction between boundary migration, carbide pinning, and oxide growth, the intergranular oxidation in 600TT is highly variable and discontinuous compared to 600SA, where DIGM is unchecked and easier oxide growth produces near-uniform coverage of the boundary. This marked decrease in boundary oxide coverage likely contributes to the improved SCC resistance of 600TT. These results demonstrate the necessity of investigating such phenomena using 3D analysis methods.