Intergranular oxidation of nickel-base alloys : potentialities of focused ion beam tomography

The present work focuses on the intergranular oxidation of Alloy 600 and its weld material Alloy 82 after exposure in simulated primary water at 340–360 °C during 1000–1400 h. The effects of intergranular chromium carbides, grain boundary character and pre-straining are studied either on industrial heats or model microstructures. Three-dimensional reconstructions of well-chosen grain boundaries are obtained using focused ion beam tomography in a scanning electron microscope. The 3D characterizations show that (1) twin boundaries are less susceptible to oxidation than high-angle grain boundaries; (2) intergranular chromium carbides delay the intergranular oxidation kinetics; (3) the intergranular oxidation depth is quite homogeneous along a given grain boundary except when strain heterogeneity exists in its vicinity, (4) strain heterogeneity tends to accelerate the intergranular oxidation kinetics; and (5) crack initiation in the oxidized grain boundary occurs at the alloy/oxide interface.