It is generally agreed upon that transport through grawing alumina scales occurs predominantly due to inward oxygen diffusion along grain boundaries (GBs). However, it is also accepted that there exists some amount of concurrent outward aluminum diffusion along the GBs. Moreover, the effect of alloying elements on oxidation behavior and how they affect diffusional fluxes still remains uncertain.
For this study different Ni-Al alloys with additions of Hf, Y and Zr were investigated. The materials were subjected to an Initial exposure in O2 at 1100 °C. The Initial exposure was continued again at 1100 °C for 5 h after a smoothing of the surface. After this double exposure SEM a growth of ridges above the GBs, as an indication for outward diffusion were observed. Scanning transmission electron tomography energy dispersive X-ray spectroscopy investigations were completed with atom probe tomography, which gave quantitative Information about the concentration of the different elements at the GBs. This helped to calculate the Gibbsian excesses and to discuss the influence of different dopants.