Grain boundary segregation of cation dopants in α-A2O3 scales

A Fe-20 atom % Cr-1O%Al matrix was dispersed with a wide range of oxide particulates in order to study the effect of oxygen-active dopants on high-temperature oxidation performance and alumina scale microstructure. The effects of these various cation dopants have been correlated with dopant ion segregation to the alpha-Al2O3 grain boundaries using analytical electron microscopy. Elements such as Mn and V showed little effect on the oxide scale microstructure and were not observed to segregate. Elements such as Y, Nd, and Gd were found to have a positive effect on oxidation performance and segregated to scale grain boundaries, resulting in finer, more columnar alpha-Al2O3 grains. However, Ti, Ta, Ca, and Nb also were found to segregate but had less of an effect on oxidation behavior. These results indicate that cation segregation to scale grain boundaries is not a sufficient condition for achieving beneficial oxidation performance. The driving force for diffusion and segregation in growing alumina scales is discussed. Based on comparisons with an alloy containing an Al2O3 dispersion, the finer scale grain sizes observed with reactive element oxide dispersions are the result of grain growth inhibition and not a heterogeneous nucleation effect.