The reactive element effect of ceria particle dispersion on alumina growth: A model based on microstructural observations

The oxidation kinetics of alumina-forming metals can be affected by adding a small amount of a reactive (normally rare earth) element oxide (RE(x)O(y)) and the segregation of the reactive element (RE) ions to the growing alumina grain boundaries (GBs) has been considered as a responsible reason. However, this interpretation remains a controversial issue as to how RE ions are produced by RE(x)O(y) which is thermodynamically and chemically stable in metals. The question is answered by a model that is based on transmission electron microscopy (TEM) investigation of a CeO(2)-dispersed nickel aluminide oxidized in air at 1100 °C. The CeO(2) dispersion is incorporated into the alumina scale by the inward growth of inner α-Al(2)O(3), where it partially dissolves producing tetravalent Ce cations which then transform to trivalent cations by trapping electrons. The trivalent cations segregate to the α-Al(2)O(3) GBs and diffuse outward along first the GBs and later the twin boundaries (TBs) in the outer γ-Al(2)O(3) layer, being precipitated as Ce(2)O(3) particles near surface.