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To provide resistance to aggressive environments at elevated temperatures, especially in excess of -1000~ alloys or coatings which develop a-A1203 scales are the best choice. It has been pointed out that the presence of highly stable rare earth oxide dispersoids in high temperature alloys leads to improvements in the corrosion-resistant properties of A1203 scales formed on such alloys. The present study is directed toward developing an understanding of how the properties of A1203 scales formed on Fe-based alloys are influenced by yttrium oxide dispersoids in the alloy. The Fe-based alloy system selected for the current study consists of -20% Cr, -4.5% A1, -0.5% Ti, and -0.5% Y20~. The oxidation kinetics of the alloy have been established at various oxygen partial pressures in the temperature range 1000~176The a-A1203 scales which result upon oxidation are observed to be columnar, ultrafine grained, and extremely adherent when thermally stressed. Platinum markers initially placed on the alloy surface are found at the oxide/gas interface at the completion of oxidation, suggesting that scale growth occurs by exclusive inward oxygen migration. The ultrafine grain size (0.5-1 ~m) suggests that grain boundaries in the oxide scale are the preferred path for oxygen migration. The fine dispersoid particles in the alloy (200-500A) transform to coarse (-0.5 ~m) yttrium aluminum garnet upon incorporation into the AI~O.~ scale, leading to a garnet-saturated scale. It is suggested that the remarkable adherence of the ~-A1203 scales is a consequence of a combination of factors. First, yttrium doping promotes the development of a fine-grained a-A1203 scale which can effectively relieve oxide growth stresses by diffusional plastic flow. Second, because the alumina scale grows by exclusive inward oxygen transport, growth stresses arising from A1203 nucleation within an existing scale are avoided.High temperature alloys or coatings designed to resist aggressive environments at elevated temperatures should be capable of developing a surface oxide layer which is thermodynamically stable, slow growing, and adherent. The three oxides which fit the requirement of slow growth are Cr203, SiO~, and A1~O3. Of these, chromia is the fastest growing and alumina is the slowest growing. The thermodynamic stability of these oxides is in the order A12Oz > SiO2 > Cr20~. At temperatures exceeding -1000~ chromium oxide scales tend to become unstable; in environments of relatively high oxygen partial pressure, the oxide can vaporize as CrO3 (1), while under highly reducing conditions the oxide can transform to other more thermodynamically stable phases (2).For applications in hostile environments at temperatures in excess of -1000~ A1203 and SiO2 scales should be preferred to provide corrosion resistance. While the kinetic stages leading to the development of a continuous-surface oxide layer are certainly of importance, once such an oxide layer is established, the most important consideration is how well the oxide layer adheres to the alloy surface. T...

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