To provide resistance to aggressive environments at elevated temperatures, especially in excess of ~1000°C, alloys or coatings which develop α-Al2O3 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 Al2O3 scales formed on such alloys. The present study is directed toward developing an understanding of how the properties of A12O3 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% Al, ~0.5% Ti, and ~0.5% Y2O3. The oxidation kinetics of the alloy have been established at various oxygen partial pressures in the temperature range 1000°-1200°C. The α-Al2O3 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 μm) 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 μm) yttrium aluminum garnet upon incorporation into the A12O3 scale, leading to a garnet-saturated scale. It is suggested that the remarkable adherence of the α-Al2O3 scales is a consequence of a combination of factors. First, yttrium doping promotes the development of a fine-grained α-Al2O3 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 A12O3 nucleation within an existing scale are avoided.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Renewable Energy, Sustainability and the Environment
- Surfaces, Coatings and Films
- Materials Chemistry