Metallic yttrium additions to high temperature alloys: Influence on Al₂O₃ scale properties

In the present study, the effect of adding yttrium to alloys is investigated. The microstructure of the cast Fe-25Cr-4Al-0.5Y alloy used in the study shows that the vttrium is present in different shapes and sizes as the intermetallic phase, (Fe, Cr)₄(Al, Y), previously unreported in the literature. Upon oxidation in dry oxygen in the 1100-1200 °C temperature range, a columnar, fine-grained (0.5-1 μm) α-Al₂O₃ scale is formed which grows predominantly by inward oxygen grain-boundary transport. The intermetallic phase, during incorporation into the oxide scale, is converted into Y₃Al₅O₁₂, the chromium and iron from the intermetallic diffusing back into the metal matrix. The Y₃Al₅O₁₂ phase saturates the oxide scale with yttrium, which segregates to oxide grain boundaries. The microstructural features of the oxide scale resemble those of the scale formed on the yttria-dispersed alloy we investigated earlier. The improved adherence of the oxide scale is a consequence of yttrium doping, which facilitates the formation of a fine-grained scale in which oxide growth stresses can be relieved by diffusional plastic flow. Further, yttrium suppresses Al transport in the oxide scale and prevents Al₂O₃ nucleation within the scale, a process which can generate compressive stresses in the scale. The yttrium doping in the oxide scale is somewhat more efficient when it is present as a dispersoid in the metal.