On the mechanism of high-temperature oxidation of ODS superalloys: Significance of yttrium depletion within the oxide scales

The oxidation behavior in air of two ODS superalloys, the chromia-forming PM 1000 and the alumina-forming PM 2000, was investigated at temperatures between 900 and 1300 degrees C. For both allays growth rates of the oxide scales obey a subparabolic time dependence and an Arrhenius-type temperature dependence of the rate constants is observed. Diffusion of yttrium to the surface of the oxide scales leads to almost complete depletion of this element within the oxide scales of both alloys. Scale growth then takes place by simultaneous inward diffusion of oxygen and outward diffusion of cations. At this stage of the oxidation process, the scale-growth mechanism is similar for both ODS alloys, at least in a qualitative way, to conventional (non-ODS) alloys of the same base composition.