Effect of theta-alumina formation on the growth kinetics of alumina-forming superalloys

Alumina-forming ODS superalloys are excellent oxidation-resistant materials. Their resistance relies upon the establishment of a stable, slow-growing, and adherent alpha-alumina. In the present investigation, these alloys exhibited unstable and relatively less adherent theta-alumina phase, which increased the oxidation rate in the transient stage and converted into alpha-alumina in the later part of the exposure. The oxide-growth process was found to depend upon various parameters such as temperature, time, and presence of an active element in the superalloy. Characterization carried out by XRD, SEM/EDAX, and AES on oxidized ODS and non-ODS alloys demonstrated a significant influence of the active element, Y on the transformation of theta- to alpha-alumina. SIMS analysis of two-stage oxidation at 900 degrees C for two different durations evidently showed that the change in the transport process is due to theta-to-alpha-alumina transformation. On the basis of these results, a new and consistent mechanism is proposed to explain the influence of theta-alumina and its transformation on growth kinetics and the effect of yttrium on the transformation leading to good scale adherence and oxidation resistance.