EFFECT OF THETA-ALUMINA FORMATION ON THE GROWTH-KINETICS OF ALUMINA-FORMING SUPERALLOYS

Alumina-forming ODS superalloys are excellent oxidation-resistant mate rials. Their resistance relies upon the establishment of a stable, slo w-growing, and adherent alpha-alumina. In the present investigation, t hese alloys exhibited unstable and relatively less adherent theta-alum ina phase, which increased the oxidation rate in the transient stage a nd 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 supera lloy. Characterization carried out by XRD, SEM/EDAX, and AES on oxidiz ed ODS and non-ODS alloys demonstrated a significant influence of the active element, Y on the transformation of theta- to alpha-alumina. SI MS 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 r esults, a new and consistent mechanism is proposed to explain the infl uence of theta-alumina and its transformation on growth kinetics and t he effect of yttrium on the transformation leading to good scale adher ence and oxidation resistance.