Using Rapid Thermal Annealing for Studying Early Stages of High-Temperature Oxidation of Superalloys

The great potential of Rapid Thermal Annealing (RTA), a widespread technique in semiconductor technology, for studying high-temperature properties of superalloys is demonstrated. As an application example, early stages of high-temperature oxidation of Co/Ni-base superalloys are investigated. Model alloys with different Co/Ni ratio are oxidized in synthetic dry air at 900 °C to understand the differences between the oxidation behavior of Ni- and Co-base alloys. Upon short exposure times (below 64 s), the two-phase γ/γ′ microstructure has a pronounced influence on both the morphology and growth kinetics of Al₂O₃ precipitates. Depending on the base element, Al₂O₃ nucleates in either the γ′ phase in the form of needles (Ni-base superalloy) or in the γ matrix as lath- and plate-like precipitates (Co-base superalloy) with the latter showing much slower formation kinetics. Observed differences in oxidation behavior can be directly correlated to changes in the partitioning of W, which acts as a γ former in Ni- and a γ′ former in Co-base superalloys. We propose that a significant amount of W in γ′ has an inhibitory effect on the diffusion of Al, suppressing the formation of Al₂O₃ in the γ′ phase of Co-base superalloys. Our approach proved to be very successful for oxidation studies and opens up new opportunities for superalloys research.