On the phase composition changes during high temperature oxidation of Pt-modified β-NiAl at 1150°C

Abstract

The phase composition of the scales growing during oxidation at 1150°C on Pt-free and Pt-containing (0, 5, 10 and 15 at.%) β-NiAl intermetallic compounds were studied for exposure periods of up to 6 hours using photoluminescence spectroscopy with spatial resolution down to ca 1 μm. Scanning electron microscopy was used to observe the evolution of the surface morphology of the growing scales. It was found that initially unstable polymorphs of Al2O3 oxide (γ, δ and/or θ) developed, which subsequently transformed into α-Al2O3. The transformation occurred relatively early, and after 1 h oxidation essentially α-Al2O3 was detected on all the materials. After 15 min reaction, different regions were found in all cases, and both types of phases, transient aluminas and α-Al2O3 were detected. However, the rate of the phase composition depended on the region: in some regions, the transient aluminas co-existed with a-phase, while in others only transient or only α-Al2O3 were found. Regions composed of only α-Al2O3 were observed on materials containing higher amounts of Pt-additions, i.e. 10 and 15 at.%. This indicates that Pt additions accelerated the phase transformation of alumina polymorphs into α-Al2O3. In the early scales, comprising different regions, α-Al2O3 development was favoured in patches, and only this phase was always detected in ridges growing in cracks formed in patches. Conclusions are inferred concerning the scale evolution on the materials studied as well as on the effect of Pt additions.