Effect of reactive elements on the segregation behavior and microstructure of (alpha)-Al(sub 2)O(sub 3) scales on (beta)-NiAl

摘要

In order to improve oxidation resistance, small amounts of oxygen- active or ''reactive'' elements (RE) such as Y, Zr, Hf, Ce are added to chromia- and alumina-forming high-temperature alloys. As an alternative to a system where the RE-ion supply is self-limiting (Y-implant), it was desired to study effect of RE source-removal on microstructure and grain boundary segregation levels. (beta)-NiAl doped with 0.23 wt% Zr was oxidized in oxygen at 1500 C for 50 h. Upon cooling, the scale spalled into pieces; these spalled (alpha)-Al(sub 2)O(sub 3) scales were then annealed in air at 1500 C for 1 and 50 h. Specimens were fabricated parallel to metal/scale interface, and grain boundary chemistries were determined using X-ray energy dispersive spectrometry. Results show that no qualitative change in microstructural scale or zirconia particle size/density occurs after annealing, which is consistent with the model in which the gas- interface microstructure development depends on a continuous supply of zirconia diffusing from the metal interface towards the gas interface.