The initial study reported here focused on the oxidation of Co-Al-W alloys containing nickel (29-34 at. %) and chromium (12-17 at. %) additions with the objective of improving the oxidation resistance of these alloys at high temperatures (800-850 °C) for high-pressure turbine discs. Starting with the analysis in the as-atomised powder, the material was then consolidated using hot-isostatic pressing (HIP) and a two-stage HIP was performed to optimise the precipitation of carbides. Solution heat-treated and aged samples were studied in terms of the effects of additional alloying elements on the alloys’ oxidation behaviour after high temperature exposures. Isotopic oxidation tracer experiments were carried out at 800-850 °C in isotopically enriched oxygen (16O- and 18O-) to elucidate the oxidation mechanisms.
Oxidation mechanisms in alloys with different Co-Ni ratio and Cr, Al contents were investigated in polycrystalline cobalt-base superalloys. Various characterisation techniques were used to investigate the oxidation mechanisms such as FEG-SEM, STEM with EDX, high resolution TEM, FIB-SEM, and FIB-SIMS were applied.
The various phases present in the initial and oxidised microstructures have been established, together with the oxidation behaviour of the alloy. After oxidation at 800 °C for 200 hours, the alloys exhibit a multi-layered structure, consisting of an outer oxide scale with a continuous alumina layer at the oxide-alloy interface. The bulk metal consisting of a cobalt solid solution containing secondary phases (Co3W and CoAl), carbides precipitates (Ta-Zr carbides) and fine γ’ (Co3(Al-W)) precipitates. The oxidation behaviour on shot-peened surfaces was studied and it was found that shot-peening does not affect the oxidation resistance directly but it does alter the sub-surface region causing recrystallisation and thus opening up diffusion paths for cations/anions diffusions.
The study of oxygen transport mechanisms using 18O2 as a tracer element has revealed that an outer chromium-rich oxide layer formed at the metal surface during the initial oxidation and continuous inner alumina layer is primarily formed subsequently. This alumina layer appears to effectively block the diffusion of oxygen to the metal/oxide interface and oxidation proceeds by outward cation diffusion. This study demonstrates that protective oxide scales can be formed in cobalt-base superalloys.