Alloying effects on oxidation mechanisms in polycrystalline Co–Ni base superalloys

•Oxygen isotopic exchange used to examine effectiveness of oxide scales.•Co/Ni superalloys achieved that produce only 1μm scales on oxidation at 800°C for 200h.•STEM characterisation of the scales performed. Oxidation mechanisms in polycrystalline Co–Ni–Cr–Al–W–Ta alloys were investigated using O16/O18 isotopic tracer analysis in the focused ion-beam secondary ion mass spectrometer (FIB-SIMS). It was found that Al additions favour the formation of a continuous alumina-rich layer, that Cr indirectly improved oxidation resistance and that increasing the Co fraction resulted in poorer oxidation performance. In the alloy containing 15 at.% Cr and 10 at.% Al, the outer scale formed after 200h oxidation at 800°C comprised oxides less than 1μm thick. It is concluded that protective oxide scales can be formed in Co–Ni base superalloys.