The effect of systematic variation of Ni:Co ratio on the oxidation behaviour of γ-γ′ Ni-Co-Al-Ti-Cr alloys

•Effect of Co:Ni ratio on the oxidation at 800 °C of (Ni,Co)75Al5Ti5Cr15 (at.%) alloys.•After 1000 h, the 28, 38 and 47 at.% Co alloys showed a flat, compact oxide scale.•These alloys showed a distinct Al2O3 subscale and less O ingress than lower Co alloys.•The 47 at.% Co alloy formed two distinct morphologies, one precluding O and N ingress.•The oxidised 56 at.% Co alloy exhibited a thick undulating porous Co-rich oxide scale.