The effect of grain boundaries on high temperature oxidation of new γ′-strengthened Co–Al–W–B superalloys

•Isothermal oxidation in air of new γ/γ′ Co–Al–W–B superalloys between 800 and 1000 °C.•Grain boundaries lead to improved oxidation resistance at 800 and 900 °C.•Grain boundary diffusion benefits the formation of a protective inner Al2O3 layer.•At 1000 °C, single crystalline superalloys are more oxidation resistant.•Grain boundaries may act as obstacles for the dominant lattice diffusion.

Isothermal oxidation in air was carried out on a γ′-strengthened single crystalline Co–Al–W–B superalloy at 800, 900, and 1000 °C. The absence of grain boundaries leads to inferior oxidation properties at 800 and 900 °C, and improved properties at 1000 °C, compared to a polycrystalline alloy of the same composition. At lower temperatures, grain boundary diffusion is dominant and assumed to be the reason for enhanced B-accumulation in the inner oxide layer and enhanced formation of the protective inner Al2O3 layer. However, at 1000 °C superior oxidation resistance of the single crystalline superalloy can be observed.