The effect of nickel and silicon addition on some oxidation properties of novel Co-based high temperature alloys

Isothermal oxidation was carried out on γ′-strengthened Co–Al–W–B–Ni and Co–Al–W–B–Si superalloys at 800, 900, and 1000 °C. Ni-addition to Co–Al–W–B alloys leads to an absence of B within the inner oxide layer and at the grain boundaries, as indicated by ToF-SIMS. As a result, also inner Al2O3-formation is inhibited, which leads to inferior oxidation properties compared to Ni-free alloys. In contrast, Si-addition enhances formation of protective Al2O3 at 900 and 1000 °C due to enhanced selective Al-oxidation. Si-containing phases at the oxide/alloy interface, at the grain boundaries, and within precipitates are of subordinate importance but may further improve oxidation resistance.

► Isothermal oxidation in air on new γ′-strengthened Co-base superalloys.
► Ni-addition leads to an absence of B in the oxide layer and at the grain boundaries.
► Formation of protective inner Al2O3 is inhibited compared to Ni-free alloys.
► Si-addition enhances the formation of protective inner Al2O3 at T > 900 °C.