High temperature oxidation of γ/γ'-strengthened Co-base superalloys

Isothermal oxidation was carried out on new γ/γ' Co-base superalloys of the system Co-Al-W-B. Appropriate B-contents lead to improved oxidation resistance and oxide layer adhesion. Oxidation at 800 and 900 °C results in formation of Co ₃O ₄, CoO, and complex oxides (containing Co, Al, and W). A continuous and protective inner alumina layer forms only at 800 °C. Furthermore, oxidation leads to phase transformation (γ/γ' to γ/Co ₃W microstructure) at the matrix/oxide layer interface due to Al-depletion. The effect of additional alloying elements such as Ta, Cr, Nb, Si, V, Mo, and Ir on the oxidation behaviour was also investigated.