Effects of Cr, W, and Mo on the High Temperature Oxidation of Ni-Based Superalloys

The oxidation behavior of Ni-9.5Co-(8~12)Cr-(2.5~5.5)Mo-(4~8)W-3Al-5Ti-3Ta-0.1C-0.01B alloys was investigated at 850 °C and 1000 °C The mass change, the phase of oxides, and the cross-sectional structure of specimens were analyzed after cyclic oxidation tests. The oxide scale was composed mainly of Cr₂O₃ and NiCr₂O₄, but NiO, TiO₂, and CrTaO₄ were also found. Al₂O₃ was formed beneath the Cr oxide layer. The Cr oxide layer and internal Al oxide acted as barriers to oxidation at 850 °C, while Al oxide was predominantly protective at 1000 °C. Cr increased the mass gain after oxidation test at both temperatures. Mo increased the oxidation rate at 850 °C but decreased the oxidation rate at 1000 °C. W slightly increased the mass gain at 850 °C but did not produce a significant effect at 1000 °C. The effects of Cr, Mo, W, and the temperature were discussed as well as the volatilization of oxides, the valence number of elements, and diffusion retardation.