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

The oxidation behavior of Ni-9.5Co-(8 similar to 12)Cr-(2.5 similar to 5.5)Mo-(4 similar to 8)W-3Al-5Ti-3Ta-0.1C-0.01B alloys was investigated at 850 degrees C and 1000 degrees 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 degrees C, while Al oxide was predominantly protective at 1000 degrees C. Cr increased the mass gain after oxidation test at both temperatures. Mo increased the oxidation rate at 850 degrees C but decreased the oxidation rate at 1000 degrees C. W slightly increased the mass gain at 850 degrees C but did not produce a significant effect at 1000 degrees 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.