Oxidation behavior of a Single-crystal Ni-base superalloy in air.: I:: At 800 and 900°C

The oxidation behavior of a Single-crystal Ni-base superalloy was studied using discontinuous thermogravimetric analysis (TGA) and prolonged exposure in air at 800 and 900degreesC. The mass gain of specimens at 900degreesC was found to be lower than that of specimens at 800degreesC because of the formation of a protective inner alpha-Al2O3 layer at 900degreesC. A subparabolic time dependence ( n = 0.16 at 800degreesC and n = 0.10 at 900degreesC) of the oxide growth rate was determined at both temperatures. At 800degreesC, the superalloy exhibited non-uniform oxidation-in some areas a thin scale with an outer NiO layer and an inner layer of an Al-rich oxide was found and, in other areas, complex oxides [CrTaO4, NiCr2O4, (Ni, Co) Al2O4, etc.] below the NiO outer layer formed by growing into the superalloy. The scale formed at 900degreesC is more uniform than that formed at 800degreesC, consisting of several layers: an NiO outer layer, spinel-rich sublayer, a CrTaO4-rich layer, and an alpha-Al2O3 inner layer. The alpha-Al2O3 inner layer provides good oxidation protection and the specimen mass gain is low for test up to 1925 hr.