Oxidation Behavior of a Single-Crystal Ni-Base Superalloy in Air. I: At 800 and 900°C

Abstract

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 900°C. The mass gain of specimens at 900°C was found to be lower than that of specimens at 800°C because of the formation of a protective inner α-Al₂O₃ layer at 900°C. A subparabolic time dependence (n=0.16 at 800°C and n=0.10 at 900°C) of the oxide growth rate was determined at both temperatures. At 800°C, the superalloy exhibited nonuniform 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 900°C is more uniform than that formed at 800°C, consisting of several layers: an NiO outer layer, spinel-rich sublayer, a CrTaO₄-rich layer, and an α-Al₂O₃ inner layer. The α-Al₂O₃ inner layer provides good oxidation protection and the specimen mass gain is low for test up to 1925 hr.