Improvement of the Oxidation Resistance of the Single-Crystal Ni-Based TMS-82+ Superalloy by Ni–Al Coatings with/without the Diffusion Barrier

Abstract

Oxidation behavior of the uncoated base, Ni–Al coated and Re–Cr-Ni plus Ni–Al coated single-crystal (SC) Ni-based TMS-82+ superalloy is studied under cyclic air at 900oC for 200 h to assess the oxidation resistance. Regardless of the coating processing, Ni–Al coating is effective in improving the oxidation resistance due to the formation of a continuous a-Al2O3 layer in the scale. For the uncoated base superalloy, the mass-gain curves are fitted by a subparabolic relationship, and complex oxide products including predominately NiO, some CrTaO4, a-Al2O3, Cr2O3, a minor of spinels of (Ni, Co)Al2O4, AlTaO4 and h-Al2O3 are detected. Time-dependence of the oxide growth rate for both coated superalloy with/without the diffusion barrier is explained by the parabolic relationship. The oxide scales consist predominately of a-Al2O3 and a minor of h-Al2O3. The diffusion barrier of r-phase plays a negligible effect on the oxidation resistance during the cyclic exposure environment. The amount of detrimental ý-phase and topologically close-packed (TCP) phases in the interdiffusion zone in the coated superalloy with the diffusion barrier is greatly reduced compared with that without the diffusion barrier due to the distinct barrier effect limiting diffusion of elements between the bond-coat and the substrate.