Critical Hafnium Content for Extended Lifetime of AM1 Single Crystal Superalloy

Additions of reactive elements are known to improve the oxidation resistance of alumina-forming materials. The critical thresholds for the reactive element to be beneficial are however dependant on the overall metallurgical environment. Therefore, this paper investigates the influence of increasing amounts of Hf on the cyclic oxidation behaviour of AM1 first generation single crystal Ni-based superalloy at 1150 °C in air. Insufficient doping and overdoping effects are clearly identified. The former occurs because of insufficient blockage of cation outward diffusion to establish and maintain protective scales of alumina. The overdoping effects arise on the one hand from the well-known internal oxidation of Hf that produces bulky oxides. On the other, an indirect effect is reported for the first time whereby the excess Hf destabilizes the Ta and Ti carbides. Therefore, Ta and Ti diffuse outwardly to form bulky and non-protective oxides that provoke the spallation of the protective layers.