Effect of quaternary additions on the oxidation behavior of Hf-doped NiAl

Cast model alloys, based on beta-NiAl+0.05at.%Hf were used to study the effects on oxidation behavior of elements that are commonly present in low-activity aluminide bond coatings on single-crystal, Ni-base superalloys. Single additions of Re, Ti, To, and Cr were examined in cyclic and isothermal exposures at 1100 to 1200degreesC in order to determine their effect on the oxide growth rate and resistance to scale spallation. With 1 at.% additions, all of these elements were found to be detrimental to the oxidation performance of the base NiAl + Hf alloy. Additions of Re and Cr were found to form second-phase precipitates in the alloy, which appeared to lead to scale spallation, while additions of Ti and Ta were internally oxidized and incorporated into the scale as grain-boundary segregants. These results suggest that it is necessary to minimize the levels of these types of elements that enter Hf-modified aluminide coatings by using process modifications or a diffusion barrier.