Effect of Quaternary Additions on the Oxidation Behavior of Hf-Doped NiAl

Abstract

Cast model alloys, based on β-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, Ta, and Cr were examined in cyclic and isothermal exposures at 1100 to 1200°C 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.