The microstructures of base/modified RR2072 SX superalloys and their effects on creep properties at elevated temperatures

Abstract

Investigation into the addition of minor grain boundary strengthening elements (C, Hf and B) on the properties of an experimented single crystal nickel superalloy has been undertaken. The alloys were subjected to a variety of conditions including thermal exposure at 950°C and creep over 850–1050°C. Detailed microstructural analysis has revealed the presence of script and blocky MC carbides in the eutectic regions. On prolonged exposure σ precipitation occurred close to the MC carbides in the eutectic areas. A significant observation with regard to creep is that the principal crack initiation sites, namely casting pores, are greatly reduced owing to the precipitation of the MC carbides during solidification. This improves the intermediate temperature rupture life, although the presence of retained eutectic regions due to incomplete solidification degrades the high temperature performance of the modified alloys. The morphology of the carbides is a strong function of composition, with the presence of significant levels of Hf leading to the more blocky morphology, which is less prone to fracture during creep.