Electron Microscopy of Hafnium-Modified Superalloys

Abstract

Cast nickel-base superalloys, used in turbine hardware, are typically composed of ~ 60 vol.% γ' phase coherently precipitated in an f.c.c. matrix, eutectic phases, and one or more carbide phases. These alloys are not heat-treatable in the sense of being able to re-solution and reprecipitate the γ' phase. Hence, the properties are governed by the as-cast microstructure which has become increasingly more complex as higher elevated-temperature strength has been required. This improvement in strength has an associated penalty in that the ambient and intermediate temperature ductility of cast superalloys is poor.Recently, it has been shown that additions of hafnium in nickel-base superalloys result in substantial improvements in ductility. The results presented here are from a study undertaken to define the role of hafnium in modifying microstructure in nickel-base alloys and the effect of this modification on properties.