A study on the effect of grain boundary elements on polycrystalline superalloys

Polycrystalline nickel-based superalloys substantially derive their mechanical properties from the strength of their grain boundaries. Carbon and boron are the main grain boundary strengthening elements and they play a critical role on the performance of superalloys. However, how these grain boundary elements enhance the mechanical properties of polycrystalline superalloys is not well understood. This thesis presents information about the impact of carbon and boron additions on the mechanical properties of two superalloys suitable for land-based gas turbine applications. New findings with high scientific and industrial relevance that will improve our understanding about the grain boundary elements during the design of new superalloys are presented. High-resolution characterisation methods were used to study the effect of boron on the grain boundary character of a new polycrystalline superalloy. Boron was found to alter substantially the grain boundary character by controlling phase precipitation. Different boron contents were also studied and correlated with the mechanical properties; creep performance was found to vary with boron content. In addition, in-situ studies at elevated temperatures were used to rationalise the effect of boron on improving the mechanical properties of superalloys. The main deformation mechanisms taking place were identified and presented. Finally, the effect of carbides on crack initiation and propagation under complex stress and temperature cycles was studied in detail for a conventional cast superalloy. Carbides were found to have a detrimental effect by serving as crack initiation sites on superalloys for power generation applications. In overall, the findings suggest that boron rather than carbon is the crucial grain boundary strengthener for superalloys used in industrial gas turbines.