The grain boundary microstructures of the base and modified RR 2072 bicrystal superalloys and their effects on the creep properties

Investigation into the addition of minor grain boundary strengthening elements (C, Hf and B) on the properties of an experimented bicrystal nickel superalloy has been undertaken. The alloys were subjected to thermal exposure at 950 °C and creep at 950 °C under stress of 210 or 290 MPa. Detailed microstructural analysis has revealed that small, coherent, blocky and closely spaced M23C6 carbides precipitate on the grain boundaries in the modified alloys, and that relatively larger, incoherent and widely spaced P particles on the grain boundaries in the base bicrystals. A significant observation with regard to bicrystal creep is that M23C6 carbides can effectively inhibit grain boundary sliding, leaving ductile dimples on fracture surfaces, while P particles are only weak strengtheners. In addition, MC carbides that formed on the grain boundaries as they pass through the interdendritic regions enhance the creep performance of the modified bicrystal alloys through inhibiting grain boundary sliding.