Microstructural and Environmental Effects During Creep Crack Growth in a Superalloy

Abstract

Two heats of a nickel-base superalloy were heat-treated to form three different grain boundary carbide microstructures, a blocky M23C6 carbide, a cellular M23C6 carbide, and a carbide-free structure. Creep crack growth rate tests were run at 650°C in four environments: high-purity helium, air, He-4CH4 and He-3SO2. Crack growth in all cases was intergranular and involved the nucleation and growth of micrometre-sized cavities on the grain boundaries. The air and He-3SO2 environments were very aggressive, increasing the crack growth rates over those in helium by more than 100-fold. The grain boundary microstructures significantly affected the growth rates, and microstructures that were best in some environments and were worse in others. The results suggest that a single, generalized cavity nucleation and growth mechanism may be responsible for crack growth in superalloys, but environment-microstructure interactions will have to be accounted for to provide a satisfactory description of the cracking behavior. The results also suggest that some of the general ideas about the role of carbides in superalloys may not be true under crack growth conditions, and that alloys optimized by tests in air may not be the most suitable for use in other environments.