Discontinuous γ′ coarsening in a Ni-Al-Mo base superalloy

Discontinuous coarsening in a Ni-AI-Mo base superalloy was studied emphasizing the influence of grain boundary misorientation. Nucleation and growth rates were determined as a function of temperature, time, and degree of misorientation for (001) twist boundaries. Both nucleation and growth were controlled by grain boundary diffusion and increased with twist angle and temperature. Grain boundary diffusivities (calculated assuming reduced γ′ surface energy as the driving force) were more than one order of magnitude greater than those reported for static boundaries of similar misorientation. Consideration of atomic jump frequencies in static and dynamic boundaries indicated that the high diffusivities cannot be attributed to motion enhanced grain boundary diffusion. Rather, the high calculated diffusivities are interpreted to indicate the presence of additional driving forces for discontinuous coarsening in the Ni-AI-Mo base superalloys. Potential sources of additional driving force are presented.