On the formation of irregular-shaped gamma prime and serrated grain boundaries in a nickel-based superalloy during continuous cooling

Interrupted cooling tests have been carried out in order to study the microstructural evolution of a high γ′ fraction nickel-based superalloy during continuous cooling from above γ′ solvus. It was found that the formation of irregular-shaped γ′ during slow cooling is the result of coarsening and coalescing of γ′ precipitates which involved dissolution of adjacent even finer precipitates and solute flux from them. The formation of serrated grain boundaries was found to be associated with the presence of large γ′ precipitates at the grain boundaries which may have pinned grain boundaries during their migration.