Overgrown grains appearing during sub-solvus heat treatment in a polycrystalline γ-γ’ Nickel-based superalloy

The present article focuses on establishing the nature of excessive grain growth in a new γ-γ’ superalloy, by investigating the underlying driving forces. Excessive grain growth upon annealing after deformation is a common phenomenon among Nickel-based superalloys. It shares common features with abnormal grain growth, but is fundamentally different, since not driven by capillarity forces. It consists in the selective and exaggerated growth of some grains, leading to the formation of heterogeneous microstructures and dramatically decreased mechanical properties. The present study demonstrates that the stored energy in the deformed matrix is the key driving force for exaggerated grain growth. The self-impingement of those grains is the main phenomenon limiting their growth, as Zener pinning by second-phase particles plays a minor role in the process. The absence of stored energy in the overgrown grains, the kinetics of their development and the existence of an incubation time before their appearance in the microstructures lead to the conclusion that excessive grain growth is a case of “critical” static recrystallization, involving a limited amount of nuclei. By evaluating the magnitude of the driving forces in presence before annealing, the microstructure evolutions during further annealing can be predicted, independently from the processing conditions.