An innovative approach towards forming the serrated grain boundaries and refining the γ′ precipitates in nickel-based superalloys

The serrated grain boundaries have been proven to improve the creep performance and reduce crack growth rate of polycrystalline nickel-based superalloys. However, a slow cooling rate to form the serrated grain boundaries often results in the coarsening of γ′ precipitates, thereby deteriorating the high temperature strength of the nickel-based superalloys. Hereof, a new heat treatment processing, high temperature pre-precipitation treatment (HTPT), is developed to simultaneously achieve the serrated grain boundaries and fine γ′ precipitates. The effects of HTPT conditions on the microstructure of the powder metallurgy nickel-based superalloys are systematically characterized. Surprisingly, the HTPTed samples possess the serrated grain boundaries and obtain even finer γ′ precipitates than those in air cooled samples. With the increasing of holding time from 10 min to 30 min and 60 min at 1140 ℃ for pre-precipitation, the average size of tertiary γ′ precipitates decreases from 43.9 nm to 34.7 nm and 30.4 nm, and the amplitude of the serrated grain boundaries increases from 0.84 µm to 1.71 µm and 1.87 µm, respectively. On the other hand, the excessively long holding time causes the abnormal growth of the secondary γ′ precipitates and the suitable holding time is considered to be 30 min. The formation of the serrated grain boundary during HTPT is related to the growth of the pre-precipitated γ′ precipitates and the hindering effect on the migration of grain boundaries. Our work demonstrates a new way to control the microstructure of nickel-based superalloys.