Precipitation characteristics of γ' precipitates of the GH4742 nickel-based superalloy at a slow cooling rate

The double cone samples of the predeformed GH4742 superalloy were treated by subsolid solution heat treatment (Sub-SHT) at 1080 ℃/8 h and then cooled at a cooling rate of 22 ℃/min. The evolution of γ' precipitates by coupling hot deformation and heat treatment was studied by in situ electron backscatter diffraction + scanning electron microscopy. This research explains the morphological variations between the recrystallized and deformed grains of the γ' precipitates, as well as the relationship between the characteristics of the grain boundaries (GBs) and γ' precipitates at the GBs. The results show that when the predeformed samples were subjected to an 8 h Sub-SHT and then cooled at 22 ℃/min, the γ' precipitates exhibit a tri-modal size distribution, and the primary γ' precipitates are dendritic. The morphology of γ' precipitates within deformed grains is related to the accumulation of dislocations and other defects. The maximum size of columnar γ' precipitates is obtained at the high-angle grain boundary, where the misorientation ranges from 30° to 40°. The length of columnar γ' precipitates results from the comprehensive effect of the GB energy determined by the GB misorientation and the GB mobility determined by the stored energy difference. Owing to the low energy and low mobility of the Σ3 twin boundary, the γ' precipitates at the Σ3 twin boundaries remain stable, and the morphology and size are no different from those within the grains. These findings may be helpful for the design and optimization of Ni-base superalloys.