Influence of cooling rate on the evolution of γˈ precipitates in a low-density CoNi-base γ/γ′ superalloy

The mechanical properties of γ/γʹ superalloys are governed by the size, shape, and distribution of the γʹ precipitates within the γ matrix. This work explores the feasibility of microstructure tuning by varying cooling rates in a low mass density γ/γ′ Co-30Ni-10Al-2Nb-4Ti-12Cr (at %) superalloy. We observe a strong cooling rate dependence on the morphology, composition, shape, and size distribution of γ′ precipitates. Cooling rates ≥6.25 K/s from super-solvus temperature (1413 K) of the alloy show unimodal size distribution of γˈ precipitates with a high number density. Whereas a slower rate of cooling (≤6.25 K/s) results in the formation of the bimodal size distribution of γˈ precipitates. For all the cooling rates explored (100, 28, 6.25, 1.625, 0.43, and 0.108 K/s), secondary γˈ precipitates exhibit nearly cuboidal morphology, and power law describes the evolution of their size with different cooling rates. Atomic-scale compositional analysis by an atom probe reveals the composition of secondary γˈ precipitates is dependent on the cooling rates. In addition, we found finer tertiary γˈ precipitates near the secondary γˈ precipitates and matrix interface, while relatively larger tertiary γˈ precipitates away from the secondary γˈ precipitates for the slow cooling rates (1.625 K/s, and 0.108 K/s). This was attributed to the concentration gradient that develops in the γ matrix region in between the secondary γʹ precipitates during continuous cooling. In the light of classical nucleation theory, the results indicate a multi-stage formation of γˈ precipitates whose morphology, size distribution, and composition are found to be dependent on the cooling rates. Hence, these experiments shows the possibility of tuning the microstructure of Co-based superalloys that is critical in governing their mechanical properties.