Multimodal γ′ phase precipitation and mechanical properties of Co-based superalloys with Ta addition

Combined with transmission electron microscopy (TEM), atom probe tomography (APT) and density functional theory (DFT) calculations, the effects of tantalum (Ta) on the microstructural features of Co-based superalloys were investigated, focusing on how multimodal γ′ phases influence the alloy's yield strength. The findings indicate that Ta strongly partitions into the primary γ′ phase and occupied the B-sites of A₃B-L1₂ structure, significantly increasing its solvus temperature, volume fraction, and antiphase boundary (APB) energy, thereby enhancing high-temperature mechanical properties. The addition of 2 at.% Ta notably inhibits secondary γ′ precipitation due to a decrease in nucleation drive caused by the reduced Gibbs free energy change. The diffuse distribution of secondary γ′ phases provide more dislocation pinning sites within the alloy, resulting in mechanical properties that are less sensitive to temperature variations. This is evidenced by a gentler decline in yield strength with increasing temperature.