Effect of Ta on the microstructure of high performance Ni-based powder metallurgy superalloys

The effect of Ta on the microstructures of nickel-based powder metallurgy superalloys with different Ta contents was investigated in experiments and thermodynamic calculations. The Ta element predominantly occupied the γ′ phase of the superalloys, with some incursion into the γ phase, and minimal presence in the MC carbide. By substituting part of the Ti-Nb alloy,the Ta element transforms(Ti-Nb)C into(Ti-Ta-Nb)C. The MC carbides and small amounts of M23C6 carbides were observed on the ribbon-like grain boundaries. The Ta addition little affected the primary γ′ phase, but greatly influenced the secondary γ′phase. Ta promoted the splitting of the secondary γ′ phase, converting it to a more stable state. Increasing the Ta content increased the lattice constants of γ′ and γ and altered the degree of misfit between γ′ and γ, indicating a morphology change. The Ta content also influenced the formation of the tertiary γ′ phase.