High-Temperature Structural Stabilities of Ni-Based Single-Crystal Superalloys Ni–Co–Cr–Mo–W–Al–Ti–Ta with Varying Co Contents

Abstract

It has been recently pointed out that the compositions of industrial alloys are originated from cluster-plus-glue-atom structure units in solid solutions. Specifically for Ni-based superalloys, after properly grouping the alloying elements into Al, Ni-like ($\overline{\text{Ni}}$), γ′-forming Cr-like ($\overline{\text{Cr}}^{{\gamma^{\prime } }}$) and γ-forming Cr-like ($\overline{\text{Cr}}^{\gamma }$), the optimal formula for single-crystal superalloys is established [Al–$\overline{\text{Ni}}$ ₁₂](Al₁ $\overline{\text{Cr}}^{{\gamma^{\prime } }}_{0.5} \overline{\text{Cr}}^{\gamma }_{1.5}$). The Co substitutions for Ni at the shell sites are conducted on the basis of the first-generation single-crystal superalloy AM3, formulated as [Al–Ni_12−x Co _x ](Al₁Ti_0.25Ta_0.25Cr₁W_0.25Mo_0.25), with x = 1.5, 1.75, 2 and 2.5 (the corresponding weight percents of Co are 9.43, 11.0, 12.57 and 15.71, respectively). The 900 °C long-term aging follows the Lifshitz–Slyozov–Wagner theory (LSW theory), and the Co content does not have noticeable influence on the coarsening rate of γ′. The microstructure and creep behavior of the four (001) single-crystal alloys are investigated. The creep rupture lifetime is reduced as Co increases. The alloy with the lowest Co (9.43 Co) shows the longest lifetime of about 350 h at 1050 °C/120 MPa, and all the samples show N-type rafting after creep tests.