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

It has been recently pointed out that the compositions of industrial alloys are originated from cluster-plus-glueatom structure units in solid solutions. Specifically for Ni-based superalloys, after properly grouping the alloying elements into Al, Ni-like ((Ni) over bar), gamma'-forming Cr-like ((Cr) over tilde (gamma')) and gamma-forming Cr-like ((Cr) over bar (gamma)), the optimal formula for single-crystal superalloys is established [ Al-(Ni) over bar (12)](Al-1(Cr) over bar (gamma')(0.5) (Cr) over bar (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-Ni12-xCox](Al1Ti0.25Ta0.25Cr1W0.25Mo0.25), 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 degrees 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 gamma'. 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 degrees C/120 MPa, and all the samples show N-type rafting after creep tests.