L1₂-Strengthened Cobalt-Base Superalloys

The discovery of the γ′-Co₃(Al,W) phase with an L1₂ structure provided Co-base alloys with a new strengthening mechanism, enabling a new class of high-temperature material: Co-base superalloys. This review discusses the current understanding of the phase stability, deformation, and oxidation behaviors of γ′ single-phase and γ + γ′ two-phase alloys in comparison with Ni-base γ′-L1₂ phase and γ + γ′ superalloys. Relatively low stacking fault energies and phase stability of the γ′ phase compared with those in Ni-base alloys are responsible for the unique deformation behaviors observed in Co-base γ′ and γ + γ′ alloys. Controlling energies of planar defects, such as stacking faults and antiphase boundaries, by alloying is critical for alloy development. Experimental and density functional theory studies indicate that additions of Ta, Ti, Nb, Hf, and Ni are effective in simultaneously increasing the phase stability and stacking fault energy of γ′-Co₃(Al,W), thus improving the high-temperature strength of Co-base γ′ phase and γ + γ′ two-phase superalloys.