L12-Strengthened Cobalt-Base Superalloys

The discovery of the gamma'-Co-3(Al,W) phase with an L1(2) 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 gamma' single-phase and gamma + gamma' two-phase alloys in comparison with Ni-base gamma'-L1(2) phase and gamma + gamma' superalloys. Relatively low stacking fault energies and phase stability of the gamma' phase compared with those in Ni-base alloys are responsible for the unique deformation behaviors observed in Co-base gamma' and gamma + gamma' 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 gamma'-Co-3(Al,W), thus improving the high-temperature strength of Co-base gamma' phase and gamma + gamma' two-phase superalloys.