A first-principles study of the effect of Ta on the superlattice intrinsic stacking fault energy of L1₂-Co₃(Al,W)

New Co-based alloys containing a L1₂ reinforcement phase display exceptional high-temperature properties. Early research has shown that the quaternary alloy Co-8.8Al-9.8W-2Ta (at.%) has a high-temperature strength comparable to single-crystal Ni-based superalloys above 1200 K. Associated with high strength is an unusual high density of intrinsic stacking faults within the γ′ precipitates. In this work, Density Functional Theory, the Axial Next Nearest Neighbor Ising model and Special Quasi-random Structures have been used to calculate the stacking fault energy of L1₂-Co₃(Al,W) and the effect of small Ta additions on the stacking fault energy. The model predicts a superlattice intrinsic stacking fault energy of 90–93 mJ/m², which increases up to 30% when one Ta atom is substituted on the Al/W sub-lattice. This effect can be explained by considering d-band effects resulting from the addition of Ta.