New Co-based alloys containing an L12 reinforcement phase exhibited exceptional high-temperature properties. Research had shown that the quaternary alloy, Co-8.8Al-9.8W-2at%Ta, had a high-temperature strength which was comparable to that of monocrystalline Ni-based superalloys above 1200K. Associated with the high strength was an unusual high density of intrinsic stacking faults within the γ′ precipitates. Density functional theory, the axial next-nearest neighbor Ising model and special quasi-random structures were used to calculate the stacking-fault energy of L12 Co3(Al,W) and the effect of small Ta additions upon the stacking-fault energy. The model predicted a superlattice intrinsic stacking-fault energy of 90 to 93mJ/m2, which increased up to 30% when one Ta atom was substituted at the Al/W sublattice. This effect could be explained by considering d-band effects resulting from the addition of Ta. A First-Principles Study of the Effect of Ta on the Superlattice Intrinsic Stacking Fault Energy of L12-Co3(Al,W). A.Mottura, A.Janotti, T.M.Pollock: Intermetallics, 2012, 28, 138-43