Partitioning and site occupancy of Ta and Mo in Co-base γ/γ′ alloys studied by atom probe tomography

•Detailing of the atomistic nature of γ′ precipitates in Co–Al–W base alloy.•Anomalous partitioning behavior of Mo, when compared to that in Ni superalloys.•Ta and Mo occupies W sub-lattice in Co3(Al, W) stoichiometry.

The detailed understanding of solute partitioning and site occupancies of these solutes within the ordered γ′ (L12) precipitates holds a key role in the development of cobalt-base γ/γ′ alloys with optimized properties. The present atom probe tomography study utilizes both structural and compositional information to determine the partitioning behavior of transition elements like Ta and Mo between γ matrix and γ′ precipitates and their site occupancy within the γ′ phase. The addition of Ta, which enhances the formation of γ′, to a ternary Co–Al–W alloy with stoichiometric Co3(Al,W) precipitates, results in the substitution of only the W in the γ′ precipitates to form Co3(Al, W, Ta) precipitates. Interestingly, Mo, typically considered a γ solid solution strengthener in nickel-base alloys, also partitions strongly to γ′ precipitates when added to the Co–Al–W alloy and displaces only the W atoms. The experimentally observed equal atomic substitution of W by both Ta and Mo, without any change in the Al content within the γ′ precipitates, gives insights into the energetics of relative site substitutions in this ordered compound.