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.