Influence of Ti and Ta on Deformation, Segregation, and Ordering Kinetics in Polycrystalline CoNi-Based Superalloys

Abstract Polycrystalline CoNi-based superalloys exhibit an attractive property profile for intermediate-temperature applications, with Ti- and Ta-rich variants showing exceptional creep resistance compared to cast & wrought Ni-based superalloys. However, the mechanisms underlying this improvement remain insufficiently understood. In this study, the deformation and segregation behavior of three CoNi-based alloys with varying Ti and Ta contents were systematically examined and benchmarked against Udimet720Li. Interrupted compression creep tests at 750 °C were combined with advanced scanning transmission electron microscopy and atomic-scale energy dispersive spectroscopy to reveal the controlling mechanisms. Our results demonstrate that Ti and Ta additions increase the anti-phase boundary energy (APB) of the γ ′ phase, facilitating a beneficial transition from APB-based shearing to a more creep-resistant, stacking fault-dominated deformation regime. A quantitative correlation between APB energy and the diffusional length of γ -forming elements was established to rationalize this transition. Strengthening through localized phase transformations along planar faults is effective but kinetically limited by solute diffusion, particularly of W. Ta proves uniquely beneficial, as its higher diffusivity enables rapid formation of ordered phases along stacking faults and microtwin boundaries, thereby suppressing microtwin thickening. By contrast, Ta-free alloys exhibit delayed transformation and an initial softening. These insights provide guidelines for the design of next-generation creep-resistant superalloys.