Revealing the Ta effect on γ' phase coarsening behavior and mechanical properties in CoNi-based superalloys

Due to excellent overall high-temperature performance, CoNi-based superalloys have drawn much attention as high-temperature structural materials. In present work, three CoNi-based superalloys with varying Ta contents were studied to investigate their impact on the microstructural evolution and mechanical properties, using atom probe tomography (APT), high-resolution X-ray diffraction (HR-XRD), and Density functional theory calculation (DFT). The results indicated that Ta increased the lattice misfit between γ/γ' phases, elevated the γ' phase solvus temperature, and maintained the volume fraction over long-term thermal exposure. APT results revealed that Ta enhanced the distribution of W to γ' phase and elevated the content of Al in γ matrix. The high quantity of Ni altered the coarsening rate-limiting elements in γ' phase from W to Al. The increased lattice misfit and volume fraction due to Ta addition accelerate γ' phase coalescence. Meanwhile, Ta addition reduces the interface width between γ/γ' phases, leading to the increases in interfacial energy, and the acceleration in coarsening kinetics. Furthermore, microstructure degradation during thermal exposure accelerates γ' phase coarsening, leading to a reduction in yield strength. This study elucidated the Ta effect on the coarsening mechanism of CoNi-based superalloys and its influence on the mechanical properties, by integrating diffusion kinetics and thermodynamics, which provided significant insights for optimizing the composition and designing CoNi-based superalloys with excellent microstructural stability.