Effect of alloying elements on the coarsening rate of γʹ precipitates in multi-component CoNi-based superalloys with high Cr content

The γʹ coarsening behavior during long-term aging at 850°C of multi-component CoNi-based superalloys with high (14 at.%) Cr content were investigated, and the effects of W, Mo or Ti (1 at.%) additions were analyzed with respect to the change in the γ/γʹ microstructure, lattice misfit, and elemental partitioning behavior. The coarsening behavior of the γʹ precipitates follows the classical Lifshitz-Slyozov-Wagner (LSW) model, where the size is proportional to t1/3. The addition of either W, Mo or Ti (1 at.%) alters the γ/γʹ elemental partitioning behavior and thus the γ/γʹ lattice misfit and interfacial energy, resulting in quite different coarsening rates of the γʹ precipitates, with Ti having the most significant effect. Cr was found to be the rate-limiting element of γʹ coarsening rather than the other alloying elements. This study could provide guidance for the design and optimization of CoNi-based superalloys with lower γ' coarsening rate.