Uncovering the effect of Mo addition on the precipitation kinetics of the γ′ phase in Ni–Al–Mo model superalloys using CALPHAD-assisted phase-field simulations

Mo is a key solid solution strengthening element that improves the mechanical properties of Ni-based superalloys, but the partitioning behavior of Mo remains controversial. Herein, we uncover the effect of Mo addition on the precipitation kinetics of the γ′ phase in Ni–(17−x) Al–x Mo at.% (x = 2, 3, 4, and 5) alloys by directly coupling accurate thermodynamic and kinetic descriptions with a phase-field model. Our simulations show that the addition of Mo would decrease the γ′ coarsening rate due to the decrease in the diffusion rate and the γ/γ′ lattice misfit. Besides, the partitioning behavior of Mo is reversed from the γ′ phase (K_Mo^γ′/γ>1) into the γ matrix (K_Mo^γ′/γ<1) with increasing Mo concentration, which is attributed to the solubility limit of Mo in the γ′ phase as well as a decrease in the γ/γ′ lattice misfit. Meanwhile, Al and Mo preferentially occupy the Al sublattice site of the γ′–Ni₃Al and form the Ni₃(Al, Mo) phase in Ni–Al–Mo ternary model alloys. Furthermore, the solid solution strengthening effect increases with increasing Mo concentration because of an increase in the Mo concentration in the γ matrix. Our results provide valuable insights into understanding the effect of Mo addition to Ni-based superalloys on the coarsening kinetics of γ′ phase and elemental partitioning behavior.