Effects of solute concentrations on kinetic pathways in Ni–Al–Cr alloys

The kinetic pathways resulting from the formation of coherent γ′-precipitates from the γ-matrix are studied for two Ni–Al–Cr alloys with similar γ′-precipitate volume fractions at 873 K. The details of the phase decompositions of Ni–7.5Al–8.5Cr at.% and Ni–5.2Al–14.2Cr at.% for aging times from 1/6 to 1024 h are investigated by atom-probe tomography, and are found to differ significantly from a mean-field description of coarsening. The morphologies of the γ′-precipitates of the alloys are similar, though the degrees of γ′-precipitate coagulation and coalescence differ. Quantification within the framework of classical nucleation theory reveals that differences in the chemical driving forces for phase decomposition result in differences in the nucleation behavior of the two alloys. The temporal evolution of the γ′-precipitate average radii and the γ-matrix supersaturations follow the predictions of classical coarsening models. The compositional trajectories of the γ-matrix phases of the alloys are found to follow approximately the equilibrium tie-lines, while the trajectories of the γ′-precipitates do not, resulting in significant differences in the partitioning ratios of the solute elements.