Splitting Instability in Superalloys: A Phase-Field Study

Precipitation-strengthened alloys, such as Ni-base, Co-base and Fe-base superalloys, show the development of dendrite-like precipitates in the solid state during aging at near-γ^' solvus temperatures. These features arise out of a diffusive instability wherein, due to the point effect of diffusion, morphological perturbations over a growing sphere/cylinder are unstable. These dendrite-like perturbations exhibit anisotropic growth resulting from anisotropy in interfacial/elastic energies. Further, microstructures in these alloys also exhibit "split" morphologies wherein dendritic precipitates fragment beyond a critical size, giving rise to a regular octet or quartet pattern of near-equal-sized precipitates separated by thin matrix channels. The mechanism of formation of such morphologies has remained a subject of intense investigation, and multiple theories have been proposed to explain their occurrence. Here, we developed a phase-field model incorporating anisotropy in elastic and interfacial energies to investigate the evolution of these split microstructures during growth and coarsening of dendritic γ^' precipitates. Our principal finding is that the reduction in elastic energy density drives the development of split morphology, albeit a concomitant increase in the surface energy density. We also find that factors such as supersaturation, elastic misfit, degree of elastic anisotropy and interfacial energy strongly modulate the formation of these microstructures. We analyze our simulation results in the light of classical theories of elastic stress effects on coarsening and prove that negative elastic interaction energy leads to the stability of split precipitates.