Evolution of nanoscale clusters in γ' precipitates of a Ni-Al-Ti model alloy.

The evolution of phase separation and ordering processes determines the structure and properties of Ni-based superalloys. Here we use atom probe tomography to clarify the origin of γ particles occurring in ordered (L12) γ' precipitates in a Ni86.1Al8.5Ti5.4 alloy. Particularly, we elucidate the evolution from nanoscaled Ni-rich heterogeneities (Ni-rich clusters) to γ spheres and then γ plates inside γ' precipitates from the compositional and the thermodynamic point of view. We find that Ni supersaturation of γ' precipitates is relieved by formation of Ni-rich clusters, which results in an energetically more favorable state. Subsequently, coalescence introduces necking between the Ni-rich clusters and leads to the formation of γ particles. Our results demonstrate that phase separation of γ' precipitates is characterized by different stages with various governing driving forces.