Abstract
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 gamma particles occurring in ordered (L1(2)) le precipitates in a Ni86.1Al8.5Ti5.4 alloy. Particularly, we elucidate the evolution from nanoscaled Ni-rich heterogeneities (Ni-rich clusters) to gamma spheres and then gamma plates inside gamma' precipitates from the compositional and the thermodynamic point of view. We find that Ni supersaturation of gamma' 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 gamma particles. Our results demonstrate that phase separation of gamma' precipitates is characterized by different stages with various governing driving forces. (C) 2015 Elsevier B.V. All rights reserved.