The temporal evolution of the nanostructures of model Ni-Al-Cr and Ni-Al-Cr-Re superalloys

The temporal evolution of γ′(L1₂)-precipitates in Ni-5.2 Al-14.2 Cr at.% and Ni-9.5 Al-8.6 Cr-2.0 Re at.% alloys are studied utilizing three-dimensional atom-probe microscopy. For the times investigated (0.0833 to 1024 h), these alloys exhibit a high number density (10 ²²-10²⁴ m^-3) of spheroidal γ′- precipitates, 0.45-30 nm in radius. For the Ni 5.2 Al-14.2 Cr at.% alloy, nucleation, growth, and coarsening of the γ′-precipitates is observed during the isothermal transformation at 873 K. Precipitate coalescence is present, and its influence on the power-law dependencies of the number density and mean radius during coarsening is discussed and compared to classical models. For Ni-9.5 Al-8.6 Cr-2.0 Re at.%, phase separation occurs during the initial quench leading to a highly interconnected γ′-nanostructure. The temporal evolution of the elemental partitioning behavior at 1073 K between the γ-matrix and γ′-precipitates is studied in detail.