  Contemporary powder-based polycrystalline nickel-base superalloys inherit
microstructures and properties that are heavily determined by the
thermo-mechanical treatments during processing. Here, the influence of a
thermal exposure alone to an alloy powder is studied to elucidate the
controlling formation mechanisms of the strengthening precipitates using a
combination of atom probe tomography and in-situ neutron diffraction. The
initial powder comprised a single-phase supersaturated gamma only; from this,
the evolution of gamma-prime volume fraction and lattice misfit was assessed.
The initial powder notably possessed elemental segregation of Cr and Co and
elemental repulsion between Ni, Al and Ti with Cr; here proposed to be a
precursor for subsequent gamma to gamma-prime phase transformations. Subsolvus
heat treatments yielded a unimodal gamma-prime distribution, formed during
heating, with evidence supporting its formation to be via spinodal
decomposition. A supersolvus heat treatment led to the formation of this same
gamma-prime population during heating, but dissolves as the temperature
increases further. The gamma-prime then reprecipitates as a multimodal
population during cooling, here forming by classical nucleation and growth.
Atom probe characterisation provided intriguing precipitate characteristics,
including clear differences in chemistry and microstructure, depending on
whether the gamma-prime formed during heating or cooling.
