  High entropy alloys are generally considered to be single phase material.
This state is, however, typically a non-equilibrium state after fabrication at
high cooling rates. Phase constitution after fabrication or heat treatment is
mostly known for isothermal annealing only and for casts as well as rapidly
quenched alloys. Knowledge on early phase separation stages of high entropy
alloys and their mechanisms are missing so far. Here, we present results on
phase separation at intermediate cooling rates, by characterization of gas
atomized powder of the AlCoCr0.75Cu0.5FeNi alloy. Although investigation by
X-ray diffraction and Electron Backscatter Diffraction indicates a single-phase
nature of the powder particles, aberration-corrected scanning transmission
electron microscopy and atom probe tomography reveal a nanoscale phase
separation into Ni-Al-rich B2 and Fe-Cr-rich A2 regions as well as a high
number density of 3.1x1024 Cu-rich clusters per m3 in the B2 matrix. The
observed phase separation and cluster formation are linked to spinodal
decomposition and nucleation processes, respectively. The study highlights that
adequate characterization techniques need to be chosen when making statements
about phase stability and structural evolution in compositionally complex
alloys.
