  The bulk quaternary equiatomic CoCrFeNi alloy is studied extensively in
literature. Under experimental conditions, it shows a single-phase fcc
structure and its physical and mechanical properties are similar to those of
the quinary equiatomic CoCrFeMnNi alloy. Many studies in literature have
focused on the mechanical properties of bulk nanocrystalline high entropy
alloys or compositionally complex alloys, and their microstructure evolution
upon annealing. The thin film processing route offers an excellent alternative
to form nanocrystalline alloys. Due to the high nucleation rate and high
density of defects in thin films synthesized by sputtering, the kinetics of
microstructure evolution is often accelerated compared to those taking place in
the bulk. Here, thin films are used to study the phase evolution in
nanocrystalline CoCrFeNi deposited on Si/SiO 2 and c-sapphire substrates by
magnetron cosputtering from elemental sources. The phases and microstructure of
the films are discussed in comparison to the bulk alloy. The main conclusion is
that second phases can form even at room temperature provided there are
sufficient nucleation sites.
