  As a single-phase alloy, CrFeCoNi is a face centered cubic (fcc) material
related to the archetypical high-entropy Cantor alloy CrFeCoNiMn. For thin
films, CrFeCoNi of approximately equimolar composition tends to assume an fcc
structure when grown at room temperature by magnetron sputtering. However, the
single-phase solid solution state is typically not achieved for thin films
grown at higher temperatures. The same holds true for Cantor alloy-based
ceramics (nitrides and oxides), where phase formation is extremely sensitive to
process parameters such as the amount of reactive gas. This study combines
theoretical and experimental methods to understand the phase formation in
nitrogen-containing CrFeCoNi thin films. Density functional theory calculations
considering three competing phases (CrN, Fe-Ni and Co) show that the free
energy of mixing, delta G of (CrFeCoNi)1-xNx solid solutions has a maximum at x
= 0.20-0.25, and delta G becomes lower when x less than 0.20, greater than
0.25. Thin films of (CrFeCoNi)1-xNx (x = 0.14-0.41) grown by magnetron
sputtering show stabilization of the metallic fcc when x lesser than or equal
to 0.22 and the stabilization of the NaCl B1 structure when x is greater than
0.33, consistent with the theoretical prediction. In contrast, films with
intermediate amounts of nitrogen (x = 0.22) grown at higher temperatures show
segregation into multiple phases of CrN, Fe-Ni-rich and Co. These results offer
an explanation for the requirement of kinetically limited growth conditions at
low temperature for obtaining single-phase CrFeCoNi Cantor-like
nitrogen-containing thin films and are of importance for understanding the
phase-formation mechanisms in multicomponent ceramics.
