  Tracer diffusion of all constituting elements is studied at various
temperatures in a series of (CoCrFeMn)$_{100-x}$Ni$_x$ alloys with compositions
ranging from pure Ni to the equiatomic CoCrFeMnNi high-entropy alloy. At a
given homologous temperature, the measured tracer diffusion coefficients change
non-monotonically along the transition from pure Ni to the concentrated alloys
and finally to the equiatomic CoCrFeMnNi alloy. This is explained by atomistic
Monte-Carlo simulations based on a modified embedded-atom potentials, which
reveal that local heterogeneities of the atomic configurations around a vacancy
cause correlation effects and induce significant deviations from predictions of
the random alloy model.
