  The diffusion kinetics in a CoCrFeMnNi high entropy alloy is investigated by
a combined radiotracer--interdiffusion experiment applied to a pseudo-binary
Co$_{15}$Cr$_{20}$Fe$_{20}$Mn$_{20}$Ni$_{25}$ /
Co$_{25}$Cr$_{20}$Fe$_{20}$Mn$_{20}$Ni$_{15}$ couple. As a result, the
composition-dependent tracer diffusion coefficients of Co, Cr, Fe and Mn are
determined. The elements are characterized by significantly different diffusion
rates, with Mn being the fastest element and Co being the slowest one. The
elements having originally equiatomic concentration through the diffusion
couple are found to reveal up-hill diffusion, especially Cr and Mn. The atomic
mobility of Co seems to follow an S-shaped concentration dependence along the
diffusion path. The experimentally measured kinetic data are checked against
the existing CALPHAD-type databases. In order to ensure a consistent treatment
of tracer and chemical diffusion a generalized symmetrized continuum approach
for multi-component interdiffusion is proposed. Both, tracer and chemical
diffusion concentration profiles are simulated and compared to the
measurements. By using the measured tracer diffusion coefficients the chemical
profiles can be described, almost perfectly, including up-hill diffusion.
