  Tracer diffusion of the substitutional components in
(CoCrFeNiMn)$_{1-x}$C$_x$ high-entropy alloys with x = 0.002, 0.005 and 0.008
(in at. fractions) is measured at elevated temperatures from 1173 to 1373 K.
Two different characteristic effects of interstitial carbon addition on
substitutional diffusion in these FCC alloys are distinguished. At the highest
temperature of 1373 K, alloying by C with relatively low concentrations (x =
0.002) retards diffusion of the substitutional elements with respect to those
in the C-free alloy. At lower temperatures and/or higher C concentrations (x >
0.005), an enhancement of the diffusion rates of all substitutional elements is
seen. A model is suggested that relates the self-diffusivities in the
CoCrFeMnNi-C alloys with the lattice distortion imposed by interstitially
dissolved carbon. The experimental results are interpreted in terms of a
decrease of the migration barriers for vacancy-mediated diffusion due to the
presence of interstitial C atoms.
