  High entropy alloys (HEAs) have emerged as a new class of multicomponent
materials, which have potential for high temperature applications. Phase
stability and creep deformation, two key selection criteria for high
temperature materials, are predominantly influenced by the diffusion of
constituent elements along the grain boundaries (GBs). For the first time, GB
diffusion of Ni in chemically homogeneous CoCrFeNi and CoCrFeMnNi HEAs is
measured by radiotracer analysis using the $^{63}$Ni isotope. Atom probe
tomography confirmed the absence of elemental segregation at GBs that allowed
reliable estimation of the GB width to be about 0.5 nm. Our GB diffusion
measurements prove that a mere increase in number of constituent elements does
not lower the diffusion rates in HEAs, but the nature of added constituents
plays a more decisive role. The GB energies in both HEAs are estimated at about
0.8-0.9 J/m$^2$, they are found to increase significantly with temperature and
the effect is more pronounced for the CoCrFeMnNi alloy.
