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Grain boundary diffusion in CoCrFeMnNi high entropy alloy: kinetic hints towards a phase decomposition

Grain boundary self-diffusion of $^{57}$Co, $^{51}$Cr, $^{59}$Fe and $^{54}$Mn in a coarse-grained, single-phase fcc CoCrFeMnNi high entropy alloy is measured in a wide temperature range of 643 to 1273~K in both C- and B-type kinetic regimes after Harrison's classification. The results suggest that the product of the pertinent segregation factors, $s$, and the grain boundary width, $δ$, is about 0.5~nm for all elements at temperatures $T>800$~K. Whereas one short-circuit contribution is observed at higher temperatures above 800~K, the penetration profiles in the C-type kinetic regime (643 -- 703~K) reveal two distinct contributions that hint towards a phase decomposition at a fraction of high-angle grain boundaries at these temperatures. A correlative microscopy combining transmission Kikuchi diffraction and atom probe tomography manifests formation of neighbouring Ni-Mn-rich and Cr-rich precipitates at a segment of high angle grain boundaries. Transmission electron microscopy revealed an increased dislocation density in the vicinity of such interfaces which is suggested to be a reason of the enhanced diffusion rates at low temperatures for such short circuits.