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

Grain boundary diffusion of the principal elements 57Co, 51Cr, 59Fe and 54Mn in a coarse-grained equiatomic 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 and the existence of a structural multiplicity of high-angle grain boundaries. A correlative microscopy combining transmission Kikuchi diffraction and atom probe tomography manifests formation of neighboring Ni-Mn-rich and Cr-rich precipitates at 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.