Radioactive isotopes reveal a non sluggish kinetics of grain boundary diffusion in high entropy alloys

TL;DR

This study uses radioactive isotopes to measure grain boundary diffusion in high entropy alloys, revealing that increased elemental complexity does not necessarily slow diffusion, which impacts high-temperature material performance.

Contribution

It provides the first direct measurement of grain boundary diffusion of Ni in specific high entropy alloys using radiotracer analysis, challenging assumptions about diffusion rates in complex alloys.

Findings

Grain boundary diffusion rates are not reduced by increased element complexity.

The grain boundary width is approximately 0.5 nm in studied HEAs.

Grain boundary energy increases with temperature, especially in CoCrFeMnNi.

Abstract

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.