Self-diffusion in carbon-alloyed CoCrFeMnNi high entropy alloys

TL;DR

This study investigates how interstitial carbon affects the self-diffusion of substitutional elements in CoCrFeMnNi high entropy alloys, revealing temperature- and concentration-dependent diffusion behaviors and proposing a model based on lattice distortion.

Contribution

It provides new insights into the effects of interstitial carbon on diffusion mechanisms in high entropy alloys and introduces a model linking lattice distortion to diffusion barriers.

Findings

Low C concentration retards diffusion at high temperature.

Higher C concentration enhances diffusion at lower temperatures.

Lattice distortion reduces vacancy migration barriers.

Abstract

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.