In this paper, a new diffusion couple approach is presented, to study phase stability and solid solution strengthening of high-entropy alloys. Diffusion couples have been fabricated using the Cantor alloy (CoCrFeMnNi) and a commercial fcc CoCrMoNi alloy (MP35N®) and then constituent elements as diffusion partners. Chemical gradients within the interdiffusion zone as well as phase stability are quantified using EDX and EBSD.
The experimental results show no general correlation between maximum solubility of individual elements and atomic size mismatch, whereas the valence electron concentration model (VEC) seems to be valid to predict the stable fcc solid solution for most of the investigated diffusion couples. Nanoindentation hardness measurements are used to quantify solid solution strengthening across the interdiffusion zone. Different elements exhibit hardening, softening or a constant hardness level with chemical composition gradient. Within the fcc structure of the base alloy, changes in hardness are related to modulus and atomic size mismatch effects. Furthermore, the hardness distributions can be described by a modified Labusch model for solid solution strengthening remarkably well, although the representation of hardness takes place far beyond the scope of the model.