The use of diffusion multiples to examine the compositional dependence of phase stability and hardness of the Co-Cr-Fe-Mn-Ni high entropy alloy system

• Diffusion Multiples are used to explore High Entropy Alloys.
• The quinary region of interdiffusion was compositionally mapped and compared to values from nanohardness indentations.
• Maximum hardness did not correlate to maximum atomic size mismatch.
• Phase stability parameters Ω, ΔHmix, ΔSmix, Δχ, and δ did not predict phase stability.
• Elastic modulus mismatch was the most effective indicator of strengthening.

High entropy alloys (HEAs) or Multi-principal element alloys (MEAs) are a relatively new class of alloys. These alloys are defined as having at least five major alloying elements in atomic percent from 5% to 35%. There are hundreds of thousands of equiatomic compositions possible and only a fraction have been explored. This project examines diffusion multiples as a method to accelerate alloy development in these systems. Co-Cr-Fe-Mn-Ni quinary diffusion multiples were successfully created. Using these multiples, a quinary region of disordered of FCC was formed and examined using EDS and nanoindentation methods. From these techniques, maps of common HEA parameters (Ω, δ, ΔSmix, ΔHmix and Δχ) proposed in literature could be calculated and directly compared to observed phase stability. Similarly, hardness was examined as function of compositional complexity and atomic mismatch in the quinary disordered region in order to directly test the severe lattice distortion hypothesis. It was found that proposed HEA parameters were ineffective at single phase stability limits in the Co-Cr-Fe-Mn-Ni system. It was also observed that hardness did not correlate well to the maximum compositional complexity or to the maximum in atomic mismatch. This indicates the severe lattice distortion hypothesis is not the primary contributor to strengthening in the Co-Cr-Fe-Mn-Ni HEA system.