Phase equilibria, mechanical properties and design of quaternary refractory high entropy alloys

Highlights: • Twelve new Refractory High Entropy Alloys (RHEAs) were produced and characterized • Experimental results were compared to different phase prediction models • Two widely used empirical parameters, δ and Ω, were reinterpreted. Examples of their use in alloy design are given • CALPHAD simulations are performed, their strengths and weaknesses for RHEA design are discussed • Poor correlation between micro-hardness and elastic and atomic size mismatch is observed in the single-phase alloys Refractory high entropy alloys (RHEAs) are candidates for replacing conventional refractory alloys. In this work, twelve new RHEAs were selected and produced. The phases present in the as-cast and heat-treated conditions were characterized and compared with CALPHAD simulations and empirical parameters. Here we propose a new interpretation for the two widely used δ and Ω empirical parameters. In this work, they are shown to be inaccurate when applied to a large group of RHEAs, but can be a powerful alloy design tool if applied on specific subsystems of alloys. Experimentally, chromium-containing alloys are shown to form Laves phases, especially when the lattice distortion (δ) is high, while aluminum-containing alloys are shown to form the A15 phase upon heat-treatment, due to their highly negative enthalpy of mixing (ΔH_mix). In addition to microstructural characterization, mechanical properties of these alloys via hardness testing were assessed. A poor correlation was observed between the hardness and the atomic size and elastic modulus mismatch in these single-phase BCC RHEAs, suggesting that core structure of the screw dislocations is a crucial parameter in understanding the strength of these alloys.