Thermodynamic modelling of the alloys for the TaMoCrTiAl system was carried out in order to achieve a single-phase A2 structure at moderate temperatures, as well as to minimize density. Thermodynamic calculations were performed in a commercial software FactSage using an in-house database with a following experimental verification. The following criteria to model the alloys for the system Ta-Mo-Cr-Ti-Al were chosen:

1. Ensure A2 phase fraction ≥ 94% between 700°C and 1000°C.

2. Maximize Cr and Al content, starting with 5 at.% Cr.

3. Minimize Ta content and maximize Ti concentration.

Thermodynamic modelling has shown that Cr governs the formation of Cr2Ta (Laves phase C15), whereas Al facilitates ordering from A2 to B2. To minimize the formation of Cr2Ta, the Cr concentration should not exceed 5 at.%. Figure 1 represents the effect of Al concentration on the phase equilibria in (TaMoTi)5CrxAl revealing maximal reasonable content of 13 at.% for 700 °C. Following up modelling aiming to achieve the lowest possible density without significant affecting the phase equilibria yields the chemical composition 16Ta-33Mo-33Ti-5Cr-13Al (at.%). Three most promising alloys have been identified and manufactured by arc melting. Experimentally observed phase composition and microstructure are in excellent agreement with the calculations. Mechanical properties at both room temperature (RT) and elevated ones will be tested in future studies.

This study provides guidelines for designing A2-type TaMoCrTiAl-based refractory high-entropy alloys to ensure a combination of good room temperature (RT) and high-temperature mechanical properties.