Microstructure stability of Ti-V-Zr-Nb-Hf-Ta refractory multi-principal-element alloys annealed at intermediate temperature

Maintaining thermal stability is a key concern for the potential application of multi-principal-element alloys (MPEAs) at elevated temperatures, particularly in the intermediate temperature range. In this regime, the thermodynamic dominance of the entropy term over enthalpy may diminish, while atomic migration remains kinetically active. In this study, the stability of a series of refractory MPEAs (RMPEAs) from the subsystems of Ti-V-Zr-Nb-Hf-Ta is investigated at 550 °C for 2–28 days. Although all eleven alloys exhibit a single solid solution phase with a body-centered cubic structure at their homogenized states, only two alloys, VNbTa and TiVNbTa, remain stable after annealing. Decomposition occurs in the other nine alloys under the spinodal manner or the nucleation and growth mechanism, including all three quinary alloys, demonstrating that configurational entropy is not a dominant factor. The phase stabilities can be well understood from the enthalpy perspectives, by combining first-principles calculations and semi-empirical models. By comparing the different contributors of formation enthalpy, the lattice distortion energy is found to be the most critical factor for this alloy system. Furthermore, the phases formed after long-term annealing are generally located at different regions in the space with the axes of chemical, structural, and lattice distortion energies. This work provides a way to interpret and control the stability of RMPEAs in the intermediate temperature regime.