Preparation of Refractory High-Entropy Alloys by Electro-Deoxidation and the Effect of Heat Treatment on Microstructure and Hardness

Equiatomic refractory high-entropy alloys TiNbTaZr and TiNbTaZrHf have been prepared by electro-deoxidizing pre-mixed and sintered solid metal oxide disks through cathodic polarization in a molten CaCl2 electrolyte. The sintering temperature employed in the preparation of the oxide disks prior to electro-deoxidation was varied between 1173 K and 1573 K, and it was found that this has a noticeable effect on the microstructure and porosity of the reduced alloys. A post-processing heat treatment at 1473 K in vacuum for 24 h was applied to the reduced alloys, and it was observed that the dual-phase bcc structure of TiNbTaZr changes into single-phase bcc while the single-phase bcc structure of TiNbTaZrHf remains unchanged. Notably, the microhardness of both alloys increased substantially, approximately doubling for TiNbTaZrHf. Overall, molten salt electro-deoxidation is a straightforward method for the preparation of refractory high-entropy alloys, and suitably selected heat treatments enable significant further optimization of their properties.