Effect of Cr and Zr on phase stability of refractory Al-Cr-Nb-Ti-V-Zr high-entropy alloys

The effect of annealing at 800 °C or 1000 °C for 100 h on the structure and mechanical properties of refractory AlCrxNbTiV and AlNbTiVZrx (x = 0-1.5) high-entropy alloys was studied. In the initial condition (annealing at 1200 °C for 24 h), the AlNbTiV and AlCr0.5NbTiV alloys were composed of the B2 phase while the AlCrNbTiV and AlCr1.5NbTiV alloys had a mixture of the B2 and Cr2Nb-type C14 Laves phases. The AlNbTiVZrx (x = 0.5-1.5) alloys in the initial condition consisted of the B2, Zr5Al3-type, and ZrAlV-type C14 Laves phases. Annealing of the AlCrxNbTiV (x = 0-1.5) alloys at 800 °C or 1000 °C resulted in (i) precipitation of the Nb2Al-type σ-phase or an increase in the fraction of the Laves phase; (ii) an increase in the microhardness and a pronounced drop in ductility. Annealings of the AlNbTiVZrx (x = 0.5-1.5) alloys at 800 °C or 1000 °C led to insignificant changes in the structure and microhardness but somewhat decreased ductility. Quasi-binary AlNbTiV-Cr and AlNbTiV-Zr phase diagrams constructed with a Thermo-Calc software and TCHEA2 database were used to analyze phase transformations in the experimental alloys. The differences in the behavior of the Cr/Zr-containing alloys and the relationships between the phase composition and mechanical properties of the alloys were discussed.