Aging behavior of two refractory Ti-Nb-(Hf, Zr)-Al high entropy alloys

Abstract In the present study, the response of Ti40Nb30Hf15Al15 and Ti40Nb30Zr20Al10 (at%) refractory high entropy alloys to annealing is reported. The alloys were produced by vacuum arc melting and had a density of 7.07 ± 0.03 g/cm3 and 6.03 ± 0.03 g/cm3, respectively. In the as-cast condition, both alloys had a coarse-grained single B2 phase structure. After cold rolling to 80% thickness reduction and annealing at T = 900 °C for 15 min, a fine-grained structure with a grain size of 4.3 ± 2.2 and 13.2 ± 5.0 µm was produced in the Ti40Nb30Hf15Al15 and Ti40Nb30Zr20Al10 alloys, respectively. In addition, a small amount of orthorhombic (O) and Zr5Al3-type phase particles formed in the alloys with Hf and Zr, respectively. Annealing at T = 500 °С up to 96 h had a weak effect on the structure and hardness of the alloys. Annealing at T = 600 and T = 700 °С resulted in the massive precipitation of the O-phase in the Ti40Nb30Hf15Al15 alloy and the Zr5Al3-type phase in the Ti40Nb30Zr20Al10 alloy. However, a pronounced increase in microhardness of the Ti40Nb30Hf15Al15 alloy was revealed after annealing at T = 600 °С only. Meanwhile, the Ti40Nb30Zr20Al10 alloy became softer after annealing at T = 600 and T = 700 °С. The phase transformations in the alloys were compared with the CALPHAD predictions, and hardening mechanisms were qualitatively analyzed.