The mechanical and oxidation properties of novel B2-ordered Ti2ZrHf0.5VNb0.5Alx refractory high-entropy alloys

A series of novel B2-ordered Ti2ZrHf0.5VNb0.5Alx refractory high-entropy alloys (RHEAs) was designed and prepared by vacuum arc-melting. The effect of Al alloying on the microstructure, mechanical properties, and oxidation properties was investigated. The Al addition triggers the transformation of the crystal structure from disordered BCC phase to ordered B2 phase, and the Al1 alloy present single B2 phase structure. This B2 ordering results in significant yield strength increase of the RHEAs, i.e., from 915 MPa to 1410 MPa at room temperature (RT, leading to an excellent specific strength of similar to 232.3 KPa.m(3)/kg for the Al1 alloy), 719 MPa to 1088 MPa at 873 K and 126 MPa to 278 MPa at 1073 K, respectively. However, in the expense, the near full and full B2-structured Al-0.75 and Al1 alloys show dramatic reduction in compressive ductility down to 31.4% and 15.1% at RT. Four parameters, VEC, Delta H-mix, Bo and Md are employed to predict the B2 ordering in the RHEAs, and the calculation shows that ordered phases are preferred in the realm of Delta H-mix <-14 kJ.mol(-1) and Bo < 0.4Md+1.9. In addition, the introducing of Al also significantly enhances the oxidation resistance of the alloys. While the Al-0 alloy shows catastrophic oxidation within 24 h in air at 1073 K, the Al1 alloy only gains 112 mg.cm(-2) after 50 h. However, the constitution of the oxide layer remains loose and complex, consisting of multiple oxides including Al2O3, TiO2, (Zr,Hf)O-2 and (Zr,Hf)V2O7.