Chemical ordering enhancing mechanical properties of Nb₂₅Ti₃₅V₅Zr₃₅Al_x refractory high-entropy alloys

Developing novel refractory high-entropy alloys (RHEAs) that achieve a balance of high strength and ductility continues to be a significant challenge for structural applications. In this study, the Nb₂₅Ti₃₅V₅Zr₃₅ RHEA with enhanced toughness was selected as the base alloy, and different amounts of Al were added to promote chemical ordering and improve the mechanical properties of RHEAs. The microstructure, chemical ordering, and mechanical properties of RHEAs with varying Al content were studied by combining experimental methods with DFT calculations. Results indicate that adding Al promotes the development of the B2 phase, and with increasing Al content, the configurations of the B2 phase undergoes the transformations: (NbTiAl)_α(NbTiVZr)_β → (NbTiVAl)_α(TiVZr)_β → (NbTiVAl)_α(TiZr)_β. The electronic structure reveals that among the bonds formed between Al and the other four elements, the Al-Zr bond is the most effective in promoting the formation of the B2 phase. In addition, the yield strength, specific yield strength, and hardness of the RHEAs exhibit a positive correlation with Al content, with precipitation strengthening of the nano-scale chemically ordered B2 phase-induced being the primary reason for the improvement in mechanical properties. The Nb₂₅Ti₃₅V₅Zr₃₅Al₂₀ alloy achieves an optimal strength-ductility balance, with a yield strength of 1379 MPa and an engineering strain greater than 50 %. This work offers new insights into the development of chemically ordered strengthened RHEAs.