Formation and Mechanical Behavior of Body-Centered-Cubic Zr(Hf)-Nb-Ti Medium-Entropy Alloys

The present work investigated the formation and mechanical behavior of body-centered-cubic (BCC) Zr(Hf)-Nb-Ti medium entropy alloys (MEAs), in which three series of alloys, [Zr-Zr₁₄](Zr,Nb)₃, [Ti-Zr₁₄](Ti,Nb)₃, and [Ti-(Hf,Zr)₁₄](Nb)₃, were designed by the cluster formula approach. With increasing the Nb content, the BCC-β structural stability of the [Zr-Zr₁₄](Zr,Nb)₃ alloys would be enhanced, as evidenced by the BCC [Zr-Zr₁₄](Nb)₃ (Zr_83.33Nb_16.67 in atomic percent at. pct) alloy containing a minor amount of ω phase. An appropriate content of Ti addition can further improve the BCC-β stability of [Ti-Zr₁₄](Nb₃) (Zr_77.77Ti_5.56Nb_16.67) alloy without any ω precipitation. The further substitution of Hf/Ti for the Zr could also render the [Ti-Zr₈Hf₄Ti₂](Nb₃) (Zr_44.44 Hf_22.22Ti_16.67Nb_16.67) alloy with a single BCC structure. All these BCC MEAs exhibit prominent mechanical properties, as exemplified by the [Ti-Zr₈Hf₆](Nb₃) (Zr_44.44Hf_33.33Ti_5.56Nb_16.67) with a higher yield strength of 662 MPa, a larger elongation to fraction of 15.2 pct, and a lower Young’s modulus of 71 GPa.