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 (Zr83.33Nb16.67 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) (Zr77.77Ti5.56Nb16.67) alloy without any ω precipitation. The further substitution of Hf/Ti for the Zr could also render the [Ti-Zr8Hf4Ti2](Nb3) (Zr44.44 Hf22.22Ti16.67Nb16.67) alloy with a single BCC structure. All these BCC MEAs exhibit prominent mechanical properties, as exemplified by the [Ti-Zr8Hf6](Nb3) (Zr44.44Hf33.33Ti5.56Nb16.67) 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.