Effects of Nb and Zr on structural stabilities of Ti-Mo-Sn-based alloys with low modulus

Multi-element alloying is of great importance to the structural stability of metastable β-Ti alloys with low Young's modulus. The present work investigates the effects of Nb and Zr alloying on the β stability of the [(Mo0.5Sn0.5)-(Ti,Zr)14](Nb,Zr)1,2 alloy series, according to the cluster formula approach for designing solid solution alloys. Alloy rods with a diameter of 6 mm were prepared by copper-mold suction-casting method. The contributions of Nb and Zr to the β stability, characterized with the equivalent coefficients in a Mo equivalent (Moeq) formula, are modified from the viewpoint of phase diagram features. Thus, the β stabilities of Ti alloys with the updated Moeq are consistent with the experimental results. The Young's moduli of the metastable β-Ti alloys decrease first to a low value with increasing Moeq, and then increase up to a constant level. It is reconfirmed that the lowest Young's modulus (48 GPa) of the β-[(Mo0.5Sn0.5)-(Ti13Zr1)]Nb1 alloy benefits from its lower Moeq (13.6 wt% Mo), which is slightly higher than the critical limit for β stabilization to inhibit α″ martensite. Multi-component alloys with Moeq below this value will show a double yielding phenomenon due to the stress-induced α″ martensite and/or β twins during the tension process.