Influence of equiatomic Zr/Nb substitution on superelastic behavior of Ti–Nb–Zr alloy

Based on a binary Ti–26Nb (at%) alloy, Ti–(26-z)at% Nb-(z)at% Zr (z=2, 6, 8 and 10) alloys via equiatomic substitution of Nb by Zr are formulated. Influence of equiatomic Zr/Nb substitution on microstructure evolution, mechanical properties and deformation mechanism of the superelastic Ti–Nb–Zr alloy are investigated. Experimental results show that the phase constitution is single β phase or β/ωath phase at 0<Zr/Nb<0.35 since the measured Ms temperature is maintained at around 250 K. The two-phase microstructure consist of α″ martensite and β phase is obtained at Zr/Nb ratio>0.4 due to the attenuation of β stabilizing effect of Zr, which is related closely to the Nb content in ternary Ti–Nb–Zr alloys. The mechanism of the superelastic behavior alters gradually from reversible β/α″ martensitic transformation to rearrangement of pre-existing α″ martensites as a function of Zr/Nb ratio increase. At Zr/Nb=0.3, the alloy of corresponding composition exhibits the best superelasticity and combined mechanical performance. A coefficient of ΔT (ΔT=Tβ-Ms) is proposed to understand the experimental results by evaluating the β instability of Ti–Nb–Zr alloys.