Effect of Zr and Sn on Young's modulus and superelasticity of Ti–Nb-based alloys

Quaternary Ti–(20–26)Nb–(2–8)Zr–(3.5–11.5)Sn (wt%) alloys were investigated to evaluate the effects of Zr and Sn on Young's modulus and superelasticity of Ti–Nb-based alloys. X-ray diffraction analysis showed that solution-treated alloys have β + α″, β + ω, α″ + ω, α″, or β microstructures. Zr and Sn increase the lattice parameters of the β phase; for orthorhombic α″ matensite, they increase the lattice parameter a but decrease both b and c. The martensitic start temperature of the α″ is depressed by Zr and Sn additions, whereas the formation of athermal ω is dependent on Zr and Sn contents. Differential scanning calorimetry (DSC) measurements show that 1 wt% of Nb, Zr or Sn addition decreases the martensitic start temperature by 17.6, 41.2 or 40.9 K, respectively, due to their negative effect on lattice parameter ratios of the martensite (c/a and b/a). Tensile tests were used to evaluate Young's modulus and superelasticity of the solution-treated alloys. Of the studied alloys Ti–24Nb–4Zr–7.5Sn with single β microstructure has the lowest Young's modulus of 52 GPa and recoverable elastic strain of about 2% at room temperature after cyclic strain.