Superelastic behavior in nickel-free Ti-Nb-Sn alloys

Superelasticity of an Ni-free Ti alloy containing 16 at.% Nb and 4.9 at.% Sn is investigated as a function of heat treatment using samples with different cold-working reductions. It is found that the alloy is readily cold rolled from 7.5 mm thickness to thin sheets of 80 pm thickness or thin wires 0.5 mm in diameter without intermediate annealing. Superelastic strain recovery appears in the thin sheets and thin wires as well as the 1 mm thick plates. Superelastic recovery increases with decreasing quenching temperature in the temperature range above beta-transus and can be made most distinct by a two-step quenching with intermediate holding at 400-600 degrees C for short times after solution treatment at 950 C. No detectable difference in X-Ray Diffraction (XRD) patterns is observed for the samples subjected to various heat treatments which exhibit different superelastic behavior. It is suggested that superelasticity is affected by thermal vacancies and/or atomic rearrangements prior to beta decomposition to alpha or omega.