Development and characterization of Ni-free Ti-base shape memory and superelastic alloys

Recently the Ni-hypersensitivity and toxicity of Ni have stimulated the development of Ni-free shape memory alloys. The β-Ti alloys are the most attractive candidates for biomedical shape memory alloys. Ti–Nb–X (X = Zr, Ta, Mo, Au, Pd, Pt, Al, Ga, Ge, O) and Ti–Mo–X (X = Ta, Nb, Zr, Au, Pd, Pt, Al, Ga, Ge) alloys have been developed and their shape memory effect and superelasticity were investigated systematically by the present authors for about 5 years. Although shape memory effect and superelasticity observed in the Ti–Nb alloys, the low critical stress for slip deformation caused the superelasticity not to reveal a large strain at room temperature. However, low temperature annealing and an aging treatment were effective in improving superelasticity. Additions of alloying elements such as Zr, Ta, Mo, Au, Pt and Al were also effective in stabilizing the superelasticity. In this paper, the basic characteristics of Ti–Nb, Ti–Nb–Zr, Ti–Nb–Ta and Ti–Nb–O are to be briefly reviewed based on the recent works of the present authors.