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

Abstract

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.

Introduction

Biomedical shape memory alloys are required to have superior corrosion resistance, biocompatibility and stable shape memory property. Among many shape memory alloys, only Ti–Ni alloys have been widely applied for biomedical uses to date, because they satisfy the above requirements [1], [2], [3]. However, it has been pointed out that pure Ni is a toxic element and causes Ni-hypersensitivity. Although clear Ni-hypersensitivity to human body in Ti–Ni alloys has not been reported, it is preferable to develop absolutely safe Ni-free Ti-base shape memory alloys for biomedical applications. Recently, β type Ti alloys composed of non-toxic elements have attracted attention as potent biomedical shape memory and superelastic materials [4], [5], [6], [7], [8], [9], [10].

It is well known that β type Ti-base alloys exhibit two stable phases, the β phase (disordered bcc) at higher temperatures and the α phase (HCP) at lower temperatures, and exhibit three metastable phases, α′, α″ and ω phases. It has been shown that the ω phase is easily formed by either quenching from high temperatures (athermal ω) or heat-treatment at intermediate temperatures (thermal ω). Ideally, both athermal and thermal ω phases have a hexagonal structure. On the other hand, quenching from β phase leads to the transformation from the β phase to either hexagonal martensite (α′) or orthorhombic martensite (α″). It has been reported that the reverse transformation from the α″ phase to the β phase causes shape recovery to appear [11], [12]. The transformation temperature from the β phase to the α″phase can be controlled by adjusting the amount of alloying elements.

Many β-type Ti-base alloys exhibiting shape memory effect have been studied by many researchers, e.g., Ti–V base alloys [12], [13], Ti–Mo base alloys [5], [6], [14], [15] and Ti–Nb base alloys [4], [7], [8], [9], [10]. It is confirmed that mechanical properties can be improved by addition of further alloying elements to these binary alloys. Ti-base ternary alloys have been studied for developing biomedical shape memory alloys as follows: (i) Ti–Mo–Ga [5], Ti–Mo–Ge [16], Ti–Mo–Sn [6], Ti–Nb–Sn [4] and Ti–Nb–Al [8] which include an α-stabilizer for the third element, (ii) Ti–Nb–Ta [17] which includes a second β-stabilizer for the third element, (iii) Ti–Nb–Pd [18] and Ti–Mo–Ag [6] which include a noble metal for the third element, and (iv) Ti–Nb–Zr [10] and Ti–Nb–Sc [19]. It has been reported that these ternary alloys exhibit considerably stabilized shape memory and superelastic behavior. Other attractive candidates for the third element to the β-type Ti-base binary shape memory alloys are interstitial elements which also act as α-stabilizers, such as oxygen, carbon and nitrogen. The effect of adding of these interstitial elements, nitrogen or oxygen, on shape memory behavior has been investigated [9], [20]. 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.