Martensitic transformation and shape memory effect of Ni-Ti-Ga alloys

NiTi alloy is the most successful commercial shape memory alloy up to now, especially in the biomedical field. The addition of third element is effective in tailoring the transformation temperatures and hysteresis as well as improving functional and other properties of NiTi alloy. Tens of third elements including the noble ones have been studied, however, the effect of Ga addition was seldom reported. Ga has been reported to improve the castability. Besides, Ga ion has the bactericidal effect without no significant cytotoxicity. Therefore, it is necessary to study the influence of Ga addition in NiTi.

In the present paper, the transformation behaviour, microstructure, mechanical properties and shape memory behaviour of Ni₅₀Ti_50-xGa_x (x = 0, 0.4, 1, 2 at.%) alloys were investigated using differential scanning calorimetry (DSC), optical and scanning electron microscopy, X-ray diffraction (XRD), tensile and bending tests.

The substitution of Ga for Ti induces the B2-R transformation, as Fe and Al do. The transformation temperatures of B2-R and R-B19ʹ are decreased with increasing Ga content. B2-R transformation exhibits a much better thermal cycling stability than that of R-B19ʹ transformation. With increasing Ga content, the room-temperature microstructure changes from single B19ʹ to dual phases consisting of B2 and Ti₂(Ni, Ga). The tensile strength was slightly enhanced while the ductility was greatly reduced. The shape memory effect (SME) is improved by the addition of Ga. Ni₅₀Ti_49.5Ga_0.5 exhibits the best SME with 83% recovery ratio when deformed to 20%. An appropriate addition of Ga improves the corrosion resistance in simulated saliva.