Effect of copper addition on the superelastic behavior of Ni-Ti shape memory alloys for orthodontic applications.

Transformation temperatures and mechanical properties such as transformation stresses at different temperatures, superelasticity characteristics, and load cycling behavior have been investigated in NiTiCu orthodontic archwires with various copper concentrations. The results have been compared with the conventional NiTi orthodontic archwires. The addition of copper was effective in narrowing the stress hysteresis and in stabilizing the superelasticity characteristics against cyclic deformation, with the result that the slope of the load-deflection unloading curve of the alloy is lower than NiTi. Moreover, it produced greater stability of both the transformation temperature and the force applied to the teeth for a determined design and wire cross-section. On the other hand, the presence of copper in NiTi orthodontic archwires reduced the ageing effect. Studies of ion release in artificial saliva showed only small quantities after long periods of immersion.