Effect of silver addition on the properties of nickel-titanium alloys for dental application.

Equiatomic and near-equiatomic nickel-titanium alloys exhibit a shape-memory effect and superelasticity. However, the properties of such alloys are extremely sensitive to the precise nickel-titanium ratio and the addition of alloying elements. High corrosion resistance is necessary for biomedical applications, especially orthodontic. The purpose of this study was to investigate the effect of silver addition to nickel-titanium alloys for dental and medical application. Arc melting, homogenization, hot rolling, and solution heat treatment were performed to prepare the nickel-titanium-silver (NiTi-Ag) specimens. The properties of the ternary NiTi-Ag alloys such as phase-transformation temperature, microstructure, microhardness, corrosion resistance, and cytotoxicity were investigated. The NiTi-Ag alloys showed low silver recovery rate for the cast alloy, due to silver's low evaporation temperature, and low silver solubility in nickel-titanium. Silver addition to nickel-titanium increased the transition temperature range to 100 degrees C and stabilized the martensitic phase (monoclinic structure) at room temperature, because the martensitic transformation starting temperature (Ms) was above room temperature. Martensitic and austenitic phases existed in X-ray diffraction patterns of solution-annealed NiTi-Ag alloys. The silver addition was considered to improve the corrosion resistance and form a stable passive film. Significantly, the mechanical properties of the silver-added alloys were dependent upon the amount of alloying addition. There was no toxicity in the NiTi-Ag alloys, as the response index showed none or mild levels.