Phase transformation analysis of varied nickel-titanium orthodontic wires

Background The shape memory effect of nickel-titanium （NiTi） archwires is largely determined by the phase transition temperature. It is associated with a reversible transformation from martensite to austenite. The aim of this study was to characterize austenite, martensite and R phase temperatures as well as transition temperature ranges of the commonly used clinical NiTi orthodontic arch wires selected from several manufacturers. Methods Differential scanning calorimetry （DSC） method was used to study the phase transformation temperatures and the phase transition processes of 9 commonly used clinical NiTi alloys （types： 0.40 mm （0.016 inch）, 0.40 mm × 0.56 mm （0.016 inch × 0.022 inch））. Results The austenite finish temperatures （Af） of 0.40 mm Smart, Ormco and 3M NiTi wires were lower than the room temperature, and no phase transformation was detected during oral temperature. Therefore, we predicted that these types of NiTi did not possess shape memory property. For 0.40 mm and 0.40 mm × 0.56 mm Youyan I NiTi wires, no phase transformation was detected during the scanning temperature range, suggesting that these two types of wires did not possess shape memory either. The Af of 0.40 mm × 0.56 mm Smart, L＆H, Youyan II Ni-Ti wires were close to the oral temperature and presented as martensitic-austenitic structures at room temperature, suggesting the NiTi wires listed above have good shape memory effect. Although the 0.40 mm × 0.56 mm Damon CuNiTi wire showed martensitic- austenitic structures at oral temperature, its Af was much higher than the oral temperature. It means that transformation from martensite to austenite for this type of NiTi only finishes when oral temperature is above normal. Conclusion The phase transformation temperatures and transformation behavior varied among different commonly used NiTi orthodontic arch wires, leading to variability in shape memory effect.