Mechanical behavior at different temperatures and stresses for superelastic nickel-titanium orthodontic wires having different transformation temperatures

Objective: The purpose of this study was to investigate the mechanical properties of superelastic nickel-titanium orthodontic wires under controlled stress and temperature. Methods: Three different superelastic nickel-titanium wires were examined using differential scanning calorimetry (DSC), three-point bending test and micro X-ray diffraction (micro-XRD). The three-point bending test was carried out at constant temperature (23, 37 and 60°C) and stepwise temperature changes (37-60°C and to 37°C) (37-2°C and to 37°C). Five specimens of each wire were tested. Micro-XRD spectra were measured at the tension side of the wire when the temperature changed from 37 to 60°C or 2°C. Results: The load during the stepwise temperature changes (37-2°C and to 37°C) was consistent with that measured at a corresponding constant temperature. The micro XRD spectrum clearly showed that the austenite phase was transformed to martensite phase when the temperature is decreased from 37 to 2°C. In a stepwise temperature change (37-60°C and to 37°C), the load became higher than the original load at each corresponding constant temperature. However, there was no detectable change in the micro-XRD spectrum when the temperature was increased from 37 to 60°C. Significance: The superelastic nickel-titanium wires exhibited complicated and unexpected mechanical properties under stepwise temperature change. This study shows the possibility of qualitative analysis using micro-XRD to understand mechanical properties of these nickel-titanium wires