The mechanical and thermal behaviors of heat treated NiTi orthodontic archwires

The near equiatomic nickel-titanium alloy is an outstanding intermetallic compound exhibiting distinctive properties associated with characteristic thermal and stress induced martensitic transformations. Some operations are applied in process of producing orthodontic wires so as to obtain the optimal shape memory behaviors. Phase transformation temperatures and load-deflection characteristics of this binary alloy are very significant variables in the application and can be manipulated by different thermomechanical treatments via inducing precipitation or dislocation networks in the matrix. More recently, heat activated NiTi wires have been marketed with clinically useful shape memory effect and higher transition temperature range (TTR). In this study, one brand of commercial heat activated nickel-titanium archwire (3M Unitek) was selected and solution treated. Then, The wires annealed at 400¢ªC for times of 10, 30 and 60 min. Thermal transformation temperatures were determined using differential scanning calorimeter and showed to alter as the time of heat treatment increased. The microstructures of wires were observed by scanning electron microscope. In order to evaluate mechanical parameters of heat treated archwires, they placed on an arch-form fixture simulating maxillary dentition and load-deflection curves were obtained by three point bending test at 37 ¢ªC. The results compared to as-received archwires and the changes were rationalized on the basis of microstructure.