Temperature-modulated DSC study of phase transformations in nickel–titanium orthodontic wires

Nickel–titanium archwire alloys are very important for clinical orthodontics because of their wide elastic range and excellent springback, along with their ability to deliver highly desirable light forces for tooth movement. Recently, nickel–titanium orthodontic alloys have been developed that possess shape memory over the temperature range of the oral environment, and these archwires have considerable promise for the clinical treatment of patients. The shape memory is associated with a reversible transformation from martensitic NiTi to austenitic NiTi that is completed at mouth temperature. While extensive conventional differential scanning calorimetry (DSC) studies have been performed on commercial nickel–titanium orthodontic wire alloys to characterize their phase transformation behavior, the present study is the first time that temperature-modulated DSC (TMDSC) has been used for this purpose. Two commercial nickel–titanium wires possessing shape memory were analyzed, along with a third commercial nickel–titanium wire lacking shape memory. The TMDSC analyses were generally performed from −130 to 100 °C for both the heating and cooling cycles, since there can be differences in the processes and the temperature ranges for the forward and reverse transformations. Improved resolution of the phase transformations was achieved with the use of TMDSC, compared to conventional DSC, and the novel results indicate the complexity of the phase transformation processes.