Micro X-ray diffraction study of superelastic nickel–titanium orthodontic wires at different temperatures and stresses

The phase transformation behavior in three commercial nickel–titanium orthodontic wires having different transformation temperatures was studied by micro X-ray diffraction (micro-XRD). Micro-XRD spectra were obtained at three different included bending angles (135°, 146° and 157°) and three different temperatures (25°C, 37°C and 60°C). The regions analyzed by micro-XRD were within the separate areas of a given wire specimen that experienced only tensile or compressive strain. The intensity ratio (M002/A110) between the 002 peak for martensitic NiTi and the 110 peak for austenitic NiTi was employed as the index to the proportions of the martensite and austenite phases. The ratio of martensite to austenite increased in all three nickel–titanium wires with decreasing included bending angle (greater permanent bending deformation), and was lower within the compression area for all wires at all bending angles than within the tension area. Micro-XRD provides an effective method for quantitative evaluation of the proportions of these two phases in nickel–titanium orthodontic wires, even though considerable preferred crystallographic orientation exists because of the wire drawing process.