X-ray diffraction study of low-temperature phase transformations in nickel-titanium orthodontic wires

Objectives: Employ conventional X-ray diffraction (XRD) to analyze three clinically important nickel-titanium orthodontic wire alloys over a range of temperatures between 25 and -110 °C, for comparison with previous results from temperature-modulated differential scanning calorimetry (TMDSC) studies. Methods: The archwires selected were 35 °C Copper Ni-Ti (Ormco), Neo Sentalloy (GAC International), and Nitinol SE (3M Unitek). Neo Sentalloy, which exhibits superelastic behavior, is marketed as having shape memory in the oral environment, and Nitinol SE and 35 °C Copper Ni-Ti also exhibit superelastic behavior. All archwires had dimensions of 0.016 in. × 0.022 in. (0.41 mm × 0.56 mm). Straight segments cut with a water-cooled diamond saw were placed side-by-side to yield a 1 cm × 1 cm test sample of each wire product for XRD analysis (Rint-Ultima⁺, Rigaku) over a 2θ range from 30° to 130° and at successive temperatures of 25, -110, -60, -20, 0 and 25 °C. Results: The phases revealed by XRD at the different analysis temperatures were in good agreement with those found in previous TMDSC studies of transformations in these alloys, in particular verifying the presence of R-phase at 25 °C. Precise comparisons are not possible because of the approximate nature of the transformation temperatures determined by TMDSC and the preferred crystallographic orientation present in the wires. New XRD peaks appear to result from low-temperature transformation in martensite, which a recent transmission electron microscopy (TEM) study has shown to arise from twinning. Significance: While XRD is a useful technique to study phases in nickel-titanium orthodontic wires and their transformations as a function of temperature, optimum insight is obtained when XRD analyses are combined with complementary TMDSC and TEM study of the wires.