Structural and morphological characterization of heat-activated nickel-titanium archwires

Heat-activated nickel-titanium (Ni-Ti) archwires are widely used in the initial stages of orthodontic treatment. These alloys exhibit a shape memory effect which can be related to displacive (martensitic) transformation, which can be induced by temperature variation or mechanical influence (stress). Here we explored the changes of archwires microstructure, chemical composition and phase transition temperatures due to prolonged regular usage. Three forms of dental archwires with dimensions 0.016" x 0.022" (3M Unitek, Monrovia, CA, USA) were investigated: as-received, as-received + autoclaved (sterilized) and used during treatment heat-activated Ni-Ti archwires. The three forms of archwires were analyzed by powder X-ray diffraction analysis (XRD), Scanning Electronic Microscopy (SEM) in combination with Energy Dispersive Spectroscopy (EDX) and Differential Scanning Calorimetry (DSC). The room temperature the XRD pattern of the archwires show typical peaks for a Ni-Ti alloy with austenite type structure. The DSC measurements were conducted in the -50 degrees C to + 50 degrees C temperature range. The DSC analyses of the Ni-Ti archwires revealed three phase transitions. Upon heating from -50 degrees C to + 50 degrees C a phase transition occurs at -12 degrees C. No additional endo/exo effects are registered. Upon cooling the DSC registers a phase transition around 10 degrees C and around -40 degrees C. The first effect is due to the formation of an intermediate rhombohedral phase (R phase) while at -40 degrees C Ni-Ti structure transforms to martensite. The results from EDX demonstrate that there is no pronounced change in the chemical composition on the surface of the investigated orthodontic archwires. Though SEM micrographs show some changes on the surface of the wires after usual use. The results obtained within this study contribute to the establishment of some peculiarities related to the thermal behavior and the shape-memory effect of the investigated archwires.