DIFFERENTIAL SCANNING CALORIMETRY (DSC) ANALYSES OF SUPERELASTIC AND NONSUPERELASTIC NICKEL-TITANIUM ORTHODONTIC WIRES

The purpose of this study was to determine the transformation temperat ures for the austenitic, martensitic, and rhombohedral (R) structure p hases in representative as-received commercial nitinol (NiTi) orthodon tic wire alloys, to reconcile discrepancies among recent publications. Specimens were examined by differential scanning calorimetry (DSC) ov er a temperature range from approximately -170 degrees C to 100 degree s C, with a scanning rate of 10 degrees C per minute. Two different pa thways, with the intermediate R structure either absent or present, we re observed for the transformation from martensitic to austenitic NiTi , whereas the reverse transformation from austenitic to martensitic Ni Ti always included the R structure. The enthalpy (Delta H) for the tra nsformation from martensite to austenite ranged from 0.3 to 3.5 calori es per gram. The lowest Delta H value for the nonsuperelastic Nitinol wire is consistent with a largely work-hardened, stable, martensitic m icrostructure in this product. The DSC results indicate that the trans formation processes are broadly similar in superelastic, body-temperat ure shape-memory, and nonsuperelastic NiTi wires. Differences in bendi ng properties for the NiTi orthodontic wires at room temperature and 3 7 degrees C are due to the relative proportions of the metallurgical p hases in the microstructures.