Differential scanning calorimetry (DSC) analyses of superelastic and nonsuperelastic nickel-titanium orthodontic wires

The purpose of this study was to determine the transformation temperatures for the austenitic, martensitic, and rhombohedral (R) structure phases in representative as-received commercial nitinol (NiTi) orthodontic wire alloys, to reconcile discrepancies among recent publications. Specimens were examined by differential scanning calorimetry (DSC) over a temperature range from approximately -170 degrees C to 100 degrees C, with a scanning rate of 10 degrees C per minute. Two different pathways, with the intermediate R structure either absent or present, were observed for the transformation from martensitic to austenitic NiTi, whereas the reverse transformation from austenitic to martensitic NiTi always included the R structure. The enthalpy (Delta H) for the transformation from martensite to austenite ranged from 0.3 to 3.5 calories per gram. The lowest Delta H value for the nonsuperelastic Nitinol wire is consistent with a largely work-hardened, stable, martensitic microstructure in this product. The DSC results indicate that the transformation processes are broadly similar in superelastic, body-temperature shape-memory, and nonsuperelastic NiTi wires. Differences in bending properties for the NiTi orthodontic wires at room temperature and 37 degrees C are due to the relative proportions of the metallurgical phases in the microstructures.