TMDSC Investigation of Transformations in NiTi Wires for Endodontic Instruments

The properties of nickel-titanium wires depend on their microstructural phases, which can be studied by differential scanning calorimetry (DSC). Temperature-modulated DSC (TMDSC) provides greater insight into phase transformations than possible with DSC. Objective: Employ TMDSC to investigate transformations in an experimental wire (M-Wire) with superior tensile and fatigue properties. Methods: Five NiTi wires for rotary endodontic instruments were analyzed: 2 batches of M-Wire (SportsWire LLC), 2 batches of superelastic (SE) wire (Tulsa Dental and Maillefer), and 1 batch of commercial NiTi blanks. Test specimens consisted of 4 wire segments having 4 – 5 mm lengths. The linear heating and cooling rate was 2°C per minute, with superimposed sinusoidal oscillation having 0.318°C amplitude and 60 second period (Q2000 and Q1000, TA Instruments). Each specimen was cooled from room temperature to -125°C, heated to 100°C, and cooled to -125°C. The DSC cell was purged with helium. Results: Two endothermic peaks were observed during the heating cycle, corresponding to transformation from martensite to R-phase, followed by transformation from R-phase to austenite. Two reverse transformations could not be resolved on the broad exothermic peak observed during the cooling cycle. Conventional SE wires had approximate austenite-finish (A_f) temperatures of 20°C (Tulsa Dental) and 10°C (Maillefer). The M-Wire batches had approximate A_f temperatures of 45°C and 50°C. The A_f temperature for the NiTi blanks was approximately 45°C. For all wires, the nonreversing heat flow associated with the transformations was much less than the reversing heat flow; enthalpy changes for the five wires were similar. Conclusions: Thermomechanical processing for M-Wire yielded much higher A_f temperatures than for conventional SE wires, indicating the presence of R-phase and possibly martensite at mouth temperature, suggested by our Micro-XRD study. These higher A_f temperatures and the heavily twinned microstructure found in our STEM study may account for the improved mechanical properties.