Micro-XRD Observations of NiTi Wires for Rotary Endodontic Instruments

A new nickel-titanium wire (M-Wire) with superior mechanical properties has been developed, using proprietary processing (SportsWire LLC). This new wire yields rotary instruments with considerably improved in vitro fatigue performance, compared to conventional instruments fabricated from superelastic (SE) wire. Our research group has investigated the microstructures of M-Wire and conventional SE NiTi wire in another study, using scanning transmission electron microscopy (STEM), but the interpretation of microstructural phases from these observations was uncertain. Objective: Employ Micro-x-ray diffraction (Micro-XRD) to determine the metallurgical phases in M-Wire and conventional SE wire (Maillefer). Methods: Three 1-cm length samples were cut from each of two 5-cm segments for one batch of SE wire and two batches of M-Wire, using a water-cooled saw. Micro-XRD (Rint-2000, Rigaku) was performed on 5 samples of each wire batch at room temperature (25°C), using CuKa radiation at 40 kV and tube current of 300 mA. The sixth sample from each batch was analyzed at temperatures of 5°C, 25°C, 37°C and 60°C. Analyses were performed at sites of approximately 100 μm dimensions, and performed for as-received wires and polished wires. Results: The conventional SE NiTi wire had the expected austenite structure. The two batches of M-Wire were predominantly austenite, with the broadened diffraction peak at 42.5° suggesting the possible presence of martensite or R-phase. The second batch contained non-austenite diffraction peaks at approximately 44.5°, 51.5° and 54.0°, which did not disappear when the sample was heated to 37°C and 60°C, and are tentatively attributed to titanium oxide(s). Conclusions: Phases at the surface of M-Wire can vary, depending upon variations in processing. Micro-XRD indicates that M-Wire is predominantly austenite at room temperature. Future transmission electron microscopy observations accompanied by electron diffraction analyses are needed to investigate martensite in M-Wire, which can have important consequences for mechanical properties.