STEM Observations of NiTi Wires for Rotary Endodontic Instruments

There is concern about fatigue performance of nickel-titanium rotary endodontic instruments. Recently, rotary instruments manufactured from a new nickel-titanium wire (M-Wire), using a proprietary procedure (SportsWire LLC), have shown considerably improved in vitro fatigue performance, compared to conventional instruments fabricated from superelastic (SE) wire. Objective: To employ electron microscopy and investigate reasons for this improved fatigue performance. Methods: Using a focused ion beam (FIB) technique, starting blanks of M-Wire and conventional SE wire (Maillefer) were prepared in foil form. The Tecnai TF-20 electron microscope (Philips) was operated at 200 kV in the scanning transmission electron microscope (STEM) mode to examine foils at room temperature (20°C) and obtain bright-field images. Results: Cross-sections of SE wire contain fine equiaxed grains of predominantly austenite, with some darker grains which may be R-phase or martensite. Grain boundaries are well-defined, and some twins can be observed. These darker grains may have a high dislocation density, whereas minimal dislocations appear to exist in the austenite grains. Cross-sections of M-wire have much coarser grains, which is consistent with annealing during processing. Triple-point junctions of grain boundaries are often found, also indicative of annealing. Considerable twinning from wire processing is also evident in M-Wire, and these may be annealing or deformation twins. Complementary Micro-x-ray diffraction analyses at room temperature have confirmed that the superelastic wire is austenite and the batch of M-Wire examined is principally austenite with perhaps some R-phase and martensite, which is largely consistent with temperature-modulated differential scanning calorimetry (DSC) analyses from another study by our group. Conclusions: The superior mechanical properties of M-Wire arise from the twinned microstructure, which provides strengthening and increased deformation without fracture. Future TEM study of M-Wire will focus on obtaining electron diffraction patterns to confirm the NiTi microstructural phases and on detailed characterization of the martensite phase.