The aim of this study was to investigate the effect of thermomechanical treatment on mechanical and metallurgical properties of nickel-titanium (NiTi) rotary instruments. Eight kinds of NiTi rotary instruments with sizes of ISO #25 were selected: ProFile, K3, and One Shape for the conventional alloy; ProTaper NEXT, Reciproc, and WaveOne for the M-wire alloy; HyFlex CM for the controlled memory- (CM-) wire; and TF for the R-phase alloy. Torsional fracture and cyclic fatigue fracture tests were performed. Products underwent a differential scanning calorimetry (DSC) analysis. The CM-wire and R-phase groups had the lowest elastic modulus, followed by the M-wire group. The maximum torque of the M-wire instrument was comparable to that of a conventional instrument, while those of the CM-wire and R-phase instruments were lower. The angular displacement at failure (ADF) for the CM-wire and R-phase instruments was higher than that of conventional instruments, and ADF of the M-wire instruments was lower. The cyclic fatigue resistance of the thermomechanically treated NiTi instruments was higher. DSC plots revealed that NiTi instruments made with the conventional alloy were primarily composed of austenite at room temperature; stable martensite and R-phase were found in thermomechanically treated instruments.

The aim of this study was to compare the cyclic fatigue resistance, torsional resistance, and metallurgical characteristics of conventional NiTi wire (V taper 2, V2) and CM wire (V taper 2H, V2H)-based files. Cyclic fatigue and torsional resistance of V2 and V2H were investigated by measuring the number of cycles to fracture, maximum torque at fracture, and maximum angle at fracture. The typical patterns of fatigue and torsional fractures were investigated using a scanning electron microscope (SEM). The metallurgical characteristics were investigated by differential scanning calorimetry (DSC) from -100 °C to 100 °C. The austenite finishing temperature (Af) of each instrument was also measured. The microstructures of the instruments were investigated by a transmission electron microscope (TEM) along with selected area diffraction pattern analysis. The results were statistically analyzed by Mann-Whitney U-test (p = 0.05). V2H showed significantly higher cyclic fatigue resistance and torsional resistance than V2. SEM images of the fractured surfaces showed typical patterns of fatigue and torsional fracture. The DSC analysis of V2 showed one small peak in both the heating and cooling curves. The Af of V2 was -0.32 °C. V2H showed two remarkable peaks in the heating curve and one remarkable peak in the cooling curve. The Af of V2H was 33.25 °C. The TEM analysis showed that both V2 and V2H are mainly composed of austenite. In conclusion, V2H showed higher cyclic fatigue resistance and torsional resistance than V2. The superior properties of V2H could be attributed to the annealing effect and possibly the martensite phase. SCANNING 38:564-570, 2016. © 2016 Wiley Periodicals, Inc.