Metallurgical Characterization of M-Wire Nickel-Titanium Shape Memory Alloy Used for Endodontic Rotary Instruments during Low-cycle Fatigue

doi:10.1016/j.joen.2011.09.028

Journal of Endodontics

Volume 38, Issue 1, January 2012, Pages 105-107

https://doi.org/10.1016/j.joen.2011.09.028 Get rights and content

Abstract

Introduction

Rotary instruments made of a new nickel-titanium (NiTi) alloy (M-Wire) have shown improved cyclic fatigue resistance and mechanical properties compared with those made of conventional superelastic NiTi wires. The objective of this study was to characterize microstructural changes of M-Wire throughout the cyclic fatigue process under controlled strain amplitude.

Methods

The average fatigue life was calculated from 30 M-Wire samples that were subjected to a strain-controlled (∼4%) rotating bend fatigue test at room temperature and rotational speed of 300 rpm. Microstructural evolution of M-Wire has been investigated by different metallurgical characterization techniques, including differential scanning calorimetry, Vickers microhardness, and transmission electron microscopy at 4 different stages (as-received state, 30%, 60%, and 90% of average fatigue life).

Results

During rotating bend fatigue test, no statistically significant difference (P > .05) was found on austenite finish temperatures between as-received M-Wire and fatigued samples. However, significant differences (P < .05) were observed on Vickers microhardness for samples with 60% and 90% fatigue life compared with as-received and 30% fatigue life. Coincidentally, substantial growth of martensite grains and martensite twins was observed in microstructure under transmission electron microscopy after 60% fatigue life.

Conclusions

The results of the present study suggested that endodontic instruments manufactured with M-Wire are expected to have higher strength and wear resistance than similar instruments made of conventional superelastic NiTi wires because of its unique nano-crystalline martensitic microstructure.

Section snippets

Materials and Methods

M-Wire instead of endodontic instruments made of M-Wire was selected in rotating bend fatigue test for the following 2 reasons: (1) for NiTi rotary instrument, the strain/stress level at the maximum flexure is difficult to calculate because of complicated file design (size/taper, cross-section geometry, helical angle, etc); (2) ISO standardized test methods and devices for cyclic fatigue resistance of NiTi rotary instruments are not available.

The average fatigue life was determined from 30

Results

The mean and standard deviation of A_f temperature and Vickers microhardness number (VHN) on M-Wire at 4 different stages (as-received, 30%, 60%, and 90% fatigue life) are shown in Table 1.

There was no statistically significant difference (P > .05) on A_f temperatures between as-received M-Wire and samples cycled at 30%, 60%, and 90% fatigue life.

For VHN, there was no significant difference between as-received and 30% fatigue life. However, microhardness increased as the low-cycle fatigue test

Discussion

Fracture of rotary NiTi instruments because of cyclic fatigue usually occurs near the apical third of a root canal with the highest curvature without any warning. The microstructure of NiTi raw materials used in endodontic instruments plays a crucial role on their mechanical behavior and root canal outcome, especially regarding strength and fatigue resistance. The present study examined metallurgical microstructures, phase transition temperatures, and microhardness of M-Wire at 4 different

Conclusion

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: This work was financially supported by Dentsply Tulsa Dental Specialties.

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Basic ResearchMetallurgical Characterization of M-Wire Nickel-Titanium Shape Memory Alloy Used for Endodontic Rotary Instruments during Low-cycle Fatigue

Abstract

Introduction

Methods

Results

Conclusions

Section snippets

Materials and Methods

Results

Discussion

Conclusion

J Endod

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J Endod

Basic Research
Metallurgical Characterization of M-Wire Nickel-Titanium Shape Memory Alloy Used for Endodontic Rotary Instruments during Low-cycle Fatigue