Basic Research—Technology

Mechanical Properties of Controlled Memory and Superelastic - Wires Used in the Manufacture of Rotary Endodontic Instruments Hui-min Zhou, PhD,* Ya Shen, DDS, PhD,† Wei Zheng, PhD,* Li Li, PhD,* Yu-feng Zheng, PhD,*‡ and Markus Haapasalo, DDS, PhD†

Abstract Introduction: The aim of this study was to investigate manufactured with CM wires than similar instruments made of conventional SE wires. the structure and mechanical properties of newly devel- (J Endod 2012;38:1535–1540) oped controlled memory (CM) nickel-titanium wires used in the manufacture of rotary endodontic instru- Key Words ments. Methods: The composition and the phase Controlled memory, endodontic instrument, mechanical property, nickel-titanium , transformation behavior of both types of wires were phase transformation examined by x-ray energy dispersive spectroscopy and differential scanning calorimetry, respectively. ost rotary nickel-titanium (NiTi) endodontic instruments are fabricated from near- Conventional superelastic (SE) nickel-titanium wire Mequiatomic NiTi alloys containing 54.5–57 wt % nickel (1), which are famous for was used as a control. The mechanical properties of the shape memory effect and superelasticity (SE) also known as prop- the wires at selected temperatures (room temperature, erties based on the thermoelastic martensitic transformation. SE of near-equiatomic NiTi 37C, and 60C) were evaluated with tensile, cyclic alloy arises from the reversible stress-induced martensitic (SIM) transformation and tensile, and cantilever bending tests by using an Ins- depends on the temperature difference between the working temperature and the tron 3365 universal testing machine. The data of finish temperature Af (2). The SE of NiTi rotary instruments provides improved austenitic transformation finishing temperature (Af) access to curved root canals during the chemomechanical preparation, leading to their were analyzed statistically by using 1-way analysis of successful and extensive application in the clinical practice. However, even though NiTi variance test at a significance level of P < .05. endodontic files provide considerable improvement compared with those made of stain- Results: The raw CM wires contained a nickel content less steel, the fracture of rotary NiTi instruments in the root canal may occur without any of 50.7% Æ 0.5% and possessed a relatively higher Af warning (3–7), even with brand-new instruments. than SE wires (P < .05). The critical plateau stress and To improve the fracture resistance of NiTi files and to increase their reliability, ultimate tensile strength of the CM wires were lower effectiveness, and safety, manufacturers either have introduced new alloys with superior than they were for the SE wires, but the maximum strain mechanical properties (8) or have developed new manufacturing processes (9–20), before fracture of the CM wires (58.4% Æ 7.5% to both of which have advanced the science of endodontic rotary instrumentation. In 84.7% Æ 6.8%) was more than 3 times higher than it the last decade, significant enhancements in the manufacture of these instruments was for SE wires (16.7% Æ 3.8% to 27.5% Æ 5.4%). have been made. These include design and control of the raw materials used relative The maximum strain of the CM wires with a diameter to their microstructure, material properties, and manufacturing processes (21). of 1.22 mm tested at room temperature (23C Æ 2C) Recently, thermal treatment of NiTi alloys (eg, M-Wire; Dentsply Tulsa Dental Special- was up to 84% Æ 6.4%. CM wires were not SE at either ties, Tulsa, OK (9–15), R-phase wire [SybronEndo, Orange, CA] (16, 17), and room temperature or 37C; however, they exhibited controlled memory wire [CM Wire; DS Dental, Johnson City, TN] (18–20)) has been superelasticity when heated to 60C. Conclusions: used to optimize the mechanical properties. Thermomechanical processing is a The raw CM wires exhibited different phase transforma- frequently used method to optimize the microstructure and transformation behavior tion behavior and mechanical properties when of NiTi alloys, which in turn have great influence on the reliability and mechanical compared with SE wires, attributing to the special properties of NiTi files (15). It is commonly accepted that the improved properties heat treatment history of CM wires. This study sug- of endodontic instruments are closely related to the use of new materials with optimized gested greater flexibility of endodontic instruments microstructures and their associated processing technologies (15).

From the *Center for Biomedical Materials and Engineering, College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin, China; †Division of Endodontics, Department of Oral Biological and Medical Sciences, Faculty of Dentistry, University of British Columbia, Vancouver, Canada; and ‡State Key Laboratory for Turbulence and Complex Systems and Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing, China. Supported by the Fundamental Research Funds for the Central Universities (no. HEUCFZ1123 and HEUCF201210008) and the Natural Science Foundation of Hei- longjiang Province of China (no. ZD201012). Address requests for reprints to Prof Markus Haapasalo, Chair, Division of Endodontics, Head, Oral Biological and Medical Sciences, UBC Faculty of Dentistry, 2199 Wesbrook Mall, Vancouver, BC, Canada V6T 1Z3. E-mail address: [email protected] 0099-2399/$ - see front matter Copyright ª 2012 American Association of Endodontists. http://dx.doi.org/10.1016/j.joen.2012.07.006

JOE — Volume 38, Number 11, November 2012 Mechanical Properties of Controlled Memory and Superelastic NiTi Wires 1535 Basic Research—Technology

With the novel NiTi endodontic files being successively developed chamber (Instron SFL) and a 5-kN full-scale load cell (accuracy, and commercialized by different manufacturers, it has become imper- Æ0.4% of reading). Specimens with a total length of 50 mm were cut ative for the clinician to understand the nature of the raw materials used from raw wires. Tests were performed under crosshead displacement to make these instruments and their relevant mechanical properties control with a strain rate of 0.01 minÀ1 at selected temperatures. engendered by innovative manufacturing processes. The stress-strain The uniaxial tensile tests were performed at both room temperature curve of NiTi wires could provide significant information about SIM and 37C until fracture occurred. The critical stress for stress- transformation, martensite reorientation (MR), and the distinct stage induced martensitic (sSIM) transformation and martensite reorienta- of yielding by plastic deformation (22). The stress plateau in the tion (sMR) were determined by extrapolating the elastic part and the stress-strain curve is associated with either SIM or MR, depending on stress plateau on the stress-strain curve (24, 25). After tensile tests, the starting structure (eg, deformed in austenitic state via SIM and the fractured surfaces were observed by SEM. The SE of each deformed in martensitic state via MR). This information is useful for specimen was evaluated by a cyclic tensile test in the mode of the study and understanding of the mechanical behavior of CM wires a loading-unloading cycle at room temperature. The cantilever- and corresponding CM rotary instruments. Therefore, the present study bending test, according to Hou et al (16), was conducted at room used various mechanical methods (tensile, cyclic tensile, and cantilever temperature (23C Æ 2C), 37C, and 60C. Each wire was loaded bending tests) and metallurgical laboratory techniques to compare the at 3.0 mm from the tip until a deflection of 3 mm was produced, and mechanical properties and phase transformation behavior of raw CM the unloading process was then started until the force was reduced wires with thermomechanical treatment and SE NiTi wires. The ultimate to 0 N. goal of this research was to provide deep insight into the connection The data of Af temperatures were analyzed statistically by using 1- between the microstructural evolution and improved mechanical prop- way analysis of variance (SPSS for Windows 11.0; SPSS, Chicago, IL) at erties of CM wires. a significance level of P < .05.

Materials and Methods Results The traditional SE wires and the CM wires investigated had The chemical composition of both the CM wires and the SE wires diameters of 1.22 mm and 0.64 mm, respectively, and were provided was nickel-rich, with the approximate nickel content of 50.8% Æ 0.7% by the DS Dental Company. The wires provided were the starting wire for SE wire and 50.7% Æ 0.5% for CM wire. There were also small blanks after thermal treatment and before machining of flutes. The amounts of other elements, mainly oxygen and carbon, in both CM wires wires with a diameter of 0.64 mm (0.025 inch) were denoted as and SE wires according to EDS results. 25, and the ones with a diameter of 1.22 mm (0.048 inch) were The typical DSC curves of CM and SE wires each with a diameter denoted as 48. The solid-solution treatment of the wires that could of 1.22 mm are displayed in Figure 1A. The raw CM wires exhibited dissolve the precipitate phases and eliminate the effect of previous 2-stage transformations during both the heating and cooling thermomechanical treatment was conducted to identify the composi- processes, corresponding to M/R/A and reverse transformation. tion of the NiTi wires. Each specimen subjected to heat treatment was In the case of the raw SE wires, a 2-stage transformation during heat- sealed in an evacuated quartz capsule and heated to 900Cin ing and 1-stage transformation during cooling were observed with a muffle furnace for 1 hour and then transferred to a water sink relatively low and broad peaks. In addition, a single clear peak ap- and broken quickly to quench it in water to maintain its high- peared on both cooling and heating curves of CM and SE wires after temperature structure (supersaturated solid solution) at low temper- heat treatment. As shown in Figure 1B, there was a shift in the endo- ature. Each capsule contained a piece of pure titanium to suppress thermic and exothermic peaks of the SE wires with a smaller diam- the oxidation of the wire (23). The composition of the raw SE and eter such that the peaks occurred at lower temperatures. The CM CM wires after solid-solution heat treatment was determined by an x- wires exhibited the same tendency as the SE wires. The transforma- ray energy dispersive spectroscopy (EDS) system (Oxford Instru- tion temperatures and enthalpies of both raw and heat-treated CM ment, Oxfordshire, UK) equipped on a JSM-6480 scanning electron and SE wires are listed in Table 1. The mean Af temperatures for microscope (SEM) (JEOL Ltd, Tokyo, Japan) by using the solid- raw CM wires with diameters of 1.22 mm and 0.64 mm were higher solution treated wires. Test specimens were carefully cut from the than those of SE wires (P < .05). raw wires by using a water-cooled, slow-speed diamond saw. The The tensile stress-strain curves of CM and SE wires tested at specimens with a length of approximately 4–5 mm and 50 mm room temperature and at 37C are shown in Figure 1C. The critical were used for differential scanning calorimetry (DSC) analyses and plateau stress of CM wires for the reorientation of the martensite vari- mechanical tests, respectively. Eight specimens for each type were ants was in the range of 128 Æ 15.3 to 251 Æ 18.6 MPa, which was subjected to the test. much lower than the critical stress sSIM of the SE wires (490 Æ 22.4 The phase transformation behavior was examined with DSC to 582 Æ 26.3 MPa) for the SIM transformation. In addition, the ulti- (PYRIS, Perkin Elmer Diamond Series DSC; PerkinElmer, Shelton, mate tensile strength for the CM wires (1094 Æ 17.5 MPa) was also CT) for both raw wires and heat-treated wires over a temperature lower than it was for the SE wires (1415 Æ 21.2 MPa). However, the ranging from À100C to 100C by using the liquid nitrogen cooling maximum strain of the CM wires before fracture (58.4% Æ 7.5% to accessory to achieve subambient temperatures. The austenitic 84.7% Æ 6.8%) was much higher than it was for that of the SE wires transformation-starting and transformation-finishing points (As,Af) (16.7% Æ 3.8% to 27.5% Æ 5.4%). The maximum strain of the CM and martensitic reverse transformation-starting and transformation- wires with a diameter of 1.22 mm tested at room temperature was up finishing points (Ms,Mf) were determined by the intersection of an to 84% Æ 6.4%. extrapolated baseline and the maximum gradient line of the lambda- The stress-strain response of SE and CM wires at room tempera- type DSC curve. This procedure has been described in detail in ture during loading and unloading tensile tests is displayed in Figure 1D. a previous article (18). SE wires possessed SE at room temperature with a residual strain of Mechanical testing was performed by using a universal testing 0.6% Æ 0.1%, whereas CM wires at room temperature had no SE machine (Instron 3365; Instron, Norwood, MA) with a temperature with a residual strain of 5% Æ 0.8%.

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Figure 1. (A) DSC curves of raw and heat-treated (HT) SE and CM wires with diameter of 1.22 mm. (B) DSC curves of raw SE wires with diameter of 1.22 mm and 0.64 mm. (C) Tensile stress-strain curves of raw CM and SE wires with diameter of 1.22 mm. Test was conducted at room temperature and oral temperature (37C). (D) Tensile stress-strain response of SE and CM wires during loading-unloading process performed at room temperature. (E) Flexural load-deflection curves of raw SE wires. (F) Flexural load-deflection curves of raw CM wires. Tests were conducted at room temperature (RT, 23C Æ 2C), oral temperature (37C), and 60C.

A plot of the load versus deflection during flexural tests of the CM and The typical surface morphology of tensile fractures of CM wires SE wires at indicated test temperatures is shown in Figure 1E and F. The is shown in Figure 2. There was no significant difference in the frac- residual strain of CM wires decreased when the test temperatures were ture morphology for the specimens tested at room temperature increased. At a test temperature of 60C, CM wires with a diameter of either (23C Æ 2C) or at 37C; both exhibited a fiber region in the 1.22 mm or 0.64 mm had a residual strain of about 2.1%, indicating that central part of the fracture and shear lips at its periphery. A large CM wires recovered their SE at 60C. In addition, the critical plateau stress number of dimples could be observed on the fracture surface, indi- of both SE and CM wires increased with an increase of the test temperature. cating a ductile fracture.

TABLE 1. Phase Transformation Temperatures and Associated Energy from DSC Plots for Raw NiTi Wires     Type Ms ( C) Mf ( C) As ( C) Af ( C) DHheating (J/g) DHcooling (J/g) 25SE 21.2 Æ 2.7 À1.1 Æ 2.6 À21.2 Æ 1.6 23.6 Æ 2.3a 1.6 Æ 1.4 À1.9 Æ 0.5 48SE 26.4 Æ 4.1 2.3 Æ 1.9 À14.6 Æ 2.1 31.0 Æ 3.6a 2.1 Æ 2.2 À3.8 Æ 0.7 25CM 21.3 Æ 3.4 À32.1 Æ 2.3 20.8 Æ 3.2 54.3 Æ 4.2b 16.7 Æ 1.7 À6.9 Æ 1.3 48CM 24.2 Æ 2.5 À29.1 Æ 3.5 25.0 Æ 2.7 48.8 Æ 3.9b 16.4 Æ 1.2 À15.7 Æ 0.9 HT 25SE À34.3 Æ 3.7 À37.2 Æ 2.6 À18.5 Æ 2.3 À12.7 Æ 3.1c 9.6 Æ 1.3 À8.1 Æ 1.6 HT 48SE À29.4 Æ 5.2 À33.3 Æ 1.5 À8.9 Æ 2.9 À0.9 Æ 2.4c 13.7 Æ 2.2 À13.3 Æ 1.4 HT 25CM À34.3 Æ 4.6 À40.8 Æ 0.9 À20.6 Æ 2.6 À13.9 Æ 3.1c 10.8 Æ 2.4 À9.4 Æ 0.8 HT 48CM À28.4 Æ 4.8 À31.5 Æ 2.4 À9.5 Æ 1.3 À1.2 Æ 3.2c 13.7 Æ 2.3 À12.3 Æ 1.1

HT, heat treatment; DH, enthalpy changes. Data are expressed as mean Æ standard deviation. Different superscript letters indicate statistically significant differences between groups (P < .05).

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Figure 2. SEM images of typical morphology of fractured surface of CM wires with diameter of 1.22 mm after tensile test at (A and B) room temperature (23C Æ 2C), (C and D) oral temperature (37C).

Discussion of the CM wires could be attributed to their special thermomechanical NiTi alloys are usually chosen for the functional properties they processing. have associated with the martensitic transformation, which are sensitive According to the DSC results, the starting temperature (As) and fin- to alloy composition and microstructure as well as external influences ishing temperature (Af) of austenite transformation of the raw CM wires (26). A NiTi alloy with a specific composition could have its mechanical were much higher than those of the SE wires, and the enthalpy changes properties optimized by altering the alloy’s microstructure via cold during the heating and cooling processes for the CM wires were work and heat treatment (27). This is why new NiTi endodontic files also larger than for SE wires. Thermomechanical treatment during with superior properties could be developed through thermomechan- manufacturing of endodontic instruments is believed to have a great ical processing, which is a metallurgical process that integrates work influence on the thermal behavior of NiTi instruments (30–34).In hardening and heat treatment into a single process. However, there is a previous study on the phase transformation behavior and still lack of understanding about the fundamental mechanisms for microstructure of several newly developed NiTi instruments and improved performance of endodontic instruments related to the ther- regular SE NiTi instruments, it was found that TYP CM (Typhoon CM; Clinician’s Choice Dental Products, New Milford, CT) endodontic files momechanical treatment. In the present study, CM and SE raw wires  with 2 different diameters were selected as specimens. Raw wires had an Af temperature exceeding 37 C (18), which is in agreement were selected for 2 reasons; it was easier to conduct the mechanical with the raw CM wires investigated in the present study. However, the tests on the wire-type specimens, which did not have taper or cutting DSC curves obtained for raw CM wires are different from those of flutes, and studying the raw wires eliminated any influence of the TYP CM files, which is probably due to the machining process of cutting complicated geometry of endodontic files and allowed us to focus on flutes and the files’ post-treatment. the influence of the thermomechanical treatment on the mechanical Although the exact thermomechanical treatment of CM wires properties. remains unknown, the mechanical deformation behavior is closely The composition of the NiTi alloy, especially the nickel content, related to the phase transformation temperature, which is sensitive has a great influence on the transformation temperatures. It has been to thermomechanical history. Flexibility is regarded as one of the reported that the phase transformation temperature shifts 12C toward most important mechanical properties of NiTi rotary instruments, a lower temperature when the nickel atom content of a Ni-rich Ni-Ti and the choice of one instrument over another often hinges on this. The tensile results showed that CM wires had a much higher alloy increases by 0.1% (28). Our EDS results indicated that the compo- s sition of CM and SE wires could be considered as the same, and the maximum strain before fracture and lower SIM values than did SE similar phase transformation temperatures of the CM and SE wires after wires (Figure 1C), indicating the superior flexibility of CM wire heat treatment also indicated this. The solid-solution heat treatment compared with regular SE wire. This may be attributed to the different perhaps eliminates the influence of the precipitates and dislocations phase composition at oral temperature between CM and SE wires. Ac- cording to the DSC results (Table 1), conventional SE wire has produced by thermomechanical processing, leading to the homogeni-  zation of the microstructure and chemical composition within the CM complete austenite structure at the oral temperature (37 C); thus, and SE wires (29). Thus, the influence of the composition could be this type of wire would exhibit SE during clinical application. ignored in this study, and the different phase transformation behavior However, CM wire is a mixture of martensite, R-phase, and a small

1538 Zhou et al. JOE — Volume 38, Number 11, November 2012 Basic Research—Technology amount of austenite and did not exhibit SE during the tensile loading- 2. McCormick PG, Liu Y, Miyazaki S. Intrinsic thermal-mechanical behaviour associ- unloading test (Figure 1D). It was found that the R-phase possesses ated with the stress-induced martensitic transformation in NiTi. Mater Sci Eng A a lower shear modulus than martensite and austenite, and the trans- 1993;167:51–6. 3. Ankrum MT, Hartwell GR, Truitt JE. K3 Endo, ProTaper, and ProFile systems: formation strain for R-phase is less than one-tenth that of martensitic breakage and distortion in severely curved roots of molars. J Endod 2004;30: transformation (16, 35). Thus, the lower plateau stress for 234–7. reorientation of the martensite variant is needed for CM wires. In 4. Pruett JP, Clement DJ, Carnes DL Jr. Cyclic testing of nickel-titanium addition, it was also revealed in a previous study that the critical endodontic instruments. J Endod 1997;23:77–85. 5. Zuolo ML, Walton RE. Instrument deterioration with usage: nickel-titanium versus plateau stress is inversely correlated with the Ms temperature . Quintessence Int 1997;28:397–402. because a lower Ms impedes the phase transformation and more 6. Mandel E, Adib-Yazdi M, Benhamou LM, Lachkar T, Mesgouez C, Sobel M. Rotary stress is required to induce martensitic transformation (36). In the Ni-Ti profile systems for preparing curved canals in resin blocks: influence of oper- ator on instrument breakage. Int Endod J 1999;32:436–43. present study, the lower sSIM value for CM wires could be explained by the higher M temperature compared with SE wires (Table 1). This 7. Arens FC, Hoen MM, Steiman HR, Dietz GC Jr. Evaluation of single-use rotary nickel- s titanium instruments. J Endod 2003;29:664–6. superior flexibility of raw CM wires as revealed from the tensile tests 8. Testarelli L, Plotino G, Al-Sudani D, et al. Bending properties of a new nickel- points toward potential improvement of the clinical properties of CM titanium alloy with a lower percent by weight of nickel. J Endod 2011;37:1293–5. endodontic files. It is consistent with previous studies that found that 9. Johnson E, Lloyd A, Kuttler S, Namerow K. Comparison between a novel nickel- NiTi instruments made from CM wires were significantly more resis- titanium alloy and 508 nitinol on the cyclic fatigue life of ProFile 25/.04 rotary instruments. J Endod 2008;34:1406–9. tant to fatigue failure than instruments made from conventional NiTi 10. Alapati SB, Brantley WA, Iijima M, et al. Metallurgical characterization of a new wire (19, 20). In addition, it was revealed that raw CM wires have nickel-titanium wire for rotary endodontic instruments. J Endod 2009;35:1589–93. a large amount of R-phase at the oral temperature, which was not 11. Larsen CM, Watanabe I, Glickman GN, He J. Cyclic fatigue analysis of a new gener- mentioned in our previous study (18). This may have been due to ation of nickel titanium rotary instruments. J Endod 2009;35:401–3. the difficulty of distinguishing the diffraction peaks of R-phase located 12. Gao Y, Shotton V, Wilkinson K, Phillips G, Johnson WB. Effects of raw material and  rotational speed on the cyclic fatigue of ProFile Vortex rotary instruments. J Endod at nearly 42 from the austenite (37), as well as due to the interme- 2010;36:1205–9. diate processing method turning raw material into a final endodontic 13. Al-Hadlaq SM, Aljarbou FA, AlThumairy RI. Evaluation of cyclic flexural fatigue of M- instrument. Wire nickel-titanium rotary instruments. J Endod 2010;36:305–7. The flexural load-deflection response of CM and SE wires at 14. Ye J, Gao Y. Metallurgical characterization of M-wire nickel-titanium shape memory alloy used for endodontic rotary instruments during low-cycle fatigue. J Endod different temperatures provided further information regarding the 2012;38:105–7. mechanical properties and their potential application in clinical prac- 15. Gambarini G, Plotino G, Grande NM, Al-Sudani D, De Luca M, Testarelli L. Mechan- tice. The increase of critical stress with respect to testing temperature ical properties of nickel-titanium rotary instruments produced with a new complied with the Clausius-Clapeyron equation (38), which defines manufacturing technique. Int Endod J 2011;44:337–41. a linear relationship between the critical stress and critical temper- 16. Hou X, Yahata Y, Hayashi Y, Ebihara A, Hanawa T, Suda H. Phase transformation behaviour and bending property of twisted nickel-titanium endodontic instruments. ature for thermoelastic martensitic transformation. The linear coeffi- Int Endod J 2011;44:253–8. cient is a material constant. Furthermore, it is worth noting that 17. Kim HC, Yum J, Hur B, Cheung GS. Cyclic fatigue and fracture characteristics of the difference in diameter of the wires resulted in a different phase ground and twisted nickel-titanium rotary files. J Endod 2010;36:147–52. transformation temperature and, accordingly, a different mechanical 18. Shen Y, Zhou HM, Zheng YF, Campbell L, Peng B, Haapasalo M. Metallurgical char- acterization of controlled memory wire nickel-titanium rotary instruments. J Endod response. 2011;37:1566–71. 19. Shen Y, Qian W, Abtin H, Gao Y, Happasalo M. Fatigue testing of controlled memory wire nickel-titanium rotary instruments. J Endod 2011;37:997–1001. Conclusions 20. Shen Y, Qian W, Abtin H, Gao Y, Happasalo M. Effect of environment on fatigue of The results of the present study indicated that the raw CM wire controlled memory wire nickel-titanium rotary instruments. J Endod 2012;38: 376–80. was a kind of Ni-rich NiTi alloy that possessed a relatively high As and A compared with regular SE wire. The critical plateau stress and ulti- 21. Gao Y, Gutmann JL, Wilkinson K, Maxwell R, Ammon D. Evaluation of the impact of f raw materials on the fatigue and mechanical properties of ProFile Vortex rotary mate tensile strength of the CM wires were lower than they were for instruments. J Endod 2012;38:398–401. the SE wires, but the maximum strain before fracture of the CM wires 22. Tan G, Liu Y. Comparative study of deformation-induced martensite stabilisation via was more than 3 times higher than it was for the SE wires. The CM martensite reorientation and stress-induced martensitic transformation in NiTi. wires had no SE at room temperature or at 37C, whereas they ex- Intermetallics 2004;12:373–81.  23. Khier SE, Brantley WA, Fournelle RA. Bending properties of superelastic and non- hibited SE when heated to 60 C. The CM wires exhibited different superelastic nickel-titanium orthodontic wires. Am J Orthod Dentofacial Orthop phase transformation behavior and mechanical properties compared 1991;99:310–8. with SE wires, attributing to the special heat treatment history of CM 24. Wang QY, Zheng YF, Liu Y. Microstructure, martensitic transformation and super- wires. This study suggested greater flexibility of endodontic instru- elasticity of Ti49.6Ni45.1Cu5Cr0.3 shape memory alloy. Mater Lett 2011;65:74–7. ments manufactured with CM wires than similar instruments made 25. Nasser SN, Guo WG. Superelastic and cyclic response of NiTi SMA at various strain rates and temperatures. Mech Mater 2006;38:463–74. of conventional SE wires. 26. Liu Y, Galvin SP. Criteria for pseudoelasticity in near-equiatomic NiTi shape memory alloys. Acta Mater 1997;45:4431–9. 27. Pelton AR, Dicello J, Miyazaki S. Optimization of processing and properties of Acknowledgments medical-grade nitinol wire. 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31. Brantley WA, Svec TA, Iijima M, Powers JM, Grentzer TH. Differential scanning calo- 35. Wu SK, Lin HC, Chou TS. A study of electrical resistivity, internal friction and shear rimetric studies of nickel titanium rotary endodontic instruments. J Endod 2002;28: modulus on an aged Ti49Ni51 alloy. Acta Metall Mater 1990;38:95–102. 567–72. 36. Miyai K, Ebihara A, Hayashi Y, Doi H, Suda H, Yoneyama T. Influence of phase trans- 32. Kuhn G, Jordan L. Fatigue and mechanical properties of nickel-titanium endodontic formation on the torsional and bending properties of nickel-titanium rotary instruments. J Endod 2002;28:716–20. endodontic instruments. Int Endod J 2006;39:119–26. 33. Hayashi Y, Yoneyama T, Yahata Y, et al. Phase transformation behaviour and 37. Alapati SB, Brantley WA, Iijima M, et al. Micro-XRD and temperature-modulated DSC bending properties of hybrid nickel-titanium rotary endodontic instruments. Int En- investigation of nickel-titanium rotary endodontic instruments. Dent Mater 2009;25: dod J 2007;40:247–53. 1221–9. 34. Yahata Y, Yoneyama T, Hayashi Y, et al. Effect of heat treatment on transformation 38. Liu Y, Mahmud A, Kursawe F, Nam TH. Effect of pseudoelastic cycling on the Clau- temperatures and bending properties of nickel-titanium endodontic instruments. Int sius–Clapeyron relation for stress-induced martensitic transformation in NiTi. Endod J 2009;42:621–6. J Alloy Compd 2008;449:82–7.

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