This work presents the results of the microstructure observation of six different types of NiTi orthodontic wires by using Transmission Electron Microscopy (TEM). Within these analyses the chemical compositions of each wire were observed in different places by applying the EDS detector. Namely, the chemical composition in the orthodontic wires is very important because it shows the dependence between the phase temperatures and mechanical properties. Microstructure observations showed that orthodontic wires consist of nano-sized grains containing precipitates of Ti<sub>2</sub>Ni and/or TiC. The first precipitated Ti<sub>2</sub>Ni are rich in Ti, while the precipitated TiC is rich in C. Further investigation showed that there was a difference in average grain size in the NiTi matrix. The sizes of grains in orthodontic wires are in the range from approximately 50 to 160 nm and the sizes of precipitate are in the range from 0,3 μm to 5 μm

B. Buchmeister

B. Budič

B. Kosec

D. Jenko

F. Rojko

J. Ferčec

R. Rudolf

This work presents the results of the microstructure observation of six different types of NiTi orthodontic wires by using Transmission Electron Microscopy (TEM). Within these analyses the chemical compositions of each wire were observed in different places by applying the EDS detector. Namely, the chemical composition in the orthodontic wires is very important because it shows the dependence between the phase temperatures and mechanical properties. Microstructure observations showed that orthodontic wires consist of nano-sized grains containing precipitates of Ti2Ni and/or TiC. The first precipitated Ti2Ni are rich in Ti, while the precipitated TiC is rich in C. Further investigation showed that there was a difference in average grain size in the NiTi matrix. The sizes of grains in orthodontic wires are in the range from approximately 50 to 160 nm and the sizes of precipitate are in the range from 0,3 μm to 5 μm

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469METALURGIJA 53 (2014) 4, 469-472J. FERČEC, D. JENKO, B. BUCHMEISTER, F. ROJKO, B. BUDIČ, B. KOSEC, R. RUDOLFMICROSTRUCTURE OF NiTi ORTHODONTIC WIRES OBSERVATIONS USING TRANSMISSION ELECTRON MICROSCOPYReceived – Prispjelo: 2014-01-07Accepted – Prihvaćeno: 2014-05-25Original Scientific Paper – Izvorni znanstveni radISSN 0543-5846METABK 53(4) 469-472 (2014)UDC – UDK 669.14.018.298:669.18=111J. Ferčec, B. Buchmeister, University of Maribor, Faculty of Mechanical Engineering, Maribor, Slovenia; D. Jenko, Institute of Metals and Tech-nology, Ljubljana, Slovenia;  F. Rojko, University of Ljubljana, Faculty of Health Sciences, Ljubljana, Slovenia; B. Budič, National Chemistry Institute, Ljubljana, Slovenia; B. Kosec, University of Ljubljana, Facul-ty of Natural Sciences and Engineering, Ljubljana, Slovenia; R. Rudolf, University of Maribor, Faculty of Mechanical Engineering, Maribor, and Zlatarna Celje d.d., Celje, SloveniaThis work presents the results of the microstructure observation of six different types of NiTi orthodontic wires by using Transmission Electron Microscopy (TEM). Within these analyses the chemical compositions of each wire were observed in different places by applying the EDS detector. Namely, the chemical composition in the orthodontic wires is very important because it shows the dependence between the phase temperatures and mechanical properties. Micro-structure observations showed that orthodontic wires consist of nano-sized grains containing precipitates of Ti2Ni and/or TiC. The first precipitated Ti2Ni are rich in Ti, while the precipitated TiC is rich in C. Further investigation showed that there was a difference in average grain size in the NiTi matrix. The sizes of grains in orthodontic wires are in the range from approximately 50 to 160 nm and the sizes of precipitate are in the range from 0,3 μm to 5 μm. Key words: NiTi orthodontic wire, microstructure, TEM, EDS, average grain size INTRODUCTION The Shape Memory Alloys (SMA-s) nickel – titani-um (NiTi) are very useful materials in medical use be-cause they have good biocompatibility and functional properties [1]. In particular, this alloy is used in the field of orthodontics for orthodontic wire in orthodontic ap-pliances. The reasons for this are that they have the low-est force delivery (small modulus of elasticity) and they have a large recoverable strain for time continuous or-thodontic treatment (superelasticity) [2,3].The commercially available wires are in superelastic and shape memory forms [4]. The properties of SMA wires depend on the characteristics of the austenitic phase (body centred cubic structure) and martensite phase (monoclinic structure) [5]. For clinical conditions the austenite - finish (Af) temperature is important. This tem-perature in the NiTi SMA for orthodontic use is usually lower than the human body temperature (37 °C). The rea-son for this is that these alloys at temperatures above Af have characteristic behaviour called superelasticity, which means a material property that has a lot of recover-able elongation. A special feature of these alloys is the transformational plateau, where due stress caused a diffu-sion-less phase transformation from austenite to de-twinned martensite while, in SMA, it greatly increases the elongation at low changes of stress. In the case when such a wire is inserted into the oral environment and de-formed it is transformed from austenite to martensite. This means that the material caused the gentle and time continuous force which we want in orthodontic treat-ment, especially in the initial stage of treatment [6-8]. In SMA-s NiTi the temperatures of a phase have a great influence on the chemical composition of the transformation. Moreover, the mechanical properties are also strongly dependent on the phase temperatures [5]. For these purposes the characterisation of micro-structure for SMA NiTi was studied using Transmission Electron Microscopy (TEM) in many studies [9, 10]. However, there are not many studies that deal with the comparable TEM microstructural analysis of orthodon-tic wires which have similar chemical composition. The aim of this work was to observe the TEM micro-structures and to determine the chemical compositions of six commercially available SMA NiTi orthodontic wires in stress free conditions and on different regions within the volume. With this analysis we want to ob-serve the starting microstructure and it final influence on the functional behaviour of each orthodontic wire. MATERIALS AND METHODSWe investigated and analysed six commercially available orthodontic wires from SMA NiTi with a di-ameter 0,014 ‘’ (0,305 mm): 1. Damon Copper NiTi (Ormco, USA), 2. Thermo NiTi (Dentaurum, Germa-ny), 3. Sentalloy (GAC, Japan), 4. Truflex (Ortho Tech-nology, USA), 5. Truflex Thermal (Ortho Technology, USA), and 6. Rematitan (Dentaurum, Germany). Previously we analysed the phase transformation temperatures of each orthodontic wire [11] and results 470  METALURGIJA 53 (2014) 4, 469-472J. FERČEC et al.: MICROSTRUCTURE OF NITI ORTHODONTIC WIRES OBSERVATIONS USING TRANSMISSION...showed that all of the wires have a phase transformation temperature Af below the body temperature in a range from approximately 14 to 27 °C. For investigation we used (TEM) equipped with the Energy - Dispersive X-ray Spectroscopy (EDS) which enables the detection of chemical composition. Namely, TEM/EDS techniques are very useful tools for the quan-titative analysis of a sample on the nanometre scale [12,13]. TEM analyses were performed at room tempera-ture. From the general analysed phase temperatures it can be seen that the material at this temperature has, in most cases, an austenitic structure with minimum martensite structure due to temperature. For the determination of the average austenite grain size we used the standard ASTM E112 – 10 [14]. We measured the grain sizes which are related to the average diameter of each grain. Because it is impossible to calculate the diameter from each indi-vidual grain in whole volume in wire, we calculated these grain sizes from the surfaces respectively from cross-sections of wires. Next we used test methods which cov-ered the measurements including the comparison proce-dure, the planimetric or Jeffries procedure. In this proce-dure we inscribed a circle on a micrograph. We consid-ered that in the circle are around 50 grains. Then we counted the number of grains included completely within this area plus one half of the number of grains which were intersected by the circumference. From the number of grains we calculated the average gain area. Finally, we calculated the average grain diameter or grain size from the average gain area. The technique used for preparation of thin foils for TEM was argon ion-slicing with an Ion Slicer (Jeol EM-09100IS). The Ion Slicer thins the speci-men partly. It is possible to prepare cross-section or plan-view thin foil specimens. The instrument irradiates an argon ion beam on the specimen, which is masked par-tially with a shield belt. After argon ion-slicing the speci-mens were examined with TEM (Jeol JEM-2100, HR) at 200 kV using conventional TEM (CTEM), high-resolu-tion TEM (HRTEM), electron diffraction, and Energy Dispersive X-ray Spectroscopy (EDS).Specimens for TEM were cut out of a bulk material (orthodontic wires) in lengths of approximately 2,8 − 3,0 mm. The thickness (or diameter) of specimens was around 351 − 360 µm which was reduced further to around 86 − 160 µm with a Jeol Handy Lap using grind-ing papers SiC 1 200, 2 400, 4 000 and distilled water. Specimens were then mounted on an Ion Slicer speci-men holder, masked partially with a shield belt (bulk cross-section preparation) and thinned additionally with an argon ion beam. The slicing process started at the pressure of 10–5 Pa or 10–4 Pa, and alternated between the front and the backside of the specimen while it was rotating. The beam was tilted between 2,0, 2,5 or 3,5 °. Accelerating voltage 6 kV, argon gas flow rate between 6,8 and 7,3 (arbitrary units) and side change interval of 30 s were chosen. After a large thin area of the speci-mens up to 500 × 500 µm or more was obtained, a hole was generated in the thinnest region of the specimens. Polishing was used with a tilt angle of 0,8 or 1,5 °, at the accelerating voltage of 2 kV and side change interval of 30 s for 10 or 12 min. The total time of slicing was be-tween 4 h and 8 min and 10 h and 19 min. An example of a specimen before and after argon ion-slicing using an Ion Slicer is shown in Figure 1. RESULTS AND DISCUSSIONResults of microstructure observations for all ortho-dontic wires are shown in Figures 2(a) to 2(f), where the bright field (BF) images technique was used, respec-tively. From the surfaces of these wires the chemical com-positions of the matrix were detected by TEM/EDS. The results of the average value for chemical composi-tion of the matrix are presented in Table 1. The first orthodontic wire contained, besides nickel (Ni) and titanium (Ti), also copper (Cu). In all other or-thodontic wires the microstructure is enrich in Ni. It is very important to mention that the original bulk compo-sition of the orthodontic wires (through the whole vol-ume) was different. Namely, to achieve a Shape Memo-ry Alloy it is expected that alloys have a composition of Ni between 50 and 51 at.% [5]. The values for Ni in these wires vary in the range from 52 to 54,4 at.%. De-tectable change occurs due to the formation of precipi-Figure 1  Images of specimen of orthodontic SMA NiTi wires before (a) and after (b) Ar ion-slicing using an Ion Slicer471METALURGIJA 53 (2014) 4, 469-472J. FERČEC et al.: MICROSTRUCTURE OF NITI ORTHODONTIC WIRES OBSERVATIONS USING TRANSMISSION...tates Ti2Ni which finally resulted in the fact that SMA orthodontic wires became rich in Ni [15]. Detailed TEM examination of orthodontic wires at room temperature indicated the existence of an austenite phase. The BF in all Figures 2 show fine-scale austenite particles which are distributed almost homogeneously in the micro-structure. The grain sizes are different for each ortho-dontic wire and are written in Table 2 including the standard deviation of the individual measurements. Or-thodontic wire number 6 had the smallest grains at 49,6 nm, while orthodontic wire number 2 had the largest size of grains at 157,2 nm. Because the measurement of determination grain size is not exact, due to having a problem with the precise calculation within the deter-mined surface, we considered that the actual average grain sizes are representative. In Table 2 is presented also the calculated percent’s relative accuracy %RA of carried out measurements.Table 2  Grain sizes of austenite for SMA orthodontic NiTi wiresOrthodontic wireAverage grain size / nmStandard deviationRelative accuracy / %RA1 50,4 2,2 5,32 157,2 10,7 8,53 95,0 7,5 9,84 71,3 5,7 9,95 97,9 7,7 9,86 49,6 3,0 7,7The general rule of RA is that the value in the range of 10 %RA or lower is considered to be acceptable pre-cision for most purposes [14]. With our calculation we obtained RA values less than 10 % (from 5,3 to 9,9 % RA) which showed the adequateness of this method. The reduction of grain size improves the pseudoelastic-ity and facilitates the orientation of neighbour grains and its stiffness is related to the number of martensite plates which are produced [16]. Figures 3(a) to 3(f) show the TEM images of Ti2Ni and TiC particles which precipitate in different positions within the volume of the orthodontic wire. The chemical compositions of these particles, in-cluding average values of the presenting chemical ele-ments, are shown in Table 3. The particles of Ti2Ni are rich with Ti - more than 64 at.% in all orthodontic wires. In Figures 3(c) and 3(e) we can see particles of titanium carbide (TiC) which contain a different amount of carbon (C). In orthodontic wire 3 the C content 73 at.% represents a maximum value, while in orthodontic wire 5 the content of C 53 at.% is minimal. In Table 4 are shown the results of particles size analysis with their extreme values: the minimal and maximum sizes. These particles are approximately from 0,3 to 5 µm in size. The size of TiC precipitations are in the range 0,2 Figure 2  TEM bright - field (BF) micrograph images of different orthodontic wires from SMA NiTi showing small austenite particles in the matrixTable 1  Results of chemical composition for SMA orthodontic NiTi wires by TEM-EDS / %Orthodontic wireChemical elementMatrixAverage Standard deviation1 Ni 46,9 0,667Ti 47,8 0,777Cu 5,3 0,4692 Ni 53,5 1,580Ti 46,5 1,5803 Ni 52,2 0,335Ti 47,8 0,3354 Ni 53,3 1,040Ti 46,7 1,0405 Ni 53,0 0,689Ti 47,0 0,6896 Ni 54,4 2,712Ti 45,6 2,712Figure 3  TEM bright - field (BF) micrographs showing precipitates in SMA orthodontic NiTi wires472  METALURGIJA 53 (2014) 4, 469-472J. FERČEC et al.: MICROSTRUCTURE OF NITI ORTHODONTIC WIRES OBSERVATIONS USING TRANSMISSION...to approximately 2 µm. The TiC particles resulted from the use of a graphite crucible during the casting and melt-ing of NiTi SMA alloys. Frenzel reported in literature [17,18] the thermodynamic and kinetics of TiC formation due to the reaction between molten NiTi and the graphite crucible and/or mould. According to literature and other findings [19], these particles did not have any role in in-fluencing the mechanical or superelastic properties. In reference [20] it is reported that with the addition of graphite the martensite transformation temperature de-creased noticeably.Table 4 Minimal and maximal size of precipitatesOrthodontic wire Precipitates dmin / μm dmax /μm1 Ti2Ni 0,7 4,92 Ti2Ni 1,7 2,73 TiC 0,2 1,64 Ti2Ni 0,6 0,95 TiC 0,3 1,96 Ti2Ni 0,3 3,3In the present TEM/EDS study we have provided determination of the chemical composition and obser-vation of NiTi orthodontic wire’s microstructure. We found out that, in orthodontic wires, different types of precipitation could be connected to the casting and melting of the NiTi SMA alloys. We observed the parti-cles of Ti2Ni and TiC, which are larger than the grain of a matrix consisting of an austenite phase. In the micro-structure no other particles were observed which could be Ni2Ti and Ni3Ti according to the literature [20].CONCLUSIONS Under the conditions of this study we can draw the three main following conclusions:•  NiTi alloy for orthodontic wire is composed of an austenitic phase and Ti2Ni and/or TiC particles.•  The particles sizes are in the range from 0,2 to ap-proximately 5 μm. •  The austenitic phase represents some kind of ma-trix and is crystallized fully with grain sizes from approximately 50 to 160 nm.REFERENCES[1] T. W. Duering, K. N. Melton, D. Stoeckl, C. M. Wayman. Engineering Aspects of Shape Memory Alloys, Butter-worth – Heinemann, London, 1990.[2]   R. Rudolf, J. Ferčec. Military Technical Courier, 61 (2013) 2, 105-122.[3] J. Ferčec, B. Glišić, I. Šćepan, E. Marković, D. Stamenković, I. Anžel, J. Flašker, R. Rudolf. Acta Physica Polonica A, 122 (2012), 659-665.[4] A. W. Brantley, G. Wenhua, A.T. William Clark, I. Masahi-ro. Dental Materials, 24 (2008), 204–210. [5] D. C. Lagoudas. Shape Memory Alloys: Modeling and En-gineering Applications. Springer, Berlin, 2010.[6] W. R. Proffit, H. W. Fields, D.M. Sarver. Contemporary Orthodontics. Mosby, St Louis, 2007.[7] A. A. Shirazdi, H. Abreu, L. Pocock, D. Klobčar, P.J. Wi-thers, H.K.D.H. Bkadeshia. International Journal of Mate-rials Research, 100 (2009) 1, 40 – 45.[8]  J. Ferčec, I. Anžel, R. Rudolf. Materials and Design, 55 (2014), 699-706.[9]   I. Budak, M. Soković, M. Barišić. Measurement, 44 (2011) 6, 1188-1200.[10] R. Delville, B. Malard, J. Pilch, P. Sittner, D. Schryvers. International Journal of Plasticity, 27 (2011) 2, 282–297.[11] J. Ferčec, R. Rudolf. IRT3000, 48 (2013), 26-29.[12] M. Nacucchi, M. Alvisi, D. Altamura, V. Pfister, M. Re, M. A. Signore, M. Vittori Antisari. Materials Science and En-gineering, 7 (2010) 1, 1 – 12.[13] S. Matera, E. Anelli. Microscopy Microanalysis Micro-structures, 6 (1995), 633-646. [14] ASTM E112 – 12. Standard Test Methods for Determining Average Grain Size: http://materials.mcmaster.ca/faculty/malakhov/4L04/ReferenceMaterials/ASTM%20stan-dards/ASTM%20E112-10%20Average%20Grain%20Size.pdf [15] K. N. Lin, S. K. Wu, L. M. Wu. Martensitic Materials Transactions, 50 (2009) 11, 2637-2642. [16]  B. Karpe, B. Kosec, A. Nagode, M. Bizjak. Journal of Mi-ning and Metallurgy, Section B: Metallurgy, 49B (2013) 1, 83-89.[17] J. Frenzel, Z. Zhang, K. Neuking, G. Eggeler. Journal of Alloys and Compounds, 385 (2004) 1-2, 214-223.[18] J. Frenzel, Z. Zhang, C. Somsen, K. Neuking, G. Eggeler, Acta Materialia, 55 (2007) 4, 1331-1341.[19] O. Mortagy, M. Farag. Effect of Heat Treatment On The Superelasticity And Hardness of NiTi. International Con-ference on Modeling and Optimization of Structures, Pro-cesses and Systems (ICMOSPTS’ 07), Durban, 2007.[20] F. Chen, Y.X. Tong, X.L. Lu, X. Wang, B. Tian, L. Li, Y.F. Zheng, C.Y. Chung. Materials Letters, 65 (2011), 1073-1075.Note:  The responsible translator for the English language is Shelagh Hedges, Faculty of Mechanical Engineering, University of Mari-bor, SloveniaTable 3  Analysis of chemical composition on precipitates in SMA orthodontic NiTi wires by TEM-EDS / %Orthodontic wireChemical elementPrecipitatesAverage Standard deviationPrecipitate1 Ni 31,9 0,222 Ti2NiTi 64,8 0,288Cu 3,3 0,0992 Ni 35,4 0,644 Ti2NiTi 64,6 0,6443 Ni 0,4 0,065 TiCTi 26,4 6,474C 73,2 6,4924 Ni 35,3 0,37 Ti2NiTi 64,7 0,375 Ni 0,5 0,467 TiCTi 46,7 0,156C 52,8 0,3116 Ni 35,8 0,362 Ti2NiTi 64,2 0,362

Microstructure of NiTi orthodontic wires observations using transmission electron microscopy

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469 METALURGIJA 53 (2014) 4, 469-472J. FERČEC, D. JENKO, B. BUCHMEISTER, F. ROJKO, B. BUDIČ, B. KOSEC, R. RUDOLFMICROSTRUCTURE OF NiTi ORTHODONTIC WIRES OBSERVATIONS USING TRANSMISSION ELECTRON MICROSCOPYReceived – Prispjelo: 2014-01-07Accepted – Prihvaćeno: 2014-05-25Original Scientific Paper – Izvorni znanstveni radISSN 0543-5846METABK 53(4) 469-472 (2014)UDC – UDK 669.14.018.298:669.18=111J. Ferčec, B. Buchmeister, University of Maribor, Faculty of Mechanical Engineering, Maribor, Slovenia; D. Jenko, Institute of Metals and Tech-nology, Ljubljana, Slovenia;  F. Rojko, University of Ljubljana, Faculty of Health Sciences, Ljubljana, Slovenia; B. Budič, National Chemistry Institute, Ljubljana, Slovenia; B. Kosec, University of Ljubljana, Facul-ty of Natural Sciences and Engineering, Ljubljana, Slovenia; R. Rudolf, University of Maribor, Faculty of Mechanical Engineering, Maribor, and Zlatarna Celje d.d., Celje, SloveniaThis work presents the results of the microstructure observation of six different types of NiTi orthodontic wires by using Transmission Electron Microscopy (TEM). Within these analyses the chemical compositions of each wire were observed in different places by applying the EDS detector. Namely, the chemical composition in the orthodontic wires is very important because it shows the dependence between the phase temperatures and mechanical properties. Micro-structure observations showed that orthodontic wires consist of nano-sized grains containing precipitates of Ti2Ni and/or TiC. The first precipitated Ti2Ni are rich in Ti, while the precipitated TiC is rich in C. Further investigation showed that there was a difference in average grain size in the NiTi matrix. The sizes of grains in orthodontic wires are in the range from approximately 50 to 160 nm and the sizes of precipitate are in the range from 0,3 μm to 5 μm. Key words: NiTi orthodontic wire, microstructure, TEM, EDS, average grain size INTRODUCTION The Shape Memory Alloys (SMA-s) nickel – titani-um (NiTi) are very useful materials in medical use be-cause they have good biocompatibility and functional properties [1]. In particular, this alloy is used in the field of orthodontics for orthodontic wire in orthodontic ap-pliances. The reasons for this are that they have the low-est force delivery (small modulus of elasticity) and they have a large recoverable strain for time continuous or-thodontic treatment (superelasticity) [2,3].The commercially available wires are in superelastic and shape memory forms [4]. The properties of SMA wires depend on the characteristics of the austenitic phase (body centred cubic structure) and martensite phase (monoclinic structure) [5]. For clinical conditions the austenite - finish (Af) temperature is important. This tem-perature in the NiTi SMA for orthodontic use is usually lower than the human body temperature (37 °C). The rea-son for this is that these alloys at temperatures above Af have characteristic behaviour called superelasticity, which means a material property that has a lot of recover-able elongation. A special feature of these alloys is the transformational plateau, where due stress caused a diffu-sion-less phase transformation from austenite to de-twinned martensite while, in SMA, it greatly increases the elongation at low changes of stress. In the case when such a wire is inserted into the oral environment and de-formed it is transformed from austenite to martensite. This means that the material caused the gentle and time continuous force which we want in orthodontic treat-ment, especially in the initial stage of treatment [6-8]. In SMA-s NiTi the temperatures of a phase have a great influence on the chemical composition of the transformation. Moreover, the mechanical properties are also strongly dependent on the phase temperatures [5]. For these purposes the characterisation of micro-structure for SMA NiTi was studied using Transmission Electron Microscopy (TEM) in many studies [9, 10]. However, there are not many studies that deal with the comparable TEM microstructural analysis of orthodon-tic wires which have similar chemical composition. The aim of this work was to observe the TEM micro-structures and to determine the chemical compositions of six commercially available SMA NiTi orthodontic wires in stress free conditions and on different regions within the volume. With this analysis we want to ob-serve the starting microstructure and it final influence on the functional behaviour of each orthodontic wire. MATERIALS AND METHODSWe investigated and analysed six commercially available orthodontic wires from SMA NiTi with a di-ameter 0,014 ‘’ (0,305 mm): 1. Damon Copper NiTi (Ormco, USA), 2. Thermo NiTi (Dentaurum, Germa-ny), 3. Sentalloy (GAC, Japan), 4. Truflex (Ortho Tech-nology, USA), 5. Truflex Thermal (Ortho Technology, USA), and 6. Rematitan (Dentaurum, Germany). Previously we analysed the phase transformation temperatures of each orthodontic wire [11] and results 470   METALURGIJA 53 (2014) 4, 469-472J. FERČEC et al.: MICROSTRUCTURE OF NITI ORTHODONTIC WIRES OBSERVATIONS USING TRANSMISSION...showed that all of the wires have a phase transformation temperature Af below the body temperature in a range from approximately 14 to 27 °C. For investigation we used (TEM) equipped with the Energy - Dispersive X-ray Spectroscopy (EDS) which enables the detection of chemical composition. Namely, TEM/EDS techniques are very useful tools for the quan-titative analysis of a sample on the nanometre scale [12,13]. TEM analyses were performed at room tempera-ture. From the general analysed phase temperatures it can be seen that the material at this temperature has, in most cases, an austenitic structure with minimum martensite structure due to temperature. For the determination of the average austenite grain size we used the standard ASTM E112 – 10 [14]. We measured the grain sizes which are related to the average diameter of each grain. Because it is impossible to calculate the diameter from each indi-vidual grain in whole volume in wire, we calculated these grain sizes from the surfaces respectively from cross-sections of wires. Next we used test methods which cov-ered the measurements including the comparison proce-dure, the planimetric or Jeffries procedure. In this proce-dure we inscribed a circle on a micrograph. We consid-ered that in the circle are around 50 grains. Then we counted the number of grains included completely within this area plus one half of the number of grains which were intersected by the circumference. From the number of grains we calculated the average gain area. Finally, we calculated the average grain diameter or grain size from the average gain area. The technique used for preparation of thin foils for TEM was argon ion-slicing with an Ion Slicer (Jeol EM-09100IS). The Ion Slicer thins the speci-men partly. It is possible to prepare cross-section or plan-view thin foil specimens. The instrument irradiates an argon ion beam on the specimen, which is masked par-tially with a shield belt. After argon ion-slicing the speci-mens were examined with TEM (Jeol JEM-2100, HR) at 200 kV using conventional TEM (CTEM), high-resolu-tion TEM (HRTEM), electron diffraction, and Energy Dispersive X-ray Spectroscopy (EDS).Specimens for TEM were cut out of a bulk material (orthodontic wires) in lengths of approximately 2,8 − 3,0 mm. The thickness (or diameter) of specimens was around 351 − 360 µm which was reduced further to around 86 − 160 µm with a Jeol Handy Lap using grind-ing papers SiC 1 200, 2 400, 4 000 and distilled water. Specimens were then mounted on an Ion Slicer speci-men holder, masked partially with a shield belt (bulk cross-section preparation) and thinned additionally with an argon ion beam. The slicing process started at the pressure of 10–5 Pa or 10–4 Pa, and alternated between the front and the backside of the specimen while it was rotating. The beam was tilted between 2,0, 2,5 or 3,5 °. Accelerating voltage 6 kV, argon gas flow rate between 6,8 and 7,3 (arbitrary units) and side change interval of 30 s were chosen. After a large thin area of the speci-mens up to 500 × 500 µm or more was obtained, a hole was generated in the thinnest region of the specimens. Polishing was used with a tilt angle of 0,8 or 1,5 °, at the accelerating voltage of 2 kV and side change interval of 30 s for 10 or 12 min. The total time of slicing was be-tween 4 h and 8 min and 10 h and 19 min. An example of a specimen before and after argon ion-slicing using an Ion Slicer is shown in Figure 1. RESULTS AND DISCUSSIONResults of microstructure observations for all ortho-dontic wires are shown in Figures 2(a) to 2(f), where the bright field (BF) images technique was used, respec-tively. From the surfaces of these wires the chemical com-positions of the matrix were detected by TEM/EDS. The results of the average value for chemical composi-tion of the matrix are presented in Table 1. The first orthodontic wire contained, besides nickel (Ni) and titanium (Ti), also copper (Cu). In all other or-thodontic wires the microstructure is enrich in Ni. It is very important to mention that the original bulk compo-sition of the orthodontic wires (through the whole vol-ume) was different. Namely, to achieve a Shape Memo-ry Alloy it is expected that alloys have a composition of Ni between 50 and 51 at.% [5]. The values for Ni in these wires vary in the range from 52 to 54,4 at.%. De-tectable change occurs due to the formation of precipi-Figure 1   Images of specimen of orthodontic SMA NiTi wires before (a) and after (b) Ar ion-slicing using an Ion Slicer471 METALURGIJA 53 (2014) 4, 469-472J. FERČEC et al.: MICROSTRUCTURE OF NITI ORTHODONTIC WIRES OBSERVATIONS USING TRANSMISSION...tates Ti2Ni which finally resulted in the fact that SMA orthodontic wires became rich in Ni [15]. Detailed TEM examination of orthodontic wires at room temperature indicated the existence of an austenite phase. The BF in all Figures 2 show fine-scale austenite particles which are distributed almost homogeneously in the micro-structure. The grain sizes are different for each ortho-dontic wire and are written in Table 2 including the standard deviation of the individual measurements. Or-thodontic wire number 6 had the smallest grains at 49,6 nm, while orthodontic wire number 2 had the largest size of grains at 157,2 nm. Because the measurement of determination grain size is not exact, due to having a problem with the precise calculation within the deter-mined surface, we considered that the actual average grain sizes are representative. In Table 2 is presented also the calculated percent’s relative accuracy %RA of carried out measurements.Table 2   Grain sizes of austenite for SMA orthodontic NiTi wiresOrthodontic wireAverage grain size / nmStandard deviationRelative accuracy / %RA15 0 , 4 2 , 2 5 , 321 5 7 , 2 1 0 , 7 8 , 539 5 , 0 7 , 5 9 , 847 1 , 3 5 , 7 9 , 959 7 , 9 7 , 7 9 , 864 9 , 6 3 , 0 7 , 7The general rule of RA is that the value in the range of 10 %RA or lower is considered to be acceptable pre-cision for most purposes [14]. With our calculation we obtained RA values less than 10 % (from 5,3 to 9,9 % RA) which showed the adequateness of this method. The reduction of grain size improves the pseudoelastic-ity and facilitates the orientation of neighbour grains and its stiffness is related to the number of martensite plates which are produced [16]. Figures 3(a) to 3(f) show the TEM images of Ti2Ni and TiC particles which precipitate in different positions within the volume of the orthodontic wire. The chemical compositions of these particles, in-cluding average values of the presenting chemical ele-ments, are shown in Table 3. The particles of Ti2Ni are rich with Ti - more than 64 at.% in all orthodontic wires. In Figures 3(c) and 3(e) we can see particles of titanium carbide (TiC) which contain a different amount of carbon (C). In orthodontic wire 3 the C content 73 at.% represents a maximum value, while in orthodontic wire 5 the content of C 53 at.% is minimal. In Table 4 are shown the results of particles size analysis with their extreme values: the minimal and maximum sizes. These particles are approximately from 0,3 to 5 µm in size. The size of TiC precipitations are in the range 0,2 Figure 2   TEM bright - field (BF) micrograph images of different orthodontic wires from SMA NiTi showing small austenite particles in the matrixTable 1   Results of chemical composition for SMA orthodontic NiTi wires by TEM-EDS / %Orthodontic wireChemical elementMatrixAverage Standard deviation1N i 4 6 , 9 0 , 6 6 7Ti 47,8 0,777Cu 5,3 0,4692N i 5 3 , 5 1 , 5 8 0Ti 46,5 1,5803N i 5 2 , 2 0 , 3 3 5Ti 47,8 0,3354N i 5 3 , 3 1 , 0 4 0Ti 46,7 1,0405N i 5 3 , 0 0 , 6 8 9Ti 47,0 0,6896N i 5 4 , 4 2 , 7 1 2Ti 45,6 2,712Figure 3   TEM bright - field (BF) micrographs showing precipitates in SMA orthodontic NiTi wires472   METALURGIJA 53 (2014) 4, 469-472J. FERČEC et al.: MICROSTRUCTURE OF NITI ORTHODONTIC WIRES OBSERVATIONS USING TRANSMISSION...to approximately 2 µm. The TiC particles resulted from the use of a graphite crucible during the casting and melt-ing of NiTi SMA alloys. Frenzel reported in literature [17,18] the thermodynamic and kinetics of TiC formation due to the reaction between molten NiTi and the graphite crucible and/or mould. According to literature and other findings [19], these particles did not have any role in in-fluencing the mechanical or superelastic properties. In reference [20] it is reported that with the addition of graphite the martensite transformation temperature de-creased noticeably.Table 4 Minimal and maximal size of precipitatesOrthodontic wire Precipitates dmin / μm dmax /μm1T i 2Ni 0,7 4,92T i 2Ni 1,7 2,73T i C 0 , 2 1 , 64T i 2Ni 0,6 0,95T i C 0 , 3 1 , 96T i 2Ni 0,3 3,3In the present TEM/EDS study we have provided determination of the chemical composition and obser-vation of NiTi orthodontic wire’s microstructure. We found out that, in orthodontic wires, different types of precipitation could be connected to the casting and melting of the NiTi SMA alloys. We observed the parti-cles of Ti2Ni and TiC, which are larger than the grain of a matrix consisting of an austenite phase. In the micro-structure no other particles were observed which could be Ni2Ti and Ni3Ti according to the literature [20].CONCLUSIONS Under the conditions of this study we can draw the three main following conclusions:•   NiTi alloy for orthodontic wire is composed of an austenitic phase and Ti2Ni and/or TiC particles.•   The particles sizes are in the range from 0,2 to ap-proximately 5 μm. •   The austenitic phase represents some kind of ma-trix and is crystallized fully with grain sizes from approximately 50 to 160 nm.REFERENCES[1]  T. W. Duering, K. N. Melton, D. Stoeckl, C. M. Wayman. Engineering Aspects of Shape Memory Alloys, Butter-worth – Heinemann, London, 1990.[2]    R. Rudolf, J. Ferčec. Military Technical Courier, 61 (2013) 2, 105-122.[3]  J. Ferčec, B. Glišić, I. Šćepan, E. Marković, D. Stamenković, I. Anžel, J. Flašker, R. Rudolf. Acta Physica Polonica A, 122 (2012), 659-665.[4]  A. W. Brantley, G. Wenhua, A.T. William Clark, I. Masahi-ro. Dental Materials, 24 (2008), 204–210. [5]  D. C. Lagoudas. Shape Memory Alloys: Modeling and En-gineering Applications. Springer, Berlin, 2010.[6]  W. R. Proffit, H. W. Fields, D.M. Sarver. Contemporary Orthodontics. Mosby, St Louis, 2007.[7]  A. A. Shirazdi, H. Abreu, L. Pocock, D. Klobčar, P.J. Wi-thers, H.K.D.H. Bkadeshia. International Journal of Mate-rials Research, 100 (2009) 1, 40 – 45.[8]   J. Ferčec, I. Anžel, R. Rudolf. Materials and Design, 55 (2014), 699-706.[9]    I. Budak, M. Soković, M. Barišić. Measurement, 44 (2011) 6, 1188-1200.[10]  R. Delville, B. Malard, J. Pilch, P. Sittner, D. Schryvers. International Journal of Plasticity, 27 (2011) 2, 282–297.[11]  J. Ferčec, R. Rudolf. IRT3000, 48 (2013), 26-29.[12]  M. Nacucchi, M. Alvisi, D. Altamura, V. Pfister, M. Re, M. A. Signore, M. Vittori Antisari. Materials Science and En-gineering, 7 (2010) 1, 1 – 12.[13]  S. Matera, E. Anelli. Microscopy Microanalysis Micro-structures, 6 (1995), 633-646. [14]  ASTM E112 – 12. Standard Test Methods for Determining Average Grain Size: http://materials.mcmaster.ca/faculty/malakhov/4L04/ReferenceMaterials/ASTM%20stan-dards/ASTM%20E112-10%20Average%20Grain%20Size.pdf [15]  K. N. Lin, S. K. Wu, L. M. Wu. Martensitic Materials Transactions, 50 (2009) 11, 2637-2642. [16]   B. Karpe, B. Kosec, A. Nagode, M. Bizjak. Journal of Mi-ning and Metallurgy, Section B: Metallurgy, 49B (2013) 1, 83-89.[17]  J. Frenzel, Z. Zhang, K. Neuking, G. Eggeler. Journal of Alloys and Compounds, 385 (2004) 1-2, 214-223.[18]  J. Frenzel, Z. Zhang, C. Somsen, K. Neuking, G. Eggeler, Acta Materialia, 55 (2007) 4, 1331-1341.[19]  O. Mortagy, M. Farag. Effect of Heat Treatment On The Superelasticity And Hardness of NiTi. International Con-ference on Modeling and Optimization of Structures, Pro-cesses and Systems (ICMOSPTS’ 07), Durban, 2007.[20]  F. Chen, Y.X. Tong, X.L. Lu, X. Wang, B. Tian, L. Li, Y.F. Zheng, C.Y. Chung. Materials Letters, 65 (2011), 1073-1075.Note:   The responsible translator for the English language is Shelagh Hedges, Faculty of Mechanical Engineering, University of Mari-bor, SloveniaTable 3   Analysis of chemical composition on precipitates in SMA orthodontic NiTi wires by TEM-EDS / %Orthodontic wireChemical elementPrecipitatesAverage Standard deviationPrecipitate1 Ni 31,9 0,222 Ti2NiTi 64,8 0,288Cu 3,3 0,0992N i 3 5 , 4 0 , 6 4 4 T i 2NiTi 64,6 0,6443N i 0 , 4 0 , 0 6 5 T i CTi 26,4 6,474C7 3 , 2 6 , 4 9 24N i 3 5 , 3 0 , 3 7 T i 2NiTi 64,7 0,375N i 0 , 5 0 , 4 6 7 T i CTi 46,7 0,156C5 2 , 8 0 , 3 1 16N i 3 5 , 8 0 , 3 6 2 T i 2NiTi 64,2 0,362

Microstructure of NiTi orthodontic wires observations using transmission electron microscopy

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