Differential scanning calorimetry of superelastic Nitinol...

Original Article

Journal of Intelligent Material Systemsand Structures2016, Vol. 27(10) 1376–1387� The Author(s) 2015

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Differential scanning calorimetry ofsuperelastic Nitinol for tunable cymbaltransducers

Andrew Feeney and Margaret Lucas

AbstractRecent research has shown that estimations of the transformation temperatures of superelastic Nitinol using differentialscanning calorimetry can be inaccurate, in part, due to the residual stress in the material. Superelastic Nitinol is selectedas the end-cap material in a tunable cymbal transducer. The differential scanning calorimetry accuracy is initially probedby comparing transformation temperature measurements of cold-worked superelastic Nitinol with the same materialafter an annealing heat treatment, administered to relieve stresses from fabrication. The accuracy is further investigatedthrough a study of the vibration response of the cymbal transducer, using electrical impedance measurements and laserDoppler vibrometry to demonstrate that the change in resonant frequencies can be correlated with the transformationtemperatures of the Nitinol measured using differential scanning calorimetry. The results demonstrate that differentialscanning calorimetry must be used with caution for superelastic Nitinol, and that an annealing heat treatment can allowsubsequent use of differential scanning calorimetry to provide accurate transformation temperature data.

KeywordsDifferential scanning calorimetry, Nitinol, phase transformation, shape memory alloy, tunable frequency, ultrasonic cym-bal transducer

Introduction

There has been increasing interest in the ability to tunethe operating frequency of ultrasonic devices such asthose for surgical applications. Nitinol, a shape mem-ory alloy (SMA), which is a binary alloy of nickel andtitanium, offers the capability for frequency tuningthrough a small change in temperature. Nitinol hasrecently been used in tunable frequency cymbal trans-ducers (Feeney and Lucas, 2014; Meyer and Newnham,2000), where the elastic modulus of the Nitinol is con-trolled by a change in the temperature of the material.A common method of measuring the transformationtemperatures is by the use of differential scanningcalorimetry (DSC). However, it has been reported(Abel et al., 2004) that DSC results can exhibit inac-curacies, especially for superelastic Nitinol, althoughthe supporting experimental evidence is not extensive.A recent publication by the authors (Feeney and Lucas,2014) on the design and characterization of Nitinolcymbal transducers showed that significant inaccuraciesin the DSC characterization of a superelastic Nitinol(Johnson Matthey Noble Metals) occurred, whereasDSC characterization of a shape memory Nitinol(Memry GmbH) did not exhibit these inaccuracies.

It has been postulated that the condition of Nitinolfrom the fabrication process, for example, cold worked,affects the ability of the DSC technique to accuratelyestimate transformation temperatures (Eaton-Evanset al., 2008). In order to investigate this phenomenon,superelastic Nitinol is subjected to an annealing heattreatment to attempt to relieve stress. The DSC methodis then applied to the heat-treated Nitinol and a cymbaltransducer is fabricated with annealed Nitinol end-caps.The accuracy of the DSC estimations is then deter-mined by measuring the vibration response of the cym-bal transducer through the phase transitions associatedwith the tunability of this transducer.

The flextensional transducer based on the traditionalClass V configuration, called a cymbal transducer, wasfirst proposed in the 1990s (Zhang et al., 1999, 2000),the name being based on the cymbal shape of the end-caps. The cymbal transducer converts the small radial

School of Engineering, University of Glasgow, Glasgow, UK

Corresponding author:

Margaret Lucas, School of Engineering, University of Glasgow, Glasgow

G12 8QQ, UK.

Email: [email protected]

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displacement amplitude of the driver disk, which is nor-mally a piezoceramic such as lead zirconate titanate(PZT), into an amplified axial displacement (Zhanget al., 2001). The cymbal was developed in order toincrease the generative force and operational displace-ment from that of an earlier moonie transducer (Silvaet al., 1999). The cymbal transducer has been very suc-cessful in low- and medium-power applications such astransdermal drug delivery (Lee et al., 2004; Maioneet al., 2002; Smith et al., 2003) and underwater acoustictransduction arrays (Newnham et al., 2000, 2001,2002). The main limitation for adoption in high-powerapplications is that the epoxy resin used to form themechanical coupling is insufficient for the higher stres-ses associated with the higher power levels (Lin, 2010).Consequently, in recent years, there have been a num-ber of proposed modifications of the cymbal transducer(Bejarano et al., 2012, 2014; Lin, 2010), for example,for incorporation in an ultrasonic orthopedic surgicaldevice (Bejarano et al., 2014).

In addition to developments of the cymbal transdu-cer for operation at high power, there is an increasingdesire to be able to operate a single device at more thanone resonant frequency (Meyer and Newnham, 2000).A novel method of achieving this tunable characteristicis to use phase changing materials. For example, in the1990s, it was demonstrated that the natural frequenciesof composite fiberglass beams could be controlled usingembedded Nitinol wires (Baz et al., 1995). Morerecently, there have been a small number of studiesreporting the incorporation of Nitinol in a cymbaltransducer as the end-cap material (Feeney and Lucas,2014; Meyer and Newnham, 2000). These investiga-tions have principally focused on the tunable frequencycapability of the transducer in response to a thermalstimulus. An example of a cymbal transducer, based onthe classical design but with superelastic Nitinol end-caps, is shown in Figure 1.

In order to develop a range of tunable high-powerultrasonic devices that can incorporate a Nitinol cym-bal transducer, it is essential that the transformationtemperatures of the end-cap material are known. IfDSC is unreliable in this respect, then there is littleopportunity to progress toward predictable and reliabledevices.

Nitinol characteristics

Nitinol is a binary alloy of nickel and titanium, whosesmart properties were recognized by Buehler et al. inthe 1960s (Gil and Planell, 1998). It has become verypopular in a wide range of applications, most notablyin the biomedical industry where it has found success incardiovascular stents (Kumar and Lagoudas, 2008;Pelton et al., 2000). The microstructure of Nitinol trans-forms between cubic austenite at high temperature andmonoclinic martensite at low temperature (Thompson,2000). An intermediate transformation can occur,called the R-phase, which is characterized by the emer-gence of a rhombohedral phase in the material micro-structure. This phase is especially common in Nitinolalloys which are rich in nickel (Shaw et al., 2008). Thetransformation between phases can be generated bystress or temperature (Thompson, 2000). The Young’smodulus of Nitinol is phase dependent, with the modu-lus of martensitic Nitinol reported to be around 28–41GPa, reaching between 60 and 90GPa in the austeni-tic condition (Meyer and Newnham, 2000). It has beenreported that the relationship between Nitinol modulusand temperature is complex, principally because themodulus is known to be affected by the strain in thematerial (Duerig, 2006). Precise modulus changesresulting from heating or cooling of the Nitinol are alsovery difficult to predict because of the difficulty of accu-rately measuring the mechanical properties of the mate-rial. The phase transformation of the material inresponse to temperature change exhibits a high level ofhysteresis. Consequently, the temperatures at which thematerial transforms between microstructure phases aredissimilar between heating and cooling. As a result, thestart (S) and finish (F) temperatures of a phase transi-tion are normally defined with reference to the respec-tive microstructure phase: austenite (A), martensite(M), or R-phase (R). Until the finish temperature of aNitinol phase has been passed (at which point one ofthe phases is eliminated), the microstructure will con-tain a mixture of more than one phase.

There are two types of Nitinol which can be fabri-cated. The first is shape memory Nitinol, whose auste-nite finish transformation temperature, AF, is at asufficiently high temperature for the material to beheated in order to generate a different microstructurephase. The shape memory effect (SME) can hence beexhibited because the material is generally martensiticat the room temperature. If shape memory Nitinol isFigure 1. A superelastic Nitinol cymbal transducer.

Feeney and Lucas 1377



deformed, its original configuration can be recoveredby heating beyond the AF transformation temperature.The second type of alloy is superelastic Nitinol. Inorder to exhibit superelasticity, the Nitinol must existabove its AF transformation temperature, where verylarge deformation is recoverable. Under sufficientstress, the loaded regions of the austenitic microstruc-ture transform to martensite. Superelastic Nitinol isoften subjected to high levels of cold working in thefabrication process (Morgan, 1999), and residual stres-ses can therefore form within the material (Kuhn andJordan, 2002).

The transformation temperatures of Nitinol can betuned by changing the chemical composition of thematerial, through certain cold working procedures, orby the application of specific heat treatments (Shawet al., 2008). The effects of including different alloyconstituents in Nitinol have been documented in detail(Pelton et al., 2003), but the influences of cold workingand heat treatment are more difficult to quantify. Anumber of studies have investigated the aging and heattreatment of Nitinol, in terms of duration and tempera-ture, in order to identify the most effective ways toinfluence the transformation temperatures (Huang andLiu, 2001; Pelton et al., 2000; Xu and Wang, 2010). Inparticular, focus has been placed on the manipulationof the AF transformation temperature (Pelton et al.,2000). Since the end-caps used in this study have beenmechanically formed prior to heat treatment, the influ-ence of annealing on the transformation temperaturesof the end-caps may be more significant than the effectof annealing a flat disk, but this is very difficult todetermine in practice. The measurement of the trans-formation temperatures of Nitinol is commonly per-formed using DSC (Alapati et al., 2009; Alexandrouet al., 2006; Kus and Breczko, 2010).

Transformation temperature measurement

Adoption of SMAs, such as Nitinol, in products hasrelied on measurement techniques that can enable char-acterization of the transformation response. The phasetransformation of Nitinol affects multiple properties.For example, the elastic modulus, electrical resistivity,and magnetic susceptibility of Nitinol are all lower inthe martensite phase compared to the austenite phase(Duerig and Pelton, 1994). The transformation tem-peratures of Nitinol can be measured using techniquessuch as dynamic mechanical analysis (DMA).However, the most commonly used method of transfor-mation temperature measurement is DSC, which is athermoanalytical technique. One of the reasons for itspopularity over other techniques is its ability to obtainaccurate and comprehensive transformation tempera-ture and specific and latent heat data, whereas, forexample, it can be difficult to interpret the change inelectrical resistivity over a temperature range (Shaw

et al., 2008). The DSC method enables all the transfor-mation temperatures to be estimated, where the heatflow in a sample of the material is measured as a func-tion of temperature. An endothermic reaction corre-lates with a transformation to austenite, whereasexothermic behavior is exhibited for a transition to amartensitic microstructure (Kus and Breczko, 2010).Despite the advantages, inaccuracies in DSC measure-ments have been reported in a number of studies(Eaton-Evans et al., 2008; Feeney and Lucas, 2014;Obaisi, 2013) and are suggested to be as a result of resi-dual stresses which arise in the fabrication of Nitinol,for example, in the tailoring of the properties of supere-lastic Nitinol (Eaton-Evans et al., 2008). However, thesupporting experimental data relating to inaccuracy ofDSC data are insufficient to draw firm conclusions(Abel et al., 2004).

In order to enable the effective design of Nitinoldevices with a tunable frequency capability, it is neces-sary to determine the transformation behavior ofNitinol accurately from DSC analysis. Therefore, basedon these previous studies, it is clear that a better under-standing is needed of how the fabricated condition ofthe Nitinol is related to the reliability of DSC analysis.

DSC of Nitinol end-caps

Cymbal transducer end-caps are often formed fromsheet metal, usually using a punch to ensure repeatabil-ity in the geometrical dimensions. Since the transforma-tion temperatures of Nitinol are extremely sensitive toheat treatment, cold working, and chemical composi-tion, DSC analysis is generally conducted on samplescut from sacrificial fabricated end-caps to ensureconsistency.

The superelastic Nitinol end-caps (Johnson MattheyNoble Metals) used in this study are composed of55.99wt% nickel balanced with titanium. As a rule, asnickel content is increased, transformation tempera-tures decrease (Kumar and Lagoudas, 2008). Smallamounts of elements C, Co, Cu, Cr, Fe, Mn, Mo, Nb,O, and Si are also included, comprising less than 0.3%of the total composition, in order to tailor the transfor-mation characteristics such that the Nitinol is supere-lastic at the room temperature.

It has been reported that a heat treatment, such asaging, at a temperature of around 450 �C can result in anoticeable increase in the AF transformation tempera-ture (Pelton et al., 2003). An annealing heat treatmentat 450 �C was therefore selected in order to comparethe accuracy of DSC estimations of transformationtemperatures of cold-worked superelastic Nitinol cym-bal end-caps before and after annealing. Two end-capswere annealed at 450 �C for 1 h, and another two wereannealed at the same temperature for 2 h. Once theNitinol end-caps were annealed, they were subjected to

1378 Journal of Intelligent Material Systems and Structures 27(10)



water quenching to ensure the cooling process for theend-caps was consistent. Small samples cut from sacrifi-cial end-caps were also heat treated for use in DSCestimation.

A DSC (Perkin Elmer Diamond) calibrated withindium was used for the estimation of the transforma-tion temperatures. Two characterizations of the Nitinolphase transformations were conducted for two differentscan rates, 10 �C/min and 20 �C/min, because both theaccuracy and the sensitivity of DSC measurement dataare dependent on the scan rate. As the scan rate isincreased, a higher temperature gradient is applied, andconsequently, the heat flow within the material isgreater. This is exhibited in the DSC thermogram as amore significant and discernible transformation event,compared to using a lower scan rate. In contrast, thedata are less accurate if the scan rate is increasedbecause of the resultant thermal lag. Additionally,because the DSC analysis method employs a heatingand cooling cycle, and it is known that thermal cycling

of Nitinol can influence the transformation tempera-tures (Zarnetta et al., 2010), the DSC process shouldideally be conducted on a sample of Nitinol once only,to ensure a high reliability of the thermogram data.

Nitinol exhibits thermal hysteresis, meaning thetransformation behavior on the heating cycle is not thesame as on the cooling cycle. Transformation eventsbetween the different microstructural phases can beclearly identified from a DSC thermogram if an appro-priate scan rate and temperature range are selected. Thesubsequent estimation of the transformation tempera-tures is known to be liable to error (Shaw et al., 2008);however, in general, the steepest portion of the thermo-gram curve at each transformation event is extrapolatedand the associated transformation temperature is at theintersection with a defined baseline.

The DSC thermograms for both the untreated(Feeney and Lucas, 2014) and treated superelasticNitinol samples, cut directly from sacrificial end-caps,are shown in Figure 2, where the upper and lower

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Figure 2. DSC thermograms for untreated superelastic Nitinol (Feeney and Lucas, 2014) at (a) 10 �C/min and (b) 20 �C/min;annealed for 1 h at (c) 10 �C/min and (d) 20 �C/min; and for 2 h at (e) 10 �C/min and (f) 20 �C/min.




curves in each image correspond to the heating andcooling cycles, respectively.

The estimated transformation temperatures for eachannealed condition and measurement scan rate,extracted from the thermograms in Figure 2, are pro-vided in Table 1, where they can be compared withthose of the cold-worked superelastic Nitinol prior toannealing. The untreated Nitinol end-caps were fabri-cated from superelastic material and the DSC thermo-grams show that the AF transformation temperature isapproximately the room temperature. However, the AF

transformation temperature is estimated and so couldbe far lower. High resistance to deformation was exhib-ited by the untreated end-caps by physical inspectionindicating the presence of a stiff microstructure, whichis consistent with superelastic Nitinol. Transformationsto the martensite phase were not identified for any ofthe Nitinol samples in the temperature range of theDSC tests. A single phase change can be observed in allthe thermograms and, based on the relatively low ther-mal hysteresis between the transformation events in theheating and cooling cycles (Shaw et al., 2008), this sin-gle transformation is between the intermediate R-phase(R) and austenite (A). It can be assumed that the super-elastic Nitinol, for both annealed conditions, trans-forms to martensite at sub-zero temperatures, andtherefore, characterization of the transformation tem-peratures associated with this phase requires a tech-nique such as liquid nitrogen DSC.

It has been previously established that superelasticNitinol does not transform at the temperatures esti-mated from DSC (Eaton-Evans et al., 2008; Feeney andLucas, 2014), as shown in Table 1. However, it is clearthat after annealing, the DSC estimations of the trans-formation temperatures are very different from those ofthe untreated sample. Each heat treatment was con-ducted at the same temperature for the two durations,and the second hour of annealing has induced a furtherslight shift of the transformation temperatures. Thissuggests that transformation temperature change is notsignificantly dependent on the duration of the annealingprocess, a conclusion which is in broad agreement witha similar observation reported in the literature (Xu andWang, 2010). In addition, the results in Table 1 showthat there is no significant difference between the

transformation temperatures measured using a scanrate of 10 �C/min and those measured at 20 �C/min,although the increase in scan rate has resulted in a smallrise in heat flow in the samples annealed for 1 h. Thecompromise between sensitivity and accuracy is notseen to be critical for the two scan rates and data fromeither can provide transformation temperatureestimates.

Although it is known that DSC is inaccurate for thecharacterization of superelastic Nitinol, what cannot beconcluded solely from Table 1 is whether DSC becomesan accurate technique for the characterization of super-elastic Nitinol once it has been subjected to a heat treat-ment, although the transformation temperatures haveclearly changed significantly. The accuracy can be con-firmed, however, through dynamic characterization ofthe resonant response of cymbal transducers fabricatedwith end-caps of the untreated and annealed superelas-tic Nitinol.

Transducer fabrication

In a cymbal transducer, metal end-caps sandwich apiezoceramic disk, bonded by an epoxy resin. The pol-ing of the piezoceramic disk is in the thickness direc-tion. The 2-h annealed Nitinol was used to fabricateone of the superelastic Nitinol cymbal transducers,which could then be compared with an identical trans-ducer fabricated from the untreated superelasticNitinol. A hard PZT, Sonox P4 (CeramTec), was cho-sen as the piezoelectric driver. A thin layer of insulatingepoxy resin (Eccobond, Ellsworth Adhesives Ltd), at aratio of three parts resin to one part hardener, was usedas the bonding agent, with insulating epoxy usedbecause of the high bond strength compared to conduc-tive epoxy resins (Zhang et al., 2001). Small solder spotswere deposited on the top and bottom surfaces of thepiezoceramic disk to ensure electrical connectivity withthe electrodes. Once the transducer was assembled, itwas supported in a custom fabrication rig where theepoxy resin was allowed to cure for 24 h. Once the cur-ing process was complete, electrodes were formed onthe flange surface of each end-cap using a silver conduc-tive epoxy (Chemtronics CW2400). Soldering was notused in this case in order to avoid any heating effects on

Table 1. Transformation temperatures estimated from DSC analysis.

Nitinol condition Transformation temperature ( �C)

10 �C/min scan rate 20 �C/min scan rate

AF AS RF RS AF AS RF RS

Untreated (Feeney and Lucas, 2014) 23 16 9 19 25 15 7 19Annealed (1 h) 41 33 30 38 43 32 28 36Annealed (2 h) 43 38 33 40 43 37 32 38




the Nitinol and also because the passive titanium oxidelayer which forms on Nitinol makes effective solderingdifficult (Hall, 1993). The final geometrical dimensionsfor the transducer are summarized in Table 2, with eachtransducer end-cap dimension illustrated in the sche-matic in Figure 3.

Dynamic characterization of thetransducer

Dynamic characterization of the cymbal transducer wasused to identify the different resonant frequencies asso-ciated with operation in the R-phase and the austenitephase. The measurements can also be used to track theresonant frequency change of the transducer as a func-tion of temperature, and hence detect the temperatureat which the phase change occurs. In practice, this isnot a highly accurate process as the phase change, andhence resonant frequency change, of Nitinol tends tooccur too rapidly for a precise transformation tempera-ture to be extracted using this technique. However, auseful estimation of the transformation temperatures isachievable by dynamic characterization and it certainlyallows the results in Table 1 to be verified.

In the assembly of a cymbal transducer, difficulty inprecise control of the amount and location of depositedepoxy resin means there are small differences in thebond profiles between the piezoceramic disk and thetwo end-caps. This asymmetry can result in a doublepeak in the measured frequency response spectrum(Ochoa et al., 2002, 2006; Tressler and Newnham,1997), representing two fundamental modes of vibra-tion of the cymbal transducer. One vibration modeexhibits out-of-phase motion of the end-caps (the sym-metric mode) and the other exhibits in-phase motion ofthe end-caps (the asymmetric mode). The appearanceof the double peak in the response is very common in

cymbal transducers (Ochoa et al., 2006) and can alsobe caused by small differences in end-cap material orgeometry (Ochoa et al., 2002, 2006; Tressler andNewnham, 1997).

The dynamic characterization experiments weretherefore used to measure and track the resonant fre-quencies of both the symmetric and asymmetric modesthrough a temperature range that incorporated thephase transformation. The resonant frequency trackingrequired initial identification and measurement of thetwo mode shapes, followed by monitoring of the displa-cement amplitude of the transducer in each mode.

The cymbal transducer was characterized using threeexperimental techniques. The frequency response spec-trum was first measured using electrical impedanceanalysis (Agilent 4294A), to identify the double peakand the resonant frequencies associated with the sym-metric and asymmetric modes. Associating the tworesonant frequencies with their mode shape (symmetricor asymmetric) was then achieved using experimentalmodal analysis (EMA), by measuring frequencyresponse functions (FRFs) from a grid of measurementpoints on the surfaces of the cymbal transducers, withthe vibration response being measured by a three-dimensional (3D) laser Doppler vibrometer (LDV)(Polytec CLV). Both electrical impedance analysis andEMA require only a very low level of excitation of thetransducer, hence a very low input voltage across thepiezoceramic. A stress-induced phase transformation inNitinol occurs when sufficient stress is generated tocause a microstructure reorientation to martensite(Kumar and Lagoudas, 2008). There is evidence thatthis can be between 200 and 800MPa, depending onboth the temperature of the material and the loadingpath (Orgeas and Favier, 1998). However, electricalimpedance analysis and EMA excite vibration of theend-caps which results in a maximum stress

Cavity apex diameterCavity depth End−cap thickness

Cavity base diameter

End−cap diameter

Figure 3. Geometrical dimensions of the cymbal transducer end-cap.

Table 2. Cymbal transducer geometrical dimensions (all units in mm).

Cavity apex diameter Cavity base diameter Cavity depth End-cap thickness PZT thickness Transducer diameter

4.50 9.10 0.31 0.24 1.00 12.72

PZT: lead zirconate titanate.




approximately two orders of magnitude lower (finiteelement models of the cymbal transducer estimated amaximum stress of around 3MPa). The measured reso-nant frequency values are therefore principally associ-ated with a change in temperature. The finalcharacterization technique comprised excitation of thetransducer through bi-directional frequency sweepswhich allowed the displacement amplitude of the trans-ducer to be measured as a function of excitation fre-quency for a fixed input excitation voltage, with the bi-directional sweep repeated for five 2V increments from2 to 10V. The normal-to-surface displacement ampli-tude at the center of each end-cap face was measuredusing a one-dimensional (1D) LDV (Polytec), and aburst-sine excitation signal was used with two secondintervals between each burst to minimize heating dueto the excitation, which could particularly affect theexperiments conducted at the lower temperatures andhigher voltages.

The electrical impedance and displacement ampli-tude response measurements were conducted for twodifferent temperatures, with the transducers placed in atemperature-controlled chamber controllable up to80 �C. A lower temperature test was conducted where itwas assumed, from the DSC thermograms, that thetransducer end-caps would exist in the softer R-phase-dominated state, and a higher temperature test, whereit was assumed the transducer end-caps were in theaustenite-dominated state. The chamber temperature ineach experiment was continuously monitored using atype K thermocouple, and measurements were onlyrecorded once the transducer end-caps reached the tem-perature of the chamber, as verified by an infrared cam-era (FLIR T425). This infrared camera measurement isimportant because the transducer self-heats, whichcould influence the phase transformation. EMA wasnot conducted for the higher temperature test becauseaccess for 3D LDV measurements was not sufficient toacquire accurate FRFs in the chamber from the entiremeasurement grid on the end-cap surfaces.

Frequency response characterization throughimpedance analysis and EMA

The impedance–frequency spectra for the frequencyresponse characterization experiments are shown inFigure 4. The DSC thermograms of the superelasticNitinol annealed for 2 h showed start and finish transi-tions to austenite at 37 �C and 43 �C, respectively, andshowed the Nitinol to exist in the R-phase at the roomtemperature. Therefore, for the impedance characteri-zations, a chamber temperature higher than 43 �C wasrequired in order to ensure complete transformation toaustenite from R-phase, and therefore, the temperaturefor the lower temperature test was set at approximately20 �C and approximately 55 �C for the higher tempera-ture test.

It is evident from Figure 4 that the transducer exhi-bits the double peak in the response spectrum, associ-ated with the resonant frequencies of the symmetric andasymmetric modes. Also, a significant increase in reso-nant frequency is achieved for both modes of vibrationat the higher temperature. The increase in resonant fre-quency for each mode is shown in Table 3.

It was not possible to capture the precise tempera-ture required for the completion of this resonant fre-quency shift from impedance analysis, and henceestimate the transformation temperature, due to thefast rate of temperature increase. However, the obser-vations indicated a good correlation with the phasetransformation temperatures obtained from the DSCanalysis.

This is quite different to the findings from impe-dance analysis of the identical cymbal transducer fabri-cated with untreated superelastic Nitinol end-caps(Feeney and Lucas, 2014). In this case, the transducerwas initially operated around 25 �C where, according tothe DSC estimations, the Nitinol end-caps would be inthe austenite condition. However, the change in reso-nant frequency did not complete at 25 �C, but con-tinued to increase until a temperature of around

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Figure 4. Impedance–frequency response of the annealed superelastic Nitinol cymbal transducer at two different temperatures.




40 �C–45 �C was reached, indicative of the completionof the transition of the superelastic Nitinol end-caps toaustenite. It is therefore clear that there is a poor corre-lation between the AF transformation temperaturefound from the DSC analysis and the AF transforma-tion temperature estimated from monitoring the changein resonant frequency, for the untreated superelasticNitinol transducer.

One reason that superelastic Nitinol might not com-plete its transition to austenite until around 45 �C is thatthe microstructure contains both austenite and R-phaseconstituents. This can occur if the Nitinol is at a tem-perature above the AS transformation temperature, butbelow the AF transformation temperature, such thatnot all the R-phase has been eliminated. The fabricationprocess of the superelastic Nitinol end-caps includedcold working, which would influence the transforma-tion temperatures of the material, thus raising the AF

transformation temperature past the room temperature.On comparison of the resonant frequencies of

annealed superelastic Nitinol transducer, it is observedthat despite the two end-caps being identical, there is adifferent frequency change through the phase transfor-mation for the symmetric mode and the asymmetricmode. This difference has been similarly reported inanother study (Meyer et al., 2002), but for a change inload media (air to water) rather than a change in tem-perature. Eliminating the asymmetric mode, whose fre-quency change is difficult to predict, from thefrequency response, may be desirable in order to designand operate a reliable tunable cymbal transducer basedon control of one modal response. However, eliminat-ing asymmetries may not be realistic for the classicalcymbal configuration and a modified version, similarto some that have previously been proposed (Bejaranoet al., 2012, 2014; Lin, 2010), may be preferable.

In order to verify the modes of vibration, EMA wasconducted at the room temperature, measured to be21 �C, and the symmetric and asymmetric mode shapesare shown in Figure 5. Cool to warm colors indicatelow to high displacement amplitudes, respectively, inthe deformation plots.

The symmetric mode shape is characterized by thetwo end-caps moving out-of-phase with each other(Figure 5(a)), whereas the asymmetric mode shape ischaracterized by the two end-caps moving in-phase(Figure 5(b)). This is typical for cymbal transducerswith asymmetry and has implications for tunable trans-ducers since the largest amplitude can only be realizedon one side of the transducer. This again is an argu-ment for incorporating a modification from the classi-cal cymbal design that can reduce asymmetry. For eachmode of vibration, one end-cap is more responsive thanthe other. The resonant frequencies of the modes ofvibration of the transducer measured using EMA corre-late well with those found from the electrical impedancemeasurements in the lower temperature conditionshown in Table 3.

The results showing the increase in resonant fre-quency with temperature in Table 3 demonstrate thattransducer end-caps stiffen from the softer R-phasemicrostructure to the austenite phase, and that the reso-nant frequency of a superelastic cymbal transducer canbe tuned to more than one frequency, in the order ofkilohertz, through a temperature change, by influencingthe phase transformation properties of the material.The change in the resonant frequencies through a phasetransformation was monitored through the electricalimpedance analysis, and for both untreated and treatedNitinol transducers, there was no further increase inresonant frequency beyond a temperature of around40 �C–45 �C. This means that the AF transformation

Figure 5. Mode shapes of the annealed superelastic Nitinol cymbal transducer using EMA at 21 �C, showing the (a) symmetricmode at 15,029 Hz and (b) asymmetric mode at 20,519 Hz.

Table 3. Resonant frequencies (Hz) of the superelastic Nitinol cymbal transducers at two different temperatures.

Mode Resonant frequencies (Hz)

Untreated Nitinol transducer (Feeney and Lucas, 2014) Annealed Nitinol transducer

22 �C 44 �C Difference 20.6 �C 55.7 �C Difference

Symmetric 32,698 33,407 + 709 (+ 2.17%) 14,925 16,600 + 1675 (+ 11.22%)Asymmetric 23,495 28,137 + 4642 (+ 19.76%) 20,475 23,375 + 2900 (+ 14.16%)




temperature from impedance analysis frequency track-ing experiments correlates well with DSC estimationsfor the treated Nitinol transducer but not for theuntreated Nitinol transducer.

Displacement amplitude response measurements

The displacement amplitude through a frequency rangeincorporating resonance, of both the symmetric andasymmetric modes in each microstructure phase, wasmeasured, as shown in Figure 6. The temperaturewithin the test chamber was monitored using a type Kthermocouple, and the temperature was recoded oncethe transducer had been allowed to thermally equili-brate, to ensure the desired Nitinol microstructurephase existed.

The resonant frequencies of the transducer weretracked using the LDV system, where it was found thatthe resonant frequencies of the transducer increasedabove the room temperature, past 40 �C. Hence, thedisplacement amplitude measurements were recordedaround 52 �C to ensure that the austenite phase wasgenerated in the microstructure of the Nitinol.

The frequencies of the resonance peaks shown inFigure 6 correlate closely with those found by electricalimpedance measurement and EMA. Additionally, byplotting the response through resonance using a bi-directional frequency sweep, the nonlinearity in the

vibration responses is clearly exhibited. Nonlinear soft-ening has been reported to occur in piezoelectric trans-ducers due to driving the transducer above somethreshold amplitude of vibration (Umeda et al., 2000).There is evidence of nonlinear softening in both Figure6(a) and (c), which are the vibration responses of thetransducer with end-caps in the R-phase, exhibited as adecrease in frequency for increased input voltage.However, for both modes in the high-temperature aus-tenite condition, nonlinear hardening is exhibited at thelower input voltage levels, while a more linear responseis measured at the higher input voltage levels, indicat-ing that the end-caps have a hardening characteristic atlow excitation that softens with increasing excitationlevel.

There are similarities in the vibration response ofthis transducer with that of the untreated superelasticNitinol cymbal transducer. The first is that the vibra-tion response of both modes exhibits an increase in dis-placement amplitude on transition to a higher modulusend-cap material phase, and this result is consistent forthe heat-treated and -untreated transducers. Thisappears to be counter-intuitive, with lower amplitudenormally expected for a stiffer material phase. It is veryprobable that the higher displacement amplitude can,at least in part, be attributed to the difference in damp-ing capacity between austenitic and R-phase (or mar-tensitic) Nitinol. It is known that martensitic Nitinol

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Figure 6. Displacement amplitude response of the annealed superelastic Nitinol cymbal transducer, for the symmetric mode at(a) 22 �C and (b) 52 �C, and the asymmetric mode at (c) 22 �C and (d) 53 �C.




exhibits higher damping capacity compared to austeni-tic Nitinol (Piedboeuf et al., 1998); hence, there is agreater energy dissipation in the lesser-ordered marten-sitic or R-phase states than in the austenitic state. Fora superelastic Nitinol transducer, this means that alarger input is required in the R-phase condition inorder to achieve comparable displacement amplitudesfor both phases of Nitinol.

Modulations in the displacement amplitude for eachinput voltage level in the austenite condition are pres-ent in the measurements for both modes of vibration.This characteristic can be attributed to the higher tem-perature of the transducer affecting the material prop-erties of the epoxy resin bond layers (Feeney andLucas, 2014). The flexural and storage moduli and theflexural strength are affected at higher temperatures(Harismendy et al., 1997), and this is a weakness of thistransducer configuration for this tunable frequencyapplication. However, alternative configurations existthat remove the epoxy layer and replace it with amechanical coupling (Bejarano et al., 2014), and thesecan be investigated for future tunable cymbaltransducers.

Superelastic Nitinol shows promise, when integratedin a cymbal transducer, for the design of ultrasonicdevices that can be tuned to operate in the same modeat two distinct resonant frequencies via a phase trans-formation through a relatively small controlled tem-perature increase. The reliance on DSC for estimatingthe phase transformation temperatures has been shownto be inaccurate for superelastic Nitinol. However, if asacrificial sample of the Nitinol is first subjected to anannealing heat treatment prior to DSC measurement,then the DSC estimates of the phase transformationtemperatures become accurate and can be reliably usedto understand how to control the temperature of thecymbal transducer to effect the excitation of the tun-able resonant frequencies.

Conclusion

This investigation has demonstrated that by subjectinguntreated superelastic Nitinol end-caps to a post-fabrication annealing heat treatment, thermoanalyticalanalysis by DSC can be adopted in order to more accu-rately estimate the transformation temperatures of thematerial. The accuracy of the DSC method wasassessed using the data obtained from electrical impe-dance and laser Doppler vibrometry measurement tech-niques on a cymbal transducer fabricated using thematerial. EMA was used to determine the mode shapesand the associated resonant frequencies of the transdu-cer with end-caps in the R-phase condition. Thehigh level of correlation between the electrical impe-dance measurement results and those from the displace-ment amplitude analysis constitutes a reliable and

comprehensive dynamic characterization. The resonantfrequencies of the modes were found to increase at atemperature which was in close accordance with the AF

transformation temperature estimated using the DSCmethod. However, this did not occur for the cymbaltransducer with superelastic Nitinol end-caps whichwere not subjected to the post-fabrication annealingheat treatment. It is very likely that the annealing pro-cess has relieved the residual stress in the end-caps,which contributed to the inaccuracy of the DSC ther-mogram data. The physical condition of the Nitinolmust be taken into account when considering the accu-racy of the DSC thermogram data. Coupling dynamiccharacterization measurements with DSC enables theeffects of temperature on the Nitinol material transfor-mation to be accurately and reliably determined. Thisis important for the future design of tunable cymbaltransducers using superelastic Nitinol.

Acknowledgements

The authors would like to acknowledge the EPSRCEngineering Instrument Loan Pool, which provided the DSC.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest withrespect to the research, authorship, and/or publication of thisarticle.

Funding

The author(s) disclosed receipt of the following financial sup-port for the research, authorship, and/or publication of thisarticle: This work was funded by the Engineering and PhysicalSciences Research Council (EPSRC) grant EP/P505534/1.

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