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DOI: 10.1002/adem.201300180

Evolution of Recovery Stress and Recovery Strain inAnnealed NiTi Thin Wire during Constrained ThermalCycling to High Temperature**

By Xiaojun Yan* and Jan Van Humbeeck

NiTi shape memory alloys (SMAs) have attracted greatattention in the past decades due to their novel performances,

1. Experimental

such as shape memory effect (SME),[1–3] pseudoelasticity,[4–6]

and high damping.[7,8] These functional properties are relatedto the reversible martensitic transformation.[9–12] The SMErefers to the ability of a SMA to undergo deformation at onetemperature, and then recover its original, undeformed shapeupon heating above its reverse transformation temperature.When this shape recovery is prevented by some externalconstraint, a recovery stress is generated. This unique propertymakes SMAs ideally suited for use as fasteners, seals,connectors, clamps, and composites.[1] The reliability andstability of the functional properties of these components aredirectly related to the magnitude and evolution of the recoverystress. The magnitude of the recovery stress can be influencedby many factors, such as thermo-mechanical treatments,[13–15]

annealing,[16,17] pre-strain,[18–20] matrix.[21–25] Depending onthe way of NiTi SMA is designed to operate, repeated heatingand cooling, long-term periods of high temperature exposure(isothermal holding) are common in some applications, e.g.different types of actuators and smart hybrid composites.These periods can last from several minutes to severalhours at temperatures significantly higher than the austenitefinish temperature. Therefore, it is desired to characterizethe evolution of the recovery stresses during such thermalcycling.

Up to now, a limited number of articles study this issue.[18,26]

In addition, those articles deal with the evolution of therecovery stress to a maximum temperature of 150 °C. Inthe present study, we investigate the influence of constrainedthermal cycling to a higher temperature (200 °C) on therecovery stress evolution of annealed NiTi thin wire.

[*] Dr. X. YanSchool of Mechanical Engineering, Dalian University, Dalian116622, ChinaE-mail: dreamto2008@sohu.comProf. J. V. HumbeeckDepartment of Metallurgy and Materials Engineering, Katho-lieke Universiteit Leuven, Kasteelpark Arenberg 44 bus 2450B-3001, Heverlee, Belgium

[**] The authors gratefully acknowledge the financial support fromFWO-project G-0652.05.

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The material used was a commercial NiTi wire purchased atSAES Getters (Italy) with a diameter of 0.076mm and a normalcomposition of 50.2 at% Ni. As-received, wires (35% cold-worked) with a length of 100mm were annealed in an argonatmosphere for 10min at 450 °C. The transformation temper-atures of the annealed specimen were Rs¼ 62 °C, Rf¼ 58 °C,Ms¼ 46 °C,Mf¼ 37 °C, As¼ 78 °C, andAf¼ 88 °C, as measuredby differential scanning calorimetry (DSC TA-Q2000).

All thermomechanical experiments were performed on adynamic mechanical analyzer (DMA) instrument (TA Q800).All samples were dipped in liquid nitrogen to ensure a fullmartensite state prior to tests. The thermomechanical loadingconsisted of:

1.

aA

Loading the sample under uniaxial tensile stress at 25 °Cup to 6% pre-strain at a strain rate of 0.5%min�1.

2.

Unloading the sample until a stress equal to 5MPa(the small stress is to keep the sample straight).

3.

Fixing the clamps after unloading and heating the sampleunder constrained condition up to 200 °C at a rate of3 °Cmin�1. Keeping the temperature at 200 °C for 60minfollowed by

4.

Cooling down to 25 °C at a rate of 3 °Cmin�1. 5. Repeating (3) and (4) for six times.

Our previous studies have shown that the 450 °C annealedsample possesses a maximum plateau strain of about 5%and the sample is in the reoriented martensite state whendeformed to 6% pre-strain. It also exhibits a high recoverystress, which steadily increases and reaches maximum value ataround 200 °C and then decreases for higher temperaturesduring heating to 300 °C.[27] Therefore, the as-receivedmaterialwas annealed at 450 °C, deformed to 6% pre-strain, and heatedto 200 °C during recovery stress measurement.

After each thermal cycling, the free shape recovery behavior,transformation behavior, transformation strain, and residualstrain of the sample were measured. The free shape recoverybehavior of the sample was measured by heating to 180 °Cand cooling down to 25 °C at a constant stress of 5MPa witha heating/cooling rate of 3 °Cmin�1. The transformationbehavior of the sample was characterized by means ofDSC. The DSC measurement was started with heating fromroom temperature. The tensile test was performed at roomtemperature at a strain rate of 0.5%min�1.

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Fig. 1. (a) Representative stress–temperature curves during thermal cycling, (b) Stress–time curves during isothermal holding at 200 °C.

Fig. 2. (a) Variation of recovery stress obtained from Figure 1 (b) at the end of isothermalholding as a function of the number of thermal cycles; (b) Recovery stress rate (ds/dT) atdifferent temperature ranges as a function of the number of thermal cycles: the uppercurve measured in the temperature range from As to Asþ 30 °C, and the lower curvemeasured in the temperature range from Asþ 30 to 200 °C.

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2. Results

2.1. Recovery StressFigure 1a and b shows the evolution of recovery stress

during heating/cooling and isothermal holding, respectively.Only the first, third, and seventh thermal cycles are shown inFigure 1a for more clarity. In general, the recovery stressincreases with temperature upon heating and decreases withtemperature upon cooling. For the first heating, the recoverystress increases slowly below 100 °C and increases drasticallyabove it. During the first isothermal holding at 200 °C, therecovery stress increases from the beginning and reaches amaximum value around 15min and then decreases slightlywith increasing time. During heating in the following thermalcycling, the recovery stress is zero at the beginning of theheating processing and starts to increase quickly between 70and 80 °C, which is almost the As temperature measured byDSC. During the further isothermal holding, the recoverystress slightly increases with increasing time, as shown inFigure 1b. It is also noted that at the very beginning of theisothermal holding, the temperature is not stable and has asmall oscillation.

Figure 2a shows the variation of recovery stress at the endof isothermal holding at 200 °C as a function of the number of

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thermal cycles. It is seen that the stress value decreasessignificantly after the first cycle and then gradually decreaseswith increasing number of thermal cycles.

The stress rate (ds/dT), which was determined bycalculating the tangent of the stress versus temperature curveshown in Figure 1a, is also a function of the number of thermalcycles. Figure 2b shows the stress rate ds/dT at differenttemperature ranges as a function of the number of thermalcycles. The stress rate value represents how fast the recoverystress builds up upon heating. It is seen that the stress ratevalue decreases gradually with increasing number of thermalcycles at the temperature range of Asþ 30 to 200 °C. It has amore complex behavior at the beginning of the stress build-up.

2.2. Free Shape Recovery Measurements after Each ThermalCycling

Figure 3a shows the representative strain–temperaturecurves during free shape recovery measurement after differentnumbers of thermal cycles. The 6% pre-strained samplewithout thermal cycling exhibits one-stage transformationupon heating. However, two-stage transformations are

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Fig. 3. (a) Representative strain–temperature curves measured during free shaperecovery during cycling; (b) Transformation and residual strains as a function of thenumber of thermal cycles; (c) Transformation temperatures as a function of the number ofthermal cycles.

Fig. 4. Representative DSC heating curves of different samples.

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observed for the sampleswith constrained thermal cycling, onestarts at around 80 °C, the other at much higher temperatures.It is seen that the sample without thermal cycling has thehigher transformation strain than others. There is a plateaubetween the two-stage decline and the residual strain is aslarge as 3.5%. The transformation strain decreases significantlyafter the first cycle and gradually decreases further withincreasing number of thermal cycles, as shown in Figure 3b.

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Figure 3c shows the transformation temperatures as afunction of the number of thermal cycles. The first transfor-mation temperature As1 decreases slightly between the firstand fifth cycles. The value has a little bit increase after the sixthcycle and then decreases again. The second transformationtemperature As2 increases gradually between the first fivecycles and then increases very slightly after five cycles.

2.3. Transformation Behaviors of the Samples after EachThermal Cycling

Figure 4 shows the representative DSC heating curves of as-annealed and pre-strained samples. It is seen that only a singlereverse transformation occurs for all samples. The pre-strainedsample without thermal cycling has a higher transformationtemperature than others. After thermal cycling, the firsttransformation appearing in Figure 3a was detected by DSCand the transformation temperature is a little lower than in theas-annealed sample and decreases slightly with increasingnumber of thermal cycles. DSCmeasurement cannot detect thesecond transformation shown in Figure 3a.

2.4. Tensile Tests after Each Thermal CyclingFigure 5a shows the representative stress–strain curves of

pre-strained samples measured at 25 °C after various thermalcycles. It is seen that the tensile strength changes slightly withincreasing number of thermal cycles. The elongation increasesafter the first cycle and changes slightly with further cycles. It isseen that the elastic modulus has a strong decrease after thefirst cycle and remains averagely constant.

3. Discussion

As is apparent from the experimental results, the recoverystress of a pre-strained NiTi wire under constrained thermalcycling exhibits an abrupt decrease after the first thermalcycling and gradually reduces with increasing the number ofthermal cycles. The degradation of the recovery stress could berelated to the decrease of transformation strain and increase of

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Fig. 5. (a) Representative stress–strain curves of pre-strained samples after differentnumbers of thermal cycles; (b) the variation of elastic modulus as a function of thenumber of thermal cycles.

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residual strain, which is caused by self-tension duringconstrained thermal cycling.

The sample was dipped in liquid nitrogen to ensure a fullmartensite state prior to tests. Thermal martensite normallyexists as self-accommodating variants. When the sample wasdeformed in tension to 6% pre-strain, which is beyond theplateau strain, as shown in Figure 5a, the thermal martensitetransforms into the oriented martensite by a martensitereorientation process. Upon heating with a fixed strain afterunloading, once the temperature exceeds As temperature,transformation from the oriented martensite to austenitecommences and the recovery stress is generated. ComparingFigure 2a with Figure 5a, one can see that the recovery stresscan easily exceed the plateau stress, so it is reasonable toassume that the remaining oriented martensite can further re-orientate and gets plastically deformed when the recoverystress exceeds the critical stress. The effect of this self-tensionprocess increases at higher temperature because the recoverystress increases with increasing temperature. Self-tension willlead to a larger strain level in the remaining orientedmartensite, which causes the increase of the residual strainand the decrease of transformation strain during free shaperecovery measurement.

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Upon cooling, the amount of the remaining orientedmartensite remains invariable. When cooling below Ms, newmartensite is generated from the austenite and the elastic strainenergy restores in the martensite. It is noted that the formationof new martensite is under the influence of recovery stress,which is gradually weakened upon cooling. Therefore, a partof the new martensite is deformed and becomes orientedmartensite when the recovery stress is high. Pure thermalmartensite forms when the recovery stress decreases to a lowerlevel. After one thermal cycling, there exist three differentmartensites, i.e. pure thermal martensite, new-orientedmartensite, and remaining oriented martensite.

The above-discussed process repeats during constrainedthermal cycling. As a result, plastic deformation accumulatesin the sample with increasing the number of thermal cycles.Accumulated plastic strain is believed to cause the decrease oftransformation strain and the increase of residual strain as wellas the degradation of recovery stress. The self-tension degree ofthe remaining martensite is higher upon the first heating anddecreases with further thermal cycling. Therefore, the recoverystress decreases drastically after the first cycle and reducesgradually further with increasing the number of thermalcycles.

The reverse transformation of the oriented martensitecauses the shape change of the sample and thus a strain–temperature curve is sensitive to the transformation of theoriented martensite. Due to the coexistence of new-orientedmartensite and remaining oriented martensite after each cycle,the strain–temperature curve shows a two-stage transforma-tion during the free shape recovery measurement. The oneoccurring at the low temperature (As1) corresponds to thetransformation from new-oriented martensite to austenite andthe other occuring at the higher temperature (As2) correspondsto the transformation from remaining oriented martensite toaustenite, respectively. However, only one transformationpeak appears in the DSC heating curve. According to previousstudies, DSC method is capable of determining the transfor-mation temperature of the thermal martensite but fails todetect the transformation of fully orientedmartensite.[28,29] It isreasonable to assume that the only peak corresponds to thetransformation from pure thermal martensite and new-oriented martensite to parent. The remaining orientedmartensite is plastically deformed by the generated recoverystress and is in fully oriented state, so it could not be detectedby DSC measurement. The decrease in the area of DSC peakswith increasing number of thermal cycles also indicates thatonly a small fraction of martensite is recovered to austenite.

The constrained phase transformations of SMAs associatedwith recovery stress have been investigated. It is widelyaccepted that the As temperature of SMAs can be elevated bydeformation.[30–34] Therefore, the As temperature of the pre-strained sample without thermal cycling is higher than that ofthe as-annealed one, as shown in Figure 3 and 4. Thetransformation temperature of remaining martensite, As2, isalso higher than that of the as-annealed one and increasesgradually due to the self-tension process with increasing

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number of thermal cycles. The transformation is not complete,even not at 180 °C. The transformation temperature of new-oriented martensite, As1, is a little lower than that of theannealed one because of the restored elastic strain energy.[28]

It is well accepted that the elastic modulus of NiTi SMAs isphase dependant and the martensite modulus is substantiallylower than the austenite modulus in tension. The significantdecrease of elastic modulus after the first cycle could be relatedto the plastically deformedmartensites, whichwere retained orunrecovered after each cycles. Our previous study showed thatthe elastic modulus plays an important role in the magnitudeof recovery stress.[16] The decrease of elastic modulus maymake a contribution to the dramatic decrease of recovery stressafter the first thermal cycle.

4. Conclusions

The effect of constrained thermal cycling on the recoverystress and recovery strain in an annealed NiTi wire to arelatively high temperature was investigated in the presentstudy. The results show that the recovery stress and recoverystrain decrease significantly after the first cycle and reducegradually further with increasing number of thermal cycles.The recovery stress exhibits a small change during isothermalholding. The evolution of recovery stress and recovery straincould be related to the accumulated plastic strain induced bygenerated recovery stress during constrained thermal cycling.

Received: May 14, 2013Final Version: August 14, 2013

Published online: October 8, 2013

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