effect of annealing and initial temperature on mechanical

Materials Science and Engineering A 432 (2006) 100107

Effect of annealing and initial temperesponse of a NiTiCr shape-

1 at-echa

e, La

May

Abstract

The effec ressivinvestigated he athe forward, transresulting str atureannealing te sses dThe highest phassingle-step t tion swith increas ealinstresses with 2006 Elsevier B.V. All rights reserved.

Keywords: Initial temperature; Transition stress; Yield stress; Clausius-Clapeyron slope; Superelastic property

1. Introdu

Shape-mous applicaerty, i.e. a ding withouproperty iscan be indutemperatursuperelastiseismic pro

The supsuppressioning the streincreasing

CorresponE-mail ad

[email protected] Present a

dong, Nam-GTel.: +82 54 2

0921-5093/$doi:10.1016/jction

emory alloys have been extensively studied for vari-tions, particularly because of their superelastic prop-issipative hysteretic response in loading and unload-t a permanent residual strain [15]. This superelastic

due to a martensitic phase transformation, whichced by the application of an external stress at some

e above Ms. Recently, it has been suggested that thisc property may be used for energy absorption (e.g.tection of structures [5,6]).erelastic property of alloys can be improved by the

of the dislocation-induced slip deformation dur-ss-induced martensite formation. This is attained bythe critical dislocation-induced slip stress [712]. In

ding author. Tel.: +1 858 534 4914; fax: +1 858 534 2727.dresses: [email protected] (J.Y. Choi),(S. Nemat-Nasser).

ddress: POSCO, Technical Research Laboratories, 1 Goedong-u, Pohang, Kyeongbuk 790-785, Republic of Korea.20 6152.

particular, Huang and Liu [13] and Nemat-Nasser and Guo [14]have studied the effect of annealing temperature on the supere-lastic response of NiTi shape-memory alloys. They point outthat the transition stress for the stress-induced martensite for-mation decreases with the increasing annealing temperature.In particular, it is reported that the dissipated energy of theNiTi shape-memory alloy, given by the area within the hys-teresis loop, attains a minimum when the annealing temperaturereaches about 600 K [14]. In addition, considerable effort hasbeen focused on studying the effect of compositional changes onthe mechanical and phase transformation properties of NiTiXternary alloy systems [1518]. It has been observed that theaddition of Fe or Nb to NiTi alloys improves their superelasticproperty by lowering the transformation temperature [17,19].Recently, Cr has been found to be a promising candidate fortailoring the superelastic property of NiTi alloys. Uchil et al.[20] have studied the transformation behavior of a NiTiCralloy in relation to its heat treatment conditions. They report thatthe phase transformation step of the alloy depends on the heat-treatment temperature: AR at annealing temperatures below673 K, ARM at temperatures 673873 K, and AM attemperatures above 873 K. Here, A denotes the austenite phase

see front matter 2006 Elsevier B.V. All rights reserved..msea.2006.05.155Jeom Yong Choi , Sia NemCenter of Excellence for Advanced Materials, Department of M

University of California, San Diego, 9500 Gilman DrivReceived 11 February 2005; received in revised form 19

t of the annealing conditions and the initial temperature on the compat a strain rate of 103/s, using an Instron hydraulic testing machine. Tfrom austenite to the stress-induced martensite, and the reverse phase

ess-induced martensite increase with the increasing annealing tempermperature of 573 K, and the yield stress for 673 K. Thereafter, both stretransition stress is attained for the smallest precipitates. In addition, theo a multiple-step with the decreasing annealing temperature. The transiing initial temperature at a rate of about 8.5 MPa/K. The optimum annout an accompanying residual strain is 573 K for 60 min.rature on mechanicalmemory alloyNasser nical and Aerospace Engineering,Jolla, CA 92093-0416, USA2006; accepted 19 May 2006

e response of a ternary NiTiCr shape-memory alloy isnnealing temperature and duration considerably influenceformation. The transition stress and the yield stress of the. The transition stress reaches its maximum value for anecrease with further increase in the annealing temperature.e transformation upon cooling and heating changes from atress for the stress-induced martensite formation increasesg condition that produces the highest transition and yield

J.Y. Choi, S. Nemat-Nasser / Materials Science and Engineering A 432 (2006) 100107 101

with a CsCl structure, R denotes the phase with a rhombohedralstructure, and M denotes the martensite phase with a mono-clinic structure. We are not aware of any study that addressesthe mechanical property of NiTiCr shape-memory alloys atvarious initial temperatures and at various annealing conditions.

In the present study, we report some recent experimentalresults on the mechanical response, especially the superelasticproperty of a NiTiCr alloy that is annealed at various tem-peratures and durations. After determining the optimum anneal-ing condition, the superelastic response of the correspondingannealed NiTiCr is investigated at a strain rate of 103/s fora range of initial temperatures.

2. Experimental procedure

A hot-rshape-memSpecial M5 mm nomby electro-annealed adurations i292 K (see

Quasi-sof 103/s awithin or ehydraulic tto attain elspecially dwithin 2attached tomeasured bibrated befends are lig

The trandeterminedMeasuremeunder a contion, the spthe minimu

The mica Philips Cdouble-tilt

Table 1Annealing tre

Temperature (As-hot-rolled4735235435736737739731123a

a 15 min.

cut with a low-speed diamond saw, parallel to the compres-sion axis. Thin foils are prepared by grinding the cut sheets toabout 150100m, and then they are thinned by twin-jet electro-polishing with an electrolyte of acetic acid and perchloric acid,9:1 volume, at 296 K.

3. Experimental results and discussion

3.1. Annealing temperature effect

To investigate the overall mechanical properties of thisNiTiCr alloy, specimens are annealed at various temperaturesfor 10 min, and then quenched in water at 292 K. Fig. 1(a) and (b)shows the effect of annealing temperature on the stressstraincurve of the alloy, deformed at a strain rate of 103/s and at

initial temperature. In the case of the specimens that areed to strains exceeding approximately 0.1 (solid lines),

essstrain curves show two transition stresses. These are

Compund

the sustrai

ture aolled 52.62 at.% Ni47.09 at.% Ti0.29 at.% Crory alloy rod of 6.35 mm diameter is secured from

etals Corporation. Circular cylindrical samples ofinal diameter and 5 mm nominal length are cutdischarge machining (EDM). The specimens aret temperatures between 473 and 1123 K for variousn air, followed by water quench to a temperature ofTable 1).tatic compression tests are performed at a strain ratend at various initial temperatures, to maximum strainsxceeding the superelastic range, using an Instron

esting machine. A high-intensity quartz lamp is usedevated temperatures. For low initial temperatures, aesigned bath is used to control the temperature toK. The temperature is measured by a thermocouple,

the sample surface. The specimen deformation isy an LVDT, mounted in the testing machine and cal-ore each test. To reduce the end friction, the samplehtly polished and greased prior to each test.sformation temperatures for the annealed alloy areusing a differential scanning calorimeter (DSC Q10).nts are carried out at temperatures from 123 to 333 Ktrolled heating and cooling rate of 10 K/min. In addi-ecimen is held for 5 min at both the maximum andm temperature.rostructure of some of the samples is examined byM200, operated at 200 kV, using a side entry typespecimen holder. Samples for TEM observations are

atment conditions

K) 10 min 30 min 60 min 120 min 240 min

296 Kdeformthe str

Fig. 1.annealedbeyonda 103/stemperaarison of the stressstrain curves for a NiTiCr alloy that iser the indicated conditions and deformed within (dotted line) andperelastic regime (solid line) at 296 K initial temperature and atn rate: (a) annealing temperature below 573 K and (b) annealingbove 573 K.

102 J.Y. Choi, S. Nemat-Nasser / Materials Science and Engineering A 432 (2006) 100107

Fig. 2. TEM at 573 K for 10 min. The arrows indicate the residual stress-inducedmartensites.

defined byand the flasition stresfor the strdata corresSIMys . Thesdeformed aresponse coite, followsite, and fimartensite.temperaturature for th

The supalso investdeformed tcurves, andstrain is reity, avoidining stresstransitionalformationThe finishinduced maEven thougmated) supual strainsin the anneregion appportion ofincreases.and finish spresent, buabove 573

Fig. 2 sbeen anneathe superelsive residua

the material has been deformed within the superelastic. This micrograph shows the presence of residual marten-

hat are left over through their interaction with the pre-g dislocations. These residual martensites in the austeniteare

roduonlyort dper

upon. 3 rensitie yieshow573

pernt wmicrographs of a superelastically deformed NiTiCr alloy that was annealed

the intersection of the lines tangent to the ascendingt sections of the stressstrain curve. The lower tran-s corresponds to the forward transformation stressess-induced martensite formation, sfr. The upperpond to the yield stress of the resulting martensite,e results are typical for shape-memory alloys that aret a temperature in the range of Ms to Md, where thensists of an initial elastic deformation of the austen-

ed by the formation of the stress-induced marten-nally the elastic-plastic deformation of the resultingHere, Ms denotes the martensite transformation-start

e on cooling, and Md denotes the maximum temper-e stress-induced martensite formation.erelastic response of the annealed specimens is

igated at 296 K initial temperature. The samples areo about 0.05 strain, estimated from the stressstrain

then unloaded to zero stress. Here, the maximumstricted to a certain level to ensure superelastic-g the dislocation-induced plastic slip. The result-strain curves (dotted lines) exhibit another lowerplateau regime, corresponding to the reverse trans-

thoughregimesites texistinmatrixbeen pture isof a shThis imstrain

Figthe traand thstressbelowing temconstafrom the stress-induced martensite to the austenite.stress for the reverse transformation from the stress-rtensite to the austenite, fre, is also shown in Fig. 1.h annealed specimens are deformed within the (esti-erelastic regime, the stressstrain curves show resid-. These residual strains decrease with the increasealing temperature. It is also observed that a plateauears in the reverse transformation on the unloadingthe stressstrain curve, as the annealing temperatureFor annealing temperatures below 573 K, the starttresses for the reverse transformation are not clearlyt they are clearly observed for annealing temperaturesK.hows the TEM micrograph of the sample that hasled at 573 K for 10 min, and then deformed within

astic regime. Residual martensites (arrow) and exten-l dislocations are clearly observed in this figure even

Fig. 3. Variatmation and thtemperaturesannealed for 1stabilized by the residual dislocations [21] that hadced in the hot-rolling process. Here, the defect struc-rearranged by the low-temperature annealing processuration, as has been pointed out by Lin and Wu [8].fect reverse transformation leaves a residual inelastictotal unloading.presents the variation with annealing temperature of

on stress for the stress-induced martensite formationld stress of the resulting martensite. The transitions an almost constant level at annealing temperaturesK and then decreases with an increase in the anneal-ature until reaching 773 K. After that, it is almosthile the annealing temperature is increased. The tran-ion of the transition stress for the stress-induced martensite for-e yield stress of the resulting martensite for various annealing

and 10 min duration. The 1123 K case (marked by dotted circle) is5 min, and triangle symbols are for those annealed for 60 min.


Fig. 4. Variat473 K for the

sition stresabout 470 Mthe yield swith the inmum valuehardeningyield stressthe annealilonging theyield stressfrom 1300the resistancipitation hthere is littl(open trian

3.2. Annea

To maxmartensitetemperaturFig. 4 show473 K for 1curves are a

induced main the anneapermanentity. Fig. 5 eat 573 K foincreases,increase, wreverse trantion increasthat were asamples, w

Fig. 6 dwere annea

Unlike tho

Variation of the stressstrain curves for a NiTiCr alloy annealed atr the indicated durations.

3 K annealing temperature the transition stresses for theinduced martensite formation and the reverse transfor-

decred rall.var

siterationsitioses w773

lighnsitioimaressnnea

ingperion of the stressstrain curves for a NiTiCr alloy annealed atindicated durations.

s deceases from 600 MPa at the as-hot-rolled state toPa at an annealing temperature of 1123 K. However,

tress of the resulting martensite increases graduallycreasing annealing temperature, reaching a maxi-of about 1450 MPa at 673 K, due to the precipitation

of the alloy. After reaching the maximum level, thedecreases continuously with a further increase in

ng temperature, because of the recrystallization. Pro-annealing at 573 K from 10 to 60 min shows that theof the resulting martensite increases substantially,

to 1700 MPa (open triangles marked by an arrow), asce to the dislocation motion is increased due to pre-ardening [22]. At an annealing temperature of 973 K,e change in the yield stress of the resulting martensitegles).

ling duration effect

imize the transition stress for the stress-inducedformation, samples are heat treated at several selectedes for various time periods, as summarized in Table 1.

Fig. 5.573 K fo

at a 77stress-mationmeasu

bly smThe

martening duthe tradecrea543 tobut a sest traapproxtion stalloy aincreasing tems the superelastic behavior of the alloy annealed at0, 120, and 240 min, respectively. The stressstrainlmost the same, but the transition stress for the stress-rtensite formation increases slightly with the increaseling duration. In addition, the annealed alloy shows aresidual strain, indicating an imperfect superelastic-xhibits the superelastic behavior of the alloy annealedr the indicated durations. As the annealing durationthe forward and the reverse transformation stresseshereas the residual strain decreases. In particular, asformation plateau emerges as the annealing dura-es. However, the superelastic response of the samplesnnealed at 773 K is quite different from that of thehich had been annealed at 573 K.isplays the superelastic behavior of samples thatled at 773 K for 10, 30, and 60 min, respectively.

se that were annealed at temperatures below 573 K,Fig. 6. Variat773 K for therease with the increasing annealing duration. Theesidual strain of all specimens, however, is negligi-

iation of the transition stress for the stress-inducedformation with the annealing temperature and anneal-n is summarized in Fig. 7. As shown in Fig. 7,n stress for the stress-induced martensite formationith an increase in the annealing temperature fromK. After that, it reaches an almost constant level;

t increase is seen for annealing at 973 K. The high-n stress is obtained at an annealing temperature of

tely 573 K. Of particular interest is that, the transi-for the stress-induced martensite formation for theled at temperatures below 573 K, increases with theannealing duration (solid arrow), whereas at anneal-atures above 573 K, the transition stress decreasesion of the stressstrain curves for a NiTiCr alloy annealed atindicated durations.


Fig. 7. Variation of the transition stress for the stress-induced martensite forma-tion as a function of annealing temperature for the indicated durations.

(dotted arrow). These variations are related to the precipitationbehavior of the NiTiCr alloy.

Fig. 8 exhibits the precipitates of undeformed samples thatwere annealed at 573, 673, and 773 K for 60 min, respectively.According to the study of the precipitation behavior in NiTi

alloy [23,24], the Ti3Ni4 precipitates strongly affect the mechan-ical property of Ni-rich NiTi alloys. Therefore, the complexprecipitate, Ti3(Ni, Cr)4, some of whose Ni is replaced byCr, is expected to be formed in the NiTiCr alloy, with theprecipitate size varying appreciably with the annealing temper-ature and duration. The precipitates are nanometer-sized clus-ters in Fig. 8(a) and micrometer-sized platelets in Fig. 8(c).The nanometer-sized clusters produce a maximum transitionstress for the stress-induced martensite formation, as shown inFig. 7. Furthermore, it is also observed that the transition stressdecreases with the increasing precipitate size. This is due to thevariation of the materials stability and the resistance to disloca-tion motion with the precipitate size. According to the study ofTi50.9 at.% Ni by Gall et al. [21,25] the transformation tem-perature, Ms, increases with the increasing precipitate size up to100 nm precipitate, and then it decreases as the precipitate sizeincreases.

DSC results of a NiTiCr alloy that was annealed at dif-ferent temperatures for 60 min, are shown in Fig. 9. The curveobtained from a sample which was annealed at 973 K, exhibitsa single transformation sequence (austenite to martensite andreverse) of this alloy, AM, as shown in Fig. 9(a). The obtainedMs and Af temperatures are 257 and 280 K, respectively. As isseen in Fig. 9(b), as the annealing temperature decreases fromFig. 8. TEM micrographs of a NiTiCr alloy annealed for 60 min at: (a) 573 K, (b) 673 K, and (c) 773 K.


Fig. 9. Trans673 K, and (c

973 to 673into a two-boidal to mheating, MMRformation curves of NiTiCr alloy annealed at: (a) 973 K, (b)) 573 K.

K, a single forward transformation, AM, evolvesstep transformation, ARM (austenite to rhom-artensite). Moreover, the reverse transformation uponA, also shows a two-step transformation behavior,A. As the annealing temperature further decreases to

Fig. 10. Trantemperatures

573 K, a twstep transfdifferent frthe ARat annealintransformaannealing tet al. [26],alloy can bprocess, anprocess. Heitates, R2 dprecipitatesphase. Thisthe inhomo[27], and bwith the prsponding ain Fig. 10.annealing tthe austenitinitial sampalso obtainwell with t

In additFig. 3 depehigher yielby Treppmpoint out thtion motionprecipitatestion value,500 nm. Inoptimum ation and yiea residual sannealing asformation temperatures of NiTiCr alloy annealed at differentfor 60 min.

o-step transformation behavior evolves into a three-ormation, as shown in Fig. 9(c). These results areom the results of Uchil et al. [20] who suggest thattransformation in NiTiCr alloy is only observedg temperatures below 673 K. A similar three-steption in NiTi alloys has also been observed whenhe material at low temperatures. According to Kimthese phase transformation steps in 50.9 at.% NiTie AR1, AR2, and R2 M2, during the coolingd M2 R2, R2 A, and R1 A, during the heatingre, R1 denotes the R-phase formed around the precip-enotes the R-phase formed in the regions away from, and M2 denotes the martensite formed from the R2-multiple-step transformation is explained in terms ofgeneity of the composition [11], internal stress fieldoth composition and internal stress [12], associatedecipitates, Ti3Ni4. The transformation and the corre-nnealing temperatures for this alloy are summarizedThe Ms temperature increases with the increasing

emperature, indicating that the relative stability ofe, i.e. the temperature difference between Ms and thele temperature, 296 K, decreases. A similar result is

ed by Gall et al. [21,25]. Moreover, this result agreeshe mechanical properties shown in Fig. 7.ion, the yield stress of the resulting martensite innds on the precipitate size. Small precipitates lead to ad stress. These trends are the same as those reportedann et al. [28] and Ishida and Miyazaki [29], whoat the critical resolved shear stress for the disloca-reaches a maximum level at approximately 10 nm

. After this, the stress decreases towards its satura-as the precipitate size increases further, up to aboutthe present work, we have sought to determine an

nnealing condition that results in an excellent transi-ld stresses for the stress-induced martensite, withouttrain. We have concluded that this can be attained byt a 573 K for 60 min.


3.3. Initial temperature effect

To study the effect of initial temperature on the supere-lastic response of our NiTiCr alloy, compression tests areperformed at a strain rate of 103/s over the temperature rangeof 230400 K. Before testing, a NiTiCr alloy is annealed at573 K for 60 min. According to the DSC result in Fig. 9(c),the specimen has a fully austenite state at temperatures above306 K, but upon cooling, it has a mixture of rhomboidal andaustenite phases in the 306277 K temperature range. A sin-gle martensite phase exists at temperatures below 239 K, and,upon heating, a mixture of rhomboidal and austenite phasesfrom 239 to 309 K, and a full austenite phase at temperaturesabove 309 K. Fig. 11 shows the effect of initial temperature on

Fig. 11. Stresat the indicatetemperature btemperature a

Fig. 12. Varia

the stresssstressstraiThe curves263 K, shownent. Theyshape-memstressstrailoop, untilthe residuaature; see FtransformaThe occurrabove 323 Ksmall amoufor 310 K,formationan initial tecurve clearreverse tran

Fig. 12 estresses forformation, and the yield stress of the austenite, at a strain rateof 103/s. The transition stress for the stress-induced marten-site formation increases with the increasing initial temperature.This is in line with the Clausius-Clapeyron equation that gives arelation between the initial temperature and the critical stress for

ss-induced martensite formation [30]. The yield stress ofstenite, however, decreases with the increase in the tem-re. The dsfr/dT at a strain rate of 103/s is approximatelya/K. This value is greater than 7.3 MPa/K in polycrys-NiTi alloys that which is obtained under quasi-static

[3133]. The Md temperature, i.e. the maximum temper-or the stress-induced martensite formation, is determinedintersection of the slope of the transition stress and thetion-induced slip stress versus temperature lines. The Md

rature at a 103/s strain rate is about 325 K, in agree-with the residual strain behavior. The finishing stress

reverse transformation, fre, is measured and plotted insstrain curves for a NiTiCr alloy annealed at 573 K, deformedd initial sample temperatures and a 103/s strain rate: (a) initial

elow 263 K, (b) initial temperature from 263 to 323 K and (c) initialbove 323 K.

the strethe auperatu8.5 MPtallinestatesature fby thedislocatempementfor thetion of the transition stress as a function of initial temperature.

train response of the annealed alloy. As is seen, then curves strongly depend on the initial temperature.in Fig. 11(a), obtained at initial temperatures belowlarge residual strains, which however are not perma-

are recovered at a temperature of 296 K, showing theory effect. At initial temperatures above 263 K, then curves display a superelastic effect, i.e. a hysteresisthe sample temperature reaches 310 K, after which,l strain increases with the increasing sample temper-ig. 11(b) and (c). An interesting point is the reverse

tion behavior in the superelastic temperature range.ence of the reverse transformation at temperatures

is unclear even though the unloading curve shows ant of recovered strain. The unloading curve obtainedhas an inflection point, indicating the reverse trans-of the stress-induced martensite to the austenite. Atmperature below 296 K, the unloading stressstrainly shows a plateau regime that corresponds to thesformation.xhibits the temperature dependence of the transitionthe stress-induced martensite and the reverse trans-


Fig. 12. The Clausius-Clapeyron slope for the reverse transfor-mation is about 9.5 MPa/K, which is greater than that of thetransition stress for the stress-induced martensite formation, i.e.8.5 MPa/K.

4. Conclusions

Compressive tests are performed on cylindrical samples toinvestigate the effect of the annealing conditions and the initialtemperaturmemory alhydraulic tas follows:

(1) For a dstress-istant fit decreand finthe reswhen t

(2) The tramationannealfor annin theannealtemper

(3) The vation anprecipiing at 5stress d

(4) The phprocesgle stechangeing anclearlythat wa

(5) The trincreasperaturrate.

(6) Considing costressefor 60and th1700 M

Acknowledgements

This work was supported by ONR (MURI) grantN000140210666 to the University of California, San Diego. Theauthors wish to express their appreciation to Mr. G.-Y Ryu atRIST, Korea, for helping with the TEM microstructural obser-vations.

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Effect of annealing and initial temperature on mechanical response of a Ni-Ti-Cr shape-memory alloyIntroductionExperimental procedureExperimental results and discussionAnnealing temperature effectAnnealing duration effectInitial temperature effect

ConclusionsAcknowledgementsReferences

effect of annealing and initial temperature on mechanical

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