Thermochimica Acta 536 (2012) 1 5

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Thermochimica Acta

jo ur n al homepage: www.elsev ier .c

Effects peparame me

S . Nevin a Firat Universib Eren Universi

a r t i c l

Article history:Received 10 OReceived in reAccepted 3 FebAvailable onlin

Keywords:Shape memorGibbs free eneElastic strain eVickers hardne

tant t CuA

s studtempory ae theriencmplehase

1. Introdu

Cu-based shape memory alloys (SMAs) have been paid moreattention in past few years owing to their low price, easy fabricationand excellent conductivity of heat and electricity. Shape memoryalloys exhibpseudoelasshape memknown andinnovative properties amartensiticphase (P) an

The shatransformaBy analogy perature ancalled mart

Several ceffect and thand microsttheir applicthe alloys isent phase) of the alloys

CorresponE-mail add

of CuCu-based SMAs are quite sensitive to alloying elements which areadded to adjust the martensitic transformation temperatures andto optimize thermal stability as well as mechanical properties.In addition, the martensitic transformation and the associated

0040-6031/$ doi:10.1016/j.it the remarkable thermomechanical properties liketicity (PE), shape memory effect (SME) and two-wayory effect (TWSME). These last two are probably the best

they make very suitable this class of the materials forapplications in various elds [13]. These remarkablere controlled by a reversible structural transformation,

transformation, between the highly symmetric parentd the less ordered martensite (M) solid phases.

pe memory effect is basically linked to a martensitiction occurring in steels and several non-ferrous alloys.with the martensitic transition of steels, the low tem-d high temperature phases of non-ferrous alloys areensite (M) and austenite (A), respectively [1,4].opper-based alloys have exhibited the shape-memorye understanding their characteristic thermal behaviorsructure evolutions has brought a signicant impact onations [5]. However, the shape memory effect (SME) of

susceptible to aging whether in austenite phase (par-or in martensite phase, which affects the applicability. There have been a lot of reports on the aging in parent

ding author.ress: nbalo@rat.edu.tr (S .N. Balo).

mechanical shape reversibility in Cu-based SMAs are stronglyinuenced by quenching and aging treatments. The paper aims toreveal aging effects in austenite of CuAlNi shape memory alloy. Themain experimental tool is differential scanning calorimetry (DSC)which provided data for thermodynamic evolution of both forwardand reverse martensitic transformations. Thus, we have focusedon this study to know the effects of the thermal aging at a constanttemperature above Af on transformation and thermodynamicsparameters in a CuAlNi shape memory alloy.

2. Experimental

A ternary Cu-rich CuAlNi SMA with a nominal composition ofCu13.5 wt.%Al4 wt.%Ni was supplied by Tremetaux, Centr deRecherce (France). The samples cut from the alloy were annealedin the phase eld for 20 min at 1203 K for betatization and rapidlyquenched in iced brine to obtained 1 martensite. The CuAlNi sam-ples were aged for 1, 2, 3, 4, 5, 6 and 7 h above 50 K of the austenitephase nish temperature (398 K). Aged alloy samples with variousperiods were directly quenched in iced brine. The transformationcharacteristics of un-aged sample and aged samples were exam-ined by a Perkin-Elmer Sapphire model thermal analyzer at heatingand cooling rates of 10 K/min in the 303373 K range.

see front matter 2012 Elsevier B.V. All rights reserved.tca.2012.02.007 of thermal aging on transformation temters of Cu13.5 wt.%Al4 wt.%Ni shape

Baloa,, Neslihan Selb

ty, Science Faculty, Department of Physics, 23169 Elazig, Turkeyty, Science and Ards Faculty, Department of Physics, Bitlis, Turkey

e i n f o

ctober 2011vised form 1 February 2012ruary 2012e 22 February 2012

yrgynergyss

a b s t r a c t

The effects of thermal aging at a consthe martensitic transformation of thethe transformation temperatures wathat the thermal aging at a constant parameters of the CuAlNi shape memshifted by the thermal annealing. Thelastic strain energy of the alloy expestructural properties of aged alloy satemperature. It was found that the pcrystallite size effect.

ction phase om/ locate / tca

ratures and some physicalmory alloy

emperature above Af (austenite phase nish temperature) onlNi shape memory alloy were investigated. The evolution ofied by differential scanning calorimetry (DSC). It was founderature above Af has a complex effect on the transformationlloy. The Ms, Mf, As, and Af transformation temperatures werermodynamic parameters such as Gibbs free energy and theed a decreasing tendency with increasing the aging time. Thes were studied by X-ray diffraction measurements at roomtransformation parameters of the alloy are controlled by the

2012 Elsevier B.V. All rights reserved.

-based SMAs [69]. The shape memory properties of

2 S .N. Balo, N. Sel / Thermochimica Acta 536 (2012) 1 5

tizatio

X-ray dRigaku Radof = 1.540formed usinwere testedForce.

3. Results

The DSCIt is well austenite cal temperatransformaheating whand nish tformation tmethod, as alloy for vaphase transtransformaincreased wFig. 2 and thin thermal trst 2 h may

Fig. 2. Changealloy.

he rands t(AfMeasermag-te

narrs tem

also tereratur

is westoree thhicbov

equstenies ofFig. 1. DSC curve of CuAlNi specimen annealed at 1203 K for beta

iffraction measurements were carried out using aB DMAX II X-ray diffractometer with Cu-K radiation56 A. The Vickers hardness measurements were per-g an Anton Paar hardness tester machine. The samples

with three indentations. The applied load was 15 g-

and discussion

curves of un-aged sample are shown in Fig. 1.known that SMAs undergo a thermally induced

martensite transformation characterized by the criti-tures; As, Af, Ms and Mf. Here As and Af refer to austenitetion start and nish temperatures, respectively, duringile Ms and Mf refer to martensite transformation startemperatures, respectively, during cooling. The trans-emperatures of the alloy are determined by tangentshown in Fig. 1. The transformation temperatures of therious aging times and the corresponding latent heats offormations (enthalpy) are given in Tables 1 and 2. The

alloy. Tand tewidth to incrtransfothe lonples isThe Maturesthe hystempeAlso, itto the lyze thview, weffect aSMA.

Thethe auenergition temperatures As, Af, Ms and Mf of CuAlNi SMA areith the thermal aging at rst 2 h, as seen in Table 1 andey gradually are decreased for 5, 6 and 7 h. The increaseransformation temperatures with the thermal aging at

be due to the instantaneous change entropy of the aged

s of transformation temperatures with various aging times in CuAlNi

dependenction,

GMA(T0)

The GM

were calculenergy is de

Fig. 3. Changen and rapidly quenched and un-aged.

nge of the phase change temperature varies with agingo decrease with increasing aging time. The hysteresiss) is varied by the thermal aging and generally tends

with aging time. The obtained results suggest that thetion range of both (MsMf) and (AfAs) is decreased byrm thermal aging, since the thermal range of aged sam-ower than that of un-aged sample, as seen in Table 1.perature is changed up to 7 K. Mf, As and Af temper-are shifted by the different amounts. This implies thatsis is changed, because the change in the transformationes Ms, Mf, As and Af is not the same for all the samples.ll known that the difference in MsMf or AfAs is relatedd elastic energy. Therefore, it seems worthwhile to ana-ermal aging effects from a thermodynamical point ofh should be included into the discussion of the aginge the austenite phase nish temperature on the CuAlNi

ilibrium temperature T0 between the martensitic andte phases is the temperature at which the Gibbs free

the two phases are equal. Thus, the Gibss free energy

e of T0 temperature is expressed by the following rela-

= GA(T0) GM(T0) = (HA T0SA) (HM T0SM)= HMA (T0SMA) (1)

A Gibss free energy values for the various aging timesated and are shown in Fig. 3. It is found that Gibbs freecreased by about 6% after 7 h thermal aging.

s of GAM and Ge compared with the value of not aged sample.

S .N. Balo, N. Sel / Thermochimica Acta 536 (2012) 1 5 3

Table 1The transformation temperature parameters of the alloy at various aging times.

Aging time (h) Ms (K) Mf (K) MsMf (K) As (K) Af (K) AfAs (K) AfMs (K) T0 (K)

0 350.9 331.7 292.2 341.3 358.5 290.2 280.6 354.71 350.7 327.8 295.9 344.4 361.1 289.7 283.4 355.92 353.4 336.1 290.3 347.0 363.5 289.5 283.1 358.43 348.3 329.5 291.8 340.9 354.2 286.3 278.9 351.24 350.2 335.2 288.0 347.1 359.5 285.4 282.3 354.85 345.3 332.6 285.7 343.8 355.5 284.7 283.2 350.46 344.1 332.1 285.0 343.4 354.5 284.1 283.4 349.37 343.9 329.3 287.6 340.6 352.6 285.0 281.7 348.2

Table 2Aging effects on the thermodynamic parameters and Vickers Hardness of the alloy.

Aging time (h) T0 (K) HM A (kJ/kg) SM A (J/kg K) GA M (J) Ge (J) Average values ofVickers hardness

0 354.7 9.15 25.79 0.095 7.347 7251 355.9 7.22 20.28 0.095 5.424 7362 358.4 8.32 23.21 0.106 6.192 8373 351.2 8.69 24.74 0.070 7.118 8014 0.115 7.212 5955 0.123 6.905 5186 0.124 6.805 5977 0.101 6.791 701

Equilibriaustenitic p

T0 =12(MS +

In additi

T0 =HM

SM

The hysting force fo

GAM(Ms

The diffein self-acco

Ge = GA

where SM

perature bein Table 2 ture, T0 of for 1 h andaging timesSMA en24.29 103tic strain enshown in Fi

Fig. 4. Effect o

h aging. The elastic strain energy is decreased by about 7% h aging time and then 8% after 7 h aging time., we have aimed to investigate the role of long-term agingAf point on martensite characteristics and stabilization ini. For this purpose, X-ray diffraction patterns were taken fromamples for various aging times, as shown in Fig. 5. Thesetograms were taken from the alloy in as-quenched case (a),

h aging (b), after 4 h aging (c), after 7 h aging (d), respec-The alloy has ordered structure in un-aged heat-treated case.

in XRD patterns, the alloy exhibits a superlattice reection354.8 9.06 25.53 350.4 8.84 25.22 349.3 8.41 24.07 348.2 8.88 25.50

um temperature between the martensitic andhases, T0 can be expressed by the following relation,

Af ) (2)

on T0 for the alloy is expressed as [10]

A

A(3)

eresis in the transformation is characterized by the driv-r the nucleation of martensite GAM(Ms) as [1113]

) = GMA(T0) GMA(Ms) = (T0 Ms)SMA (4)rence in MfMs is related to the elastic energy Ge storedmmodated martensitic variants by [11]

M(Ms) GAM(Mf ) = (Ms Mf )SMA (5)A entropy change and T0 is the equilibrium tem-tween the martensitic and austenitic phases. As seenand Fig. 4, the thermodynamic equilibrium tempera-the martensitic and the austenitic phases is increased

2 h aging and then, is decreased with 4 h and up

after 6after 6

Alsoabove CuAlNalloy sdiffracafter 2tively. As seen. The latent heat HMA of phase transformation andtropy values vary around 8.57 103 0.617 J/kg and 1.83 J/kg K respectively, as shown in Table 2. The elas-ergy values Ge, for the samples were calculated and areg. 3. The elastic strain energy shows a minimum point

f various aging times on the equilibrium temperature of the alloy.

Fig. 5. X-ray pand (b) aging aheat treatmeniece diffractograms of CuAlNi shape memory alloy; (a) heat treatedt 2 h, (c) aging at 3 h (d) aging at 4 h, (e) aging at 7 h at 398 K after thet.

4 S .N. Balo, N. Sel / Thermochimica Acta 536 (2012) 1 5

Table 3Lattice Parameters according to aging time of the alloy.

Aging time (h) a () b () c () a/b

0 1 2 3 45 67

in quenchehas the atothe orthorotion peaks sin the marteindexed onculated latt

1d2

= 12

(

a/b ratios anculated fromin Table 3. atomic sizeis evaluatedThe changeduration wthe similar of some diffof the impoevaluated tis based onbasal plane(or L21) typand it transthe martenture while shape memaging at 39occur whicequilibriumalter the chaustenite atransformastructure coquenching austenite bno marked after quenc

The crysrelation [19

D = 0.9b cos

where D is broadeningcrystallite sdeterminedsize is incredecreased w

The chanformation tthat the phcrystallite s

Fig

Fig.

rma to th

vari in Fed bsed uh houss va. Thnicald by

clusions

effects of the aging time on transformation temperaturesermodynamic parameters of the CuAlNi shape memory alloynvestigated. A change in transformation temperatures wased due to thermal aging. It is found that the elastic strain

and Gibss free energy are decreased by about 8 and 6% h aging time, respectively. The hardness value is increased

2 h aged and then at seventh hour is near to value hard- un-aged sample. It was found that the phase transformationeters of the alloy are controlled by the crystallite size effect.

wledgment

s study was supported by Firat University Research FundF) under Project No. 1419.4.2001 5.1592 38.6607 84.19 0.81414.4433 5.3826 38.6207 85.41 0.82554.1964 5.1580 38.8988 84.07 0.81364.4498 5.4069 38.4020 84.82 0.82304.1958 5.1456 38.5043 84.42 0.81544.4958 5.3153 38.8157 85.32 0.84584.4814 5.3760 38.4944 92.92 0.83364.1804 5.1604 38.3599 86.48 0.8101

d case and even after 7 h aging. Miller indices whichm distribution in the form of 18R structure refer tombic unit cell of -type martensite. Indexed diffrac-how that the alloy has the orthorhombic 18R structurensitic condition and the diffraction patterns have been

the basis of orthorhombic unit cell. Thus, we have cal-ice parameters using the following relation [14],

h2

sin2

)+ k

2

b2+ 1

c2

(l2

sin2

) 2hl cos

ac sin2 (6)

d lattice parameters of 18R martensite phase were cal- the X-ray diffractograms with aging duration are given

This ratio is less than

3/2 in the ordered case due tos of the constituent atoms for 18R martensite [15,16]. It

that the X-ray results conrm a monoclinic 18R basis.s in peak characteristics of the XRD patterns with agingere investigated. Although all the XRD patterns exhibitcharacteristics, it was observed that the peak locationsraction planes were changed. Structure ordering is onertant factors for formation of martensitic [16,17]. It ishat the martensitic phase in Cu-based -phase alloys

one of the (1 1 0) planes of austenite phase called for martensite. A (1 1 0) plane in the -phase of DO3e ordered structure is rectangular, as in original case,forms to a hexagon with hexagonal distortion duringsitic transformation [18]. 18R is a metastable struc-the equilibrium structure of Cu13.5 wt.%Al4 wt.%Niory alloy is formed from , and 2 phases. After 7 h8 K, it is expectable that some atomic migration wouldh would enable the precipitation of any of the above

phases. This diffusion controlled phenomena wouldemical composition of austenite, since we deal with

ging, and this could explain the change in the criticaltion temperatures. But if, it is evaluated that the entiremprises only metastable 18R martensite, even after theof a 7 h-aged sample, this means there is no retained ecause it totally transformed into 18R martensite anductuations of chemical concentration were observedhing.tallite size for the alloy was determined by the following,20]

(7)

the crystallite size, is the wavelength, b is the peak at full width at half maximum and is the angle. The

transfobe due

Theshownincreasdecreaat sixthardnesamplemechachange

4. Con

Theand thwere iobservenergyafter 7at rstness ofparam

Ackno

Thi(FUNAize values for the samples aged various times were and are shown in Fig. 6. As seen in Fig. 6, the crystalliteased up to 3 h and indicates a maximum and then isith increasing time.ge in crystallite size indicates the similar trend of trans-emperatures with a minimum for 3 h. This suggestsase transformation parameters are controlled by theize effect. It is evaluated that the increase in thermal

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Effects of thermal aging on transformation temperatures and some physical parameters of Cu13.5wt.%Al4wt.%Ni shape memory...1 Introduction2 Experimental3 Results and discussion4 ConclusionsAcknowledgmentReferences