Materials Science and Engineering A 521522 (2009) 182185

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Materials Science and Engineering A

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Ageing effects after heat treatment in CuAlMn

AgnieszkInstitute of Ma -3867

a r t i c l

Article history:Received 31 MAccepted 7 Oc

Keywords:Shape memorCuAlMnHigh dampingAgingPhase transfor

eat try waand Mincreathe inmpeof thtransnt. Stationt leasemoruringratur

1. Introduction

The maireductionothispurposfound in poical strengthigh dampidevelopeddamping, pticles, or malloys (SMAand CuAlcompared t

Cast paafter castinto improvenization hetemperaturtially changtemperatur

CorresponE-mail add

1 Now with:2 Now with:

highdamping should stay stable for longer times after homogeniza-tion heat treatment contrary to previous ndings, showing severe

0921-5093/$ doi:10.1016/j.mn goal of the application of high damping materials is af strongmechanical vibrationsbyenergydissipation. Fore thematerialmustprovidehighdampingcapacityoftenlymers or other organic materials. If also good mechan-h or good thermal or electric conductivities are neededng metals (HiDaMets) can be applied. HiDaMets can befrom ferromagnetic metals using magneto-mechanicalure metals reinforced by high strength bres or par-artensiticaly transforming alloys, e.g. shape memory) [1]. The most investigated and utilized SMA are TiNiNi. These alloys are expensive and complex to produceo CuAlMn SMA, developed in recent years [2,3].rts of CuAlMn SMA are inhomogeneous directlyg and should be homogenized at high temperaturestheir mechanical and damping properties. Homoge-at treatments increase grain size, change transitiones and phase composition, and therefore can substan-e thedamping level of theCuAlMnSMAat applicatione, e.g. room temperature [4,5]. For technical application

ding author. Fax: +49 5323 723148.ress: werner.riehemann@tu-clausthal.de (W. Riehemann).VW AG, Letter box 011/1499, 38436 Wolfsburg, Germany.RWTH, IOT, Jlicher Strae 344a, 52070 Aachen, Germany.

decrease of damping at room temperature after homogenizationheat treatment in CuAlMn SMA [4], called ageing. For this paperthe aging processes in CuAlMn alloys was investigated in moredetail. Aging in CuAlMn SMA is a very complex process dueto many possible martensitic phases. Below approximately 500 CCuAlMn alloys have the ordered structure L21 (bcc), which trans-forms martensitically to 3R-fcc, 18R (small martensite plates) or 2H(largemartensiteplates) structures [6,7]Additionally theagingpro-cesses, which can be caused by transformation, can lead to volumechanges, because during the transition frommartensite to austenitethe lattice transforms from hexagonal or rhombohedral symme-try to body centred cubic crystal structure depending on the Mncomposition [8,9].

2. Experimental

The investigated alloys were obtained by melting copper(99.99wt.%), aluminium (99.7wt.%) and manganese (99.85wt.%) inamid-frequency induction furnace innormal atmosphere, followedby casting in steel moulds preheated to 300 C. This state will bealways referred to an as cast state. The chemical compositions ofthe alloys were determined by inductively coupled plasma spec-troscopy (ICP) [10]. Specimens were thermally (heated or cooled)treated and subsequently aged at different temperatures and times.Normally, as aged without further description means heat treated

see front matter 2009 Elsevier B.V. All rights reserved.sea.2008.10.066a Mielczarek1, Nils Kopp2, Werner Riehemann

terials Engineering and Technology, Clausthal University of Technology, Agricolastr. 6, D

e i n f o

ay 2008tober 2008

y alloy

mation

a b s t r a c t

Different types of homogenization h800 to 920 C for 1h and subsequentlalloys with Al content 11.412.3wt.%of the investigated alloys lead to anmicrostructure and phase fractions ofture and cooling rate. The transition teMoreover, narrowing and increasingmeasurements. Different martensiticments depending on the heat treatmecapacities directly after all homogenizdependent damping measurements. Aageing effects in CuAlMn shape mdefects diffusing to the boundaries ddecrease of the transformation tempem/locate /msea

shape memory alloys

8 Clausthal-Zellerfeld, Germany

eatments, with homogenization temperatures varying fromter quenching, were performed on CuAlMn shape memoryn-content 5.06.9wt.%. All homogenization heat treatmentsse of isothermal damping with increasing temperature. Thevestigated samples depend on the homogenization tempera-

ratures increased after every homogenization heat treatment.e transformation peaks were observed by acoustic emissionformations could be detected by acoustic emission measure-rong and fast aging effects, decreasing the very high dampingheat treatments were found in room temperature amplitudet, there are three different possible explanations for the foundy alloys: (1) pinning of twin and phase boundaries by pointageing, (2) partial phase transformation during ageing, (3)

es. 2009 Elsevier B.V. All rights reserved.

A. Mielczarek et al. / Materials Science and Engineering A 521522 (2009) 182185 183

Table 1Names and chemical compositions of the investigated alloys.

Name Cu inwt.% Al inwt.% Mn inwt.%

CuAl11.4Mn6.9 81.70 11.40 6.90CuAl12.1Mn5.1 82.7 12.3 5.0

at 850 C for 1h water quenched (w.q.) and aged for 2 years at roomtemperature (RT).

In Table 1 the chemical compositions of the investigated alloysare given. Transition temperatures are strongly dependent on thetreatment of the alloys and the transitions themselves are verycomplex due to occurrence of many martensitic variants, thereforethey are not given in this table. The transition temperatures weremeasured by acoustic emission. An acoustic 2 channel AcousticEmission Signal analyser (Physical acoustics Corporation, PrincetonJct, USA) was used [10]. The temperature range of these mea-surements was 26 to 120 C. The cooling and heating rate was20K/min. The start transformation temperatures were dened bythe temperature when 100 acoustic events per second take placeduring heating or cooling with this rate.

Internal friction versus maximum strain amplitude was mea-sured at room temperature (RT=22 C) in vacuum (10Pa) as thelogarithmic decrement of free decaying bending vibrations of asingle clamped vibrating beam with a vibrating length of 97mm,a width of 10mm, and a thickness of 2mm. The specimens wereexcited torange 3843the end of ting amplitucalculated.given in ref

3. Results

3.1. Recover

The ampcast and he[4]. The amafter this heFor the presimately 2 y

Fig. 1. Strain athermal treatmdown. w.q. =w

years the damping decreased and became much smaller than thedamping of as cast state for all measured strains. In ref. [4] it wasshown for the same specimen that during 120 days the dampinglevel after (1) homogenization at 850 C decreased to the dampinglevel of theafter 120 dmen two mwith this swards (3) hwater quencould restorheat treatmbe recovereFig. 1 after alevel as thealways canlike homog

After agments at loheat treatm50 C and 1for this oppbelow 150

above 150

dence of ththis could m

ariesof thampiion tal. Thlingraturent ae, soeatm

eing

2 s) cu0 =

ing.3Mnre haf allg. 2athe s0. ThFig.andMnig. 2ageiuencens

at trey pro

CuAent)heates. A

mpinresonant vibrations with resonant frequencies in theHz depending on themass of the beamand themass at

he bending beams. After stopping excitation, the decay-des were measured, and the strain dependent dampingMore details about the used damping apparatus ares. [11,12].

and discussion

y of the damping for as aged specimens

litude dependence of damping for CuAl11.4Mn6.9, asat treatment at 850 C for 1h, has been reported in ref.plitude dependence of damping increased signicantlyat treatment. Some aging effectswere reported, aswell.ent work the same specimen was tested after approx-

ears once again. The results are shown in Fig. 1. After 2

boundrationhigh dformatmateriing cootempetreatmare gonheat tr

3.2. Ag

Fig.the (groundincreasCuAl12peratuaging o(see Fishowsment line insampleCuAl

In Fversustial inThe diming hedirectltreatedtreatmture ofincreasing damplitude dependence of damping for CuAl11.4Mn6.9 after differentents. Legend corresponds to the thermal treatment sequence top

ater quenched.

detectablethat the Yomartensite,as cast material. So, the aging process was not nishedays. After the damping measurement of as aged speci-ore homogenization heat treatments were performedample, (2) heat treatment at 800 C for 1h and after-eat treatment at 850 C for 1h, both with subsequentching. The results show that all these heat treatmentse thedampingproperties after therst homogenizationent as published in ref. [4] before 2 years. So ageing cand by homogenization heat treatments. As can be seen inpproximately 60min the damping curve is on a similardamping of the specimen aged for 2 years. So ageingbe activated again after annealing at high temperatureenization.eing damping can be increased again by thermal treat-w temperatures too. This can be seen in Fig. 1 for aent at 150 C for 10min, whereas heat treatments at00 C did not increase the damping again. The reasonsosed effects of temperature at low temperature (ageingC) and high temperature (restoration of high dampingC) could be kinetic (diffusion) and temperature depen-ermodynamic equilibrium, respectively. More concrete

ean (1) that diffusing solid solutes to phase or twinat low temperature causes ageing and that the evapo-ese atoms at higher temperatures causes restoration ofng. But this could also mean (2) that the phase trans-o austenite takes place between 100 and 150 C for agede reverse transformation in this case would occur dur-and restore high damping before measurement at roome. It is important to notice that between the third heatt 850 C and the rst damping measurements 30minpresumably the damping immediately after a suitableent is much higher.

mechanisms and change of transition temperatures

hows max, the height of the relative maximum inrve at (c) = max (compare with [4]), damping back-(=0) and square of resonant frequency f2 against

time during ageing obtained for different heat treated5.0. It can be clearly seen that the heat treatment tem-s a big inuence on ageing. In particular the completeoy CuAl12.3Mn5.0 heat treated at 920 C is maximum). The same heat treatment temperature dependencetrain independent component of the logarithmic decre-e lowdamping of as cast state,marked by the horizontal2a, is due to a high austenite volume fraction in thea low grain size. It was already published [4,5,13] for

SMA that grain size can inuence the damping level.b the square of the resonant frequencies are plottedng time after different heat treatments. Here an ini-e of the homogenization temperatures can be observed.ions and weights of the specimens did not change dur-atments. Therefore a measured resonant frequency isportional to the Youngs modulus for the variously heatl12.3Mn5.0 specimen. The initial (directly after heatYoungs modulus decreases with increasing tempera-treatmentwhile the initial strain independent dampinggeing leads to increasing Youngsmodulus and decreas-g whereas the nal values depend only weakly or noton the homogenization temperature. It is well knownungs modulus in SMA is higher in austenite than inand is minimum, when stress-induced phase transfor-

184 A. Mielczarek et al. / Materials Science and Engineering A 521522 (2009) 182185

Fig. 2. (a) relaand dampingageing time of

mations canMD no stres

Homogeence nottemperaturand can be dIn Fig. 3 thof CuAl12.3

Fig. 3. Tempecooling for Cuing runcoolinheat treated 8temperature, (

sus temperature. Heating and cooling runs are shown in differentgrey-scales. The occurring peaks indicate phase transitions duringheating or cooling, respectively. Depending on the heat treatmentup to tree peaks corresponding to tree phase transitions clearly sep-arated by thknown for Cthat austentype or one[6,7,12]. Thtwo differeindicationsformation crun after hing run). Thtime at RT.was reprodhigher temfound in op

The founenization hisothermalassumed apresent acothat peaks,higher temthe marten

le reatioy poice forestolained 150ing.tive maximum of logarithmic decrement max (i.e. peak in ()) curvebackground 0 versus ageing time in log-log-plot, and (b) f2 versusCuAl12.3Mn5.0 after different heat treatments for 1h.

probabstabilizaries beviden150 Cbe exp100 anquenchoccurwithin the temperature rangeMF

A. Mielczarek et al. / Materials Science and Engineering A 521522 (2009) 182185 185

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(1996) 4597.[8] Y. Sutou, T. Omori, T. Okamoto, R. Kainuma, K. Ishida, J. Phys. IV 11 (2001), Pr8-

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[10] S. Vogelgesang, H. Zak, B. Tonn, A. Mielczarek, W. Riehemann, Int. Found. Res.59 (2007) 2.

[11] J. Gken, W. Riehemann, Technisches Messen 68 (2001) 535.[12] A. Mielczarek, M. Marczyk, W. Riehemann, Solid State Phenom. 137 (2008)

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Eng. A 438440 (2006) 369.

Ageing effects after heat treatment in Cu-Al-Mn shape memory alloysIntroductionExperimentalResults and discussionRecovery of the damping for as aged specimensAgeing mechanisms and change of transition temperatures

ConclusionsAcknowledgmentsReferences