Materials Science and Engineering A334 (2002) 49–52

Structural change due to martensite aging of CuZnAlMnNi shapememory alloy

Yu-Jun Bai a,b,*, Xian-Gang Xu b, Yu-Xian Liu b, Li-Mei Xiao b, Gui-Li Geng b

a Mechanical Department, Shandong Uni�ersity of Science and Technology, Jinan 250031, Shandong, People’s Republic of Chinab Institute of Materials Science and Engineering, Shandong Uni�ersity, Jinan 250061, Shandong, People’s Republic of China

Received 27 March 2001; received in revised form 23 July 2001

Abstract

The microstructures of a CuZnAlMnNi shape memory alloy after one year aging in martensite phase were investigated bytransmission electron microscopy. It was found that the substructure of stacking faults in the original martensite plates becomesindistinct, and the amount decreases. However, the equilibrium face centered cubic (fcc) �-phase can be observed forming atmartensite plate boundaries or inside the plates. The structural variation during aging is responsible for the degradation of shapememory property and transformation temperatures when Cu-based SMA actuators are in use. © 2002 Elsevier Science B.V. Allrights reserved.

Keywords: Martensite aging; CuZnAlMnNi shape memory alloy; �-phase; Transmission electron microscopy

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1. Introduction

Cu-based shape memory alloys (SMA) have beenpaid more attention in the past few years owing to theirlow price, easy fabrication and excellent conductivity ofheat and electricity. However, the shape memory effect(SME) of the alloys is susceptible to aging whether inparent phase or in martensite phase, which affects thewide application of the alloys. There have been a lot ofreports on the aging in parent phase of Cu-based SMAs[1–3]. Wei et al. [1] investigated the structural change ofCuAlNiMnTi alloy during parent phase aging, andfound that the ordered parent phase first transformsinto the solute-depleted bainite with a M18R typeordered structure and then the bainite decomposes intofcc �-phase and the remaining parent matrix into cubic�2 phase, resulting in a final destruction of long rangeorder and ultimate loss of the shape memory capacityof the alloy. It is reported that martensite stabilizationoccurs during martensite aging of Cu-based SMAs,which leads to the rise of reverse martensitic transfor-mation temperature and the decrement of SME [4–6].However, few reports are available concerning the

structural change during martensite aging of Cu-basedSMAs. We observed by transmission electron mi-croscopy (TEM) the structural change of a CuZ-nAlMnNi SMA after one year aging in martensitephase, and found some new microstructure features.The investigation contributes to interpreting the degra-dation of shape memory property and transformationtemperatures during long duration of application of thealloys.

2. Experimental

The alloy investigated has a composition of Cu–23.6Zn–4.47Al–0.23Mn–0.17Ni (wt.%). The samplewas solution treated at 840 °C for 20 min, quenchedinto boiling water for 30 min and then air cooled toambient temperature. The purpose of step quenching isto obtain fully the ordering martensite. The characteris-tic transformation temperatures measured by differen-tial scanning calorimeter (DSC) at the rate of 10 °Cmin−1 are: Ms=39 °C, Mf=22 °C, As=45 °C, Af=63 °C. The quenched sample was cut into two parts—one for the immediate test and the other for aging atambient temperature. The microstructures were ob-served on a H800 transmission electron microscope,

* Corresponding author.E-mail address: byj97@263.net (Y.-J. Bai).

0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0921 -5093 (01 )01761 -0

Y.-J. Bai et al. / Materials Science and Engineering A334 (2002) 49–5250

whose specimens were jet-polished with 33.3% nitricacid and 66.7% methanol solution.

3. Results and discussion

Fig. 1 shows the bright field image of TEM mi-crostructure and the corresponding diffraction patternof the CuZnAlMnNi alloy immediately after quench-ing. The stacking faults in martensite plates are clearlydiscernible, and they arrange approximately parallelmutually in the same plate. This type of substructure isfavorable to the SME of the alloy [7–9]. From thediffraction pattern depicted in Fig. 1(b), the microstruc-ture quenched is M18R martensite, which is consistentwith the X-ray diffraction (XRD) results [10]. The zoneaxis by calculation is [292� ]M.

Fig. 2(a) exhibits the bright field image of TEMmicrostructure after one year aging in martensite phaseof the CuZnAlMnNi alloy quenched. It is difficult todistinguish the stacking faults in martensite plates, in-stead a new phase can be observed forming in theplates. Fig. 2(b) is the related diffraction pattern to Fig.2(a), where two sets of spots can be seen. The analysisresults demonstrate that one set of the spots arises fromthe M18R martensite matrix, whose zone axis is [292� ]M,the other from the fcc �-phase with the zone axis of[111]�. The orientation relationships between the twophases are: (2� 02)�//(1� 28)M, [111]�//[292� ]M. After slightinclining of the same sample, the spots produced byM18R martensite are almost invisible and only thosefrom �-phase with the zone axis of [110]� remains, asdescribed in Fig. 2(c). The fact that the microstructurechanges occur during martensite aging demonstratesthat the microstructure quenched is unstable even afterfull ordering. It is known that martensite obtainedduring rapid quenching is a supersaturated solid solu-

tion, i.e. a metastable phase, aging in martensite phaseis in fact a process during which the solute atomssegregate slowly from martensite and martensite tendsto transform gradually into equilibrium phase. The�-phase observed in the alloy after one year aging atambient temperature is just the equilibrium phase trans-formed from martensite. The variation of microstruc-ture inevitably gives rise to the decrement of theamount of reversible martensite, and further to thereduction of SME of the alloy. Meanwhile, the orderdegree of the alloy decreases with the formation of�-phase because the �-phase has a disordered fcc struc-ture. With the segregation of solute atoms from thesupersaturated solid solution during aging, the amountof the solute atoms retaining in martensite phase re-duces gradually, resulting in the rise of reverse marten-sitic transformation temperature. As a result, thestructural change during martensite aging is the originof the degradation of shape memory property andtransformation temperatures after long duration of ap-plication of Cu-based SMA actuators.

Fig. 3(a) is the bright field image of TEM microstruc-ture in another selected area after 12 months aging. Thesubstructure of stacking faults becomes indistinct,meanwhile �-phase forms at martensite plateboundaries and grows inside the plates, which is partic-ularly evident in the dark field image, as shown in Fig.3(b). Fig. 4(a) exhibits a spherical microstructure occur-ring in the alloy aged for 1 year. The contrast ofstacking faults almost disappears. From the corre-sponding diffraction pattern shown in Fig. 4(b), it canbe known that the spherical phase is also the equi-librium fcc �-phase, the zone axis is [111]�. Fig. 5 showsthat most of the martensite phase has transformed into�-phase. The substructure of stacking faults arrayedtidily in the original martensite plates can hardly beseen. It is thus evident from these results that the

Fig. 1. TEM microstructure of the CuZnAlMnNi alloy immediately after quenching: (a) bright field image; (b) the corresponding diffractionpattern with zone axis of [292� ]M.

Y.-J. Bai et al. / Materials Science and Engineering A334 (2002) 49–52 51

Fig. 2. TEM microstructure after one year aging in martensite phase of the CuZnAlMnNi alloy quenched: (a) bright field image; (b) and (c) thecorresponding diffraction pattern with zone axis of [292� ]M, [111]� and [110]�, respectively.

Fig. 3. �-phase forms at martensite plate boundaries and grows inside the plates: (a) bright field image; (b) dark field image.

reduction of SME during martensite aging of Cu-basedSMAs results partly from the substructure change.

4. Conclusion

After martensite aging for 1 year, the microstructureof CuZnAlMnNi SMA quenched changes greatly. The

substructure of stacking faults in the original martensiteplates becomes indistinct, and the amount decreases.Instead the equilibrium fcc �-phase can be observedforming at martensite plate boundaries or inside theplates. The structural variation during aging providesdirect evidence to explain the degradation of shapememory property and transformation temperatureswhen Cu-based SMA actuators are in use.

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Fig. 4. Spherical microstructure formed during martensite aging: (a) bright field image; (b) the corresponding diffraction pattern with zone axisof [111]�.

Fig. 5. Most of martensite has transformed into �-phase in theselected zone.

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