Journal of Alloys and Compounds 396 (2005) 128–132

Some results on shape memory properties ofTi50Ni25Cu25 melt-spun ribbons

G.P. Cheng, Z.L. Xie∗

School of Materials Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore

Received 9 November 2004; received in revised form 1 December 2004; accepted 6 December 2004Available online 11 January 2005

Abstract

The effect of annealing temperature on shape memory properties of Ti50Ni25Cu25 melt-spun ribbons was studied by using differentialscanning calorimetry (DSC) and thermomechanical analyzer (TMA). All the annealed ribbons had B19 structure at room temperature (RT)and B2 above 75◦C. They all showed a well-defined shape memory effect (SME) during thermal cycling from−40 to 120◦C. After annealingat 500◦C, the fully crystallized ribbon showed a shape recovery strain of 1.5% under 45 MPa. With the increase of annealing temperature therecovery strain decreased significantly.©

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2004 Elsevier B.V. All rights reserved.

eywords:TiNiCu alloy; Melt-spun ribbon; Martensitic transformation; Shape memory property

. Introduction

The equiatomic Ti–Ni alloy is well known for the uniqueature of its shape memory effect (SME). Substituting Cu fori in the binary Ti–Ni shape memory alloy lowers the trans-

ormation hysteresis, alters the transformation temperatures,educes the sensitivity of martensitic transformation temper-tureMs to compositional changes and prevents Ti3Ni4 pre-ipitation [1–3]. Fabrication of Ti–Ni-Cu ribbon by melt-pinning technique has been proved suitable for producinglloys with controllable initial structures, such as amorphousr crystalline. Ti50Ni25Cu25 ribbon has been widely stud-

ed because of its small transformation hysteresis and one-tage B2↔ B19 transformation[4]. However, research oni50Ni25Cu25 ribbon has been focused on the microstruc-

ure, crystallization behavior and transformation characteris-ics under different annealing conditions[5–7]. Shape mem-ry properties of this ribbon were reported by Liu[8] andantamarta et al.[9,10]. Santamarta et al.[9,10] compared

he shape recovery strain of Ti–Ni with that of Ti50Ni25Cu25

ribbons, and found that it decreased with the increase ocontent. Liu[8] found that the ribbon annealed at 500◦Cshowed a good superelasticity, but at higher annealingperatures it had poor mechanical properties. Little workbeen devoted to how annealing affects the shape meproperty. And in view of further application it is very impotant to understand the relationship between shape meproperties and the microstructure of this ribbon. In thisper, the effect of annealing temperature on the shape mory properties of Ti50Ni25Cu25 ribbons was studied by usinTMA under different tensile loads and DSC, focusing ontransformation behavior and shape memory effect.

2. Experimental procedures

Ti50Ni25Cu25 (at.%) melt spun ribbon was fabricatedmelt-spinning method. The ribbon had an amorphous instructure. The crystallization temperature was measurbe 450◦C [11]. The as-spun ribbons were annealed atferent temperatures from 500 to 750◦C with an interval o

∗ Corresponding author. Tel.: +65 6790 6921; fax: +65 6790 9081.E-mail address:zlxie@ntu.edu.sg (Z.L. Xie).

50◦C for 15 min, followed by water quench to room temper-ature (RT). Phase transformation characteristics of the rib-

925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.

oi:10.1016/j.jallcom.2004.12.010

G.P. Cheng, Z.L. Xie / Journal of Alloys and Compounds 396 (2005) 128–132 129

bons were determined using a differential scanning calorime-try (DSC) 2920 of TA instrument. Microstructure of the an-nealed ribbons was observed using a JEOL TEM2010 mi-croscope operated at 200 KV with a double-tilt specimenstage.

A thermomechanical analyzer (TMA) was used to deter-mine the shape recovery strain under different constant bi-asing loads during thermal cycling from−40 to 120◦C. Aspecimen was cut along the rolling direction, with a widthof 1 mm and length of 15 mm. Gauge length was 8.5 mm.The specimens were mounted in the grips, which were con-nected to linear variable differential transformers (LVDTs)via quartz rods. They were stressed under a tensile load. Thestrain was measured by LVDT with an accuracy of 0.1�m.Stress was measured by a load cell with an accuracy of 1.0 g.Specimen was kept in a chamber, in which the temperaturewas controlled by utilizing liquid nitrogen and an electricheater. The heating and cooling rate was 10◦C/min.

3. Results and discussions

3.1. Transformation behavior

Transformation temperature, hysteresis and shape recov-ery strain are main properties of shape memory materials. Int e hys-t er-a so res.F spec-t d at5 dur-i angew alingt ature.T sr

A an-n sten-i ess in-cb angesf de-c tureta be-tI is isv

theD1 andc er

Fig. 1. DSC curves of Ti50Ni25Cu25 ribbons annealed at different tempera-tures (a) cooling and (b) heating.

than the second peak during cooling. TEM observation inour previous study[11] has revealed that the microstructureof ribbon annealed at 500◦C consists of mainly nanocrys-talline of about 10 nm in size, with some granular particlesof about 200 nm superimposed in the nanocrystalline ma-trix. The two-stage phase transformation observed should bedue to the grain size difference in the ribbon annealed at500◦C. The first transformation peak inFig. 3 during cool-ing is due to the martensitic transformation of B2 to B19in the larger size particles. The second one is due to thesame transformation in the nano-crystallites. Because thevolume fraction of larger grains is much less than that ofthe nanograins, the exothermic heat of the first transforma-tion is less than that of the second. This results in the firstpeak on the DSC curve of the cooling process inFig. 3 be-ing lower than the second one. Using the same extrapola-tion, we can see that the austenitic transformation also oc-curred at a lower temperature in the nanoscaled grains duringheating.

As reported in our previous TEM study[11], in the rib-bon annealed at 550◦C, the grain size of martensite increasesrapidly to about 500 nm. In addition, the B11-TiCu phase wasfound to precipitate at 550◦C. The grain size of the marten-site and the amount of precipitates were found to increase

he present study the transformation temperature and theresis of Ti50Ni25Cu25 ribbon annealed at different tempture were studied systematically.Fig. 1 is the DSC curvef Ti50Ni25Cu25 ribbon annealed at different temperatuig. 1a and b represent cooling and heating process, re

ively. It can be seen that except for the ribbon anneale00◦C, all the ribbons exhibit a one-stage transformation

ng heating and cooling. Transformation temperatures chith the annealing temperature. With increase of anne

emperature, the DSC peaks are shifted to higher temperhis one-stage transformation is B2↔ B19 transformation aeported in our previous x-ray diffraction study[11].

Fig. 2a shows the transformation temperatures (Ms, Mf ,s andAf ) as a function of annealing temperature. All theealed ribbons have a martensitic structure at RT and au

te above 75◦C. All the four transformation temperaturhow a similar tendency that first increases quickly withreasing annealing temperature from 500 to 550◦C, followedy a slight decrease when annealing temperature ch

rom 600 to 700◦C. The transformation temperaturesrease quickly with further increasing annealing temperao 750◦C. The transformation hysteresis between B2→ B19nd B19→ B2 transformation is defined as the difference

ween peak temperatures of austeniteAp and martensiteMp.n Fig. 2(b), we can see that the transformation hysteresery small and has an average value of 7◦C.

Different from that annealed at higher temperature,SC curve of Ti50Ni25Cu25 ribbon annealed at 500◦C for5 min shows two-stage transformation during heatingooling as shown inFig. 3. The first peak is lower and broad

130 G.P. Cheng, Z.L. Xie / Journal of Alloys and Compounds 396 (2005) 128–132

Fig. 2. Dependence of transformation temperatures (a) and hysteresis (b) onannealing temperatures.

with the annealing temperature. The change in transforma-tion behavior can be directly attributed to the variation ofgrain size and microstructure. At lower annealing tempera-ture, smaller grain size in nano-scale suppresses the marten-sitic transformation to lower temperature. At higher anneal-ing temperature, most grains are in micro-scale. The effectof grain size on the martensitic transformation temperatureis less significant, while the precipitates of B11-TiCu play animportant role in shifting the transformation temperatures tolower values.

Fig. 3. DSC curve of Ti Ni Cu ribbon annealed at 500◦C for 15 min.

Fig. 4. TMA curves of Ti50Ni25Cu25 ribbon annealed at 500◦C under dif-ferent stress.

3.2. Shape memory effect

Thermal cycling tests on annealed Ti50Ni25Cu25 ribbonwere carried out under different tensile biasing loads: 6.3,11.3, 25, 37.5 and 45 MPa. Strain-temperature curves weremeasured under various constant stresses during a thermalcycle from belowMf (−40◦C) to aboveAf (120◦C). Fig. 4shows the strain-temperature curves of the 500◦C-annealedribbon under different loads. The elongation on cooling andshrinkage on heating indicates that the annealed ribbon ex-hibits a well-defined shape memory effect. Under a constantstress, the specimen starts to elongate due to the martensitictransformation atMs and finishes the deformation atMf uponcooling. While it starts to recover the deformation and shrinkdue to the reverse transformation atAs and finishes the re-covery atAf upon heating.

The shape recovery strain was calculated as a differencebetween the elongation in martensitic and austenite phase.The annealed ribbon shows a very small recovery strain underlower applied load (0.2% under 6.3 MPa). With the increaseof applied load, the shape recovery strain increases. Under45 MPa, a recoverable strain of 1.5% is obtained.Ms andAsdetermined from the strain-temperature loop are almost thesame as those determined by DSC (Fig. 2a) under stress-freecondition.

Under a constant stress, the volume fraction of marten-s rmale . Thes acro-s on ist cro-s call cov-e tedm reas-i f thef

50 25 25

ite can be split into a self-accommodating one (pure theffect) and a re-oriented one (thermomechanical effect)elf-accommodating martensite does not produce any mcopic strain, while the re-oriented one whose orientatihe most consistent with the stress tensor will induce macopic transition strain[12]. In the case of thermomechanioading, two types of martensite may be created. The rery strain is proportional to the volume fraction of re-orienartensite. The increase in recoverable strain with inc

ng stress is attributed to an increase in volume ratio oavorably reoriented martensites[1].

G.P. Cheng, Z.L. Xie / Journal of Alloys and Compounds 396 (2005) 128–132 131

Fig. 5. TMA curves of Ti50Ni25Cu25 ribbon annealed at 500◦C underthermo-cycling.

Fig. 5 shows the effect of thermal cycling on the strain-temperature behavior of Ti50Ni25Cu25 ribbon annealed at500◦C under a constant load of 45 MPa. After 10 cy-cles of thermal cycling, the strain-temperature curves donot change significantly. It may be due to the low stressor small number of cycles. It suggests that under such astress there is no formation or rearrangement of lattice de-fects.

The strain-temperature curves under 45 MPa ofTi50Ni25Cu25 ribbon annealed at different tempera-tures are shown inFig. 6. For the ribbon annealed at 500◦C,the recoverable strain is 1.5%. It decreases drastically withthe increase of annealing temperature. Annealed at 750◦C,the ribbon shows a recoverable strain of 0.38%.Fig. 7provides the shape recovery strain as a function of annealingtemperatures under different biasing loads. Under lowerload (6.3 and 11.3 MPa), recovery strain decreases slowlywith the annealing temperature. It may be due to the smallvolume of reoriented martensite under low stress. When

F ra-t

Fig. 7. Shape recovery strain under different stress for Ti50Ni25Cu25 ribbonannealed at different temperatures.

stressed under higher load (>11.3 MPa), there is a sharpdecrease of recovery strain with annealing temperatureincreasing from 500 to 550◦C. The recovery strain increasesa little with further increasing annealing temperature to750◦C. Higher temperature annealing increasesMs, butdeteriorates the shape recovery strain. So it is better tokeep the annealing temperature below 550◦C for a goodcombination of shape memory properties (highMs andrecovery strain).

Since recoverable deformation in shape memory alloys isgenerally accomplished by means of reorientation of marten-site variants, the recoverable elongation decreases with de-creasing volume fraction of favorably oriented variants[1].The decrease of shape recovery strain with the annealingtemperatures can be attributed to several factors such asgrain boundaries (or grain size), crystal structure, marten-site microstructure, texture and internal stress field existingin the ribbons. As reported in our previous paper[11], asingle-pair martensite variant with strong preferred orienta-tion was observed in most of the grains. Martensite grainsize increased with increasing annealing temperatures. Atlower annealing temperature the ribbons consisted mainlyof the B19 phase with (1 1 1) preferential orientation. Highertemperature annealing reduced the texture in the B19 phaseand assisted the preferential orientation of the (1 1 0) B11-TiCu phase. Strong orientation-dependent behavior of trans-f lsobI them wera xturei hera tatesc tudyi verys

ig. 6. TMA curves of Ti50Ni25Cu25 ribbon annealed at various tempeures under 45 MPa.

ormation strain for B2 to B19 in Ti–Ni-Cu alloys has aeen predicted by Sehitoglu et al.[13] and Nam et al.[1].

n this study, grain size, texture and precipitates areain factors affecting the shape recovery strain. At lonnealing temperature, small grain size and strong te

n martensite favor the recovery strain. While at hignnealing temperature preferentially orientated precipionfine the reorientation of martensite twin. Further ss needed to understand how texture affects the recotrain.

132 G.P. Cheng, Z.L. Xie / Journal of Alloys and Compounds 396 (2005) 128–132

4. Conclusions

All the annealed ribbons exhibited a well-defined shapememory effect. Under 45 MPa, a recovery strain of 1.5% wasobtained in the ribbon annealed at 500◦C. The recovery straindecreased with the increase of annealing temperature. Afterannealing at 750◦C, the recovery strain was 0.38%.Ms firstincreased and then decreased with annealing temperatures.Annealing below 550◦C is good for a highMs and a goodshape memory effect.

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