Materials Science and Engineering A 425 (2006) 268–271

Texture and shape memory property of annealedTi50Ni25Cu25 ribbons

G.P. Cheng a, Z.L. Xie a,∗, Y. Liu b

a School of Materials Science and Engineering, Nanyang Technological University, Nanyang Avenue, 639798, Singaporeb School of Mechanical and Aerospace Engineering, Nanyang Technological University, Nanyang Avenue, 639798, Singapore

Received 3 March 2006; received in revised form 23 March 2006; accepted 24 March 2006

Abstract

Texture development and shape recovery strain of the annealed Ti50Ni25Cu25 ribbons were studied by using XRD and TMA. A pure strong[2 1 1] fiber texture existed in the 450 ◦C-annealed sample. Annealing at 550 ◦C and above changed the texture component in B19 martensite to(1 1 1)[7 1 5] and caused the formation of B11-TiCu precipitates. Shape recovery strain of the annealed ribbons decreased with the increase ofannealing temperature.© 2006 Elsevier B.V. All rights reserved.

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eywords: Shape memory alloy; Ti–Ni–Cu ribbon; Texture; Shape recovery strain

. Introduction

Shape memory effect (SME) is a unique property of shapeemory alloys (SMAs). TiNi alloys are important shape mem-

ry alloys with good SME. It is reported that shape recoverabletrain induced by martensitic transformation showed strong ori-ntation dependence in Ti–Ni alloys [1,2].

Melt-spinning technique is widely used to produce Ti–Ni–Culloys with Cu content more than 20%. The rapid quenchinguring spinning causes grain refinement and changes grain mor-hology from equi-axial to columnar [3]. This results in theormation of texture and affects the mechanical properties. Thehape memory properties of Ti50Ni25Cu25 melt-spun ribbonsere studied by Santamarta et al. [4,5] and Liu [6]. Santamarta

t al. [4,5] compared shape recovery strain of Ti–Ni alloys withhat of Ti–Ni–Cu ribbons and found textures affected SME.iu [6] found it had good super-elasticity at lower annealing

emperature, but poor mechanical properties at higher anneal-ng temperature. Little work has been devoted to the effect ofnnealing on texture development in the ribbon. In this paper, theffect of annealing temperature on the evolution of texture and

by using XRD and TMA under different tensile loads. The rela-tionship between texture components and shape memory effectwas investigated.

2. Experimental

Ti50Ni25Cu25 (at.%) ribbon was fabricated by melt-spinningmethod. It had an amorphous initial structure. Crystallizationtemperature was measured to be 450 ◦C [7]. The as-spun ribbonwas annealed at different temperatures from 450 to 750 ◦C withan interval of 50 ◦C for 15 min, followed by water quenching toroom temperature (RT).

Incomplete pole figures were measured at RT using CuK� radiation for the reflection method with tilting angle from30◦ to 90◦. Sample size was 18 mm × 18 mm. In pole figuremeasurement, RD is the spinning direction and TD is perpen-dicular to the spinning direction. Texture components weredetermined by comparing the measured pole figure with thestandard stereographic projection of the corresponding crystalstructure.

Shape recovery strain of the annealed ribbons was deter-

hape memory properties of Ti50Ni25Cu25 ribbons was studied

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

mined by measuring the change of elongation versus tem-perature under constant tensile load, using thermo-mechanicalanalyzer (TMA). Load was applied along the spinning direc-tion. Tensile test sample was 1 mm in width and 15 mm inlength.

921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2006.03.054

G.P. Cheng et al. / Materials Science and Engineering A 425 (2006) 268–271 269

Fig. 1. Pole figures of the 450 ◦C-annealed Ti50Ni25Cu25 ribbon.

3. Results

3.1. Texture in the 450 ◦C-annealed ribbon

After annealing at 450 ◦C for 15 min, all the Ti50Ni25Cu25ribbons showed a crystalline structure. It was reported in ourprevious study [7] that only B19 martensite existed in the 450 ◦C-annealed ribbon. The high intensity ratio of 1 1 1/0 0 2, 1 1 1/0 2 2and 1 1 1/1 3 1 revealed a strong preferred orientation developedin this crystallized ribbon.

The pole figures of {0 0 2}, {1 1 1}, {0 1 2} and {0 2 2} ofB19 are shown in Fig. 1. The center of the pole figures corre-sponds to the normal direction of the sample surface (ND). Thepole figures showed how the respective pole density distributedin the specimen coordinate system RD–TD–ND. The maximumrelative pole intensity Imax was indicated in the figures.

From Fig. 1, it is known that the pole figures of {0 0 2},{0 1 2} and {0 2 2} planes showed a considerably uniform poledensity distribution. Only {1 1 1} pole figure showed a clus-tered density distribution with two peaks symmetrically locatedwithin 5◦ from ND. The maximum relative intensity was 31.7.By comparing the measured pole figures with the standard stere-ographic projection of B19 lattice, a [2 1 1] fiber texture wasdetermined. The ribbon surface was nearly parallel to {1 1 1}planes of B19 (about 5◦ away). This indicates that B19 marten-site grains grew column-like in the ribbon. And they distributedrp

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Fig. 2. {1 1 1} pole figure of Ti50Ni25Cu25 ribbon annealed at 500 ◦C.

above 500 ◦C showed a mixture of B19 martensite and B11-TiCuphase. The position of diffraction peaks did not change withannealing temperature, which means lattice parameters of B19and B11-TiCu phase were identical for all the annealed ribbons.However, the relative intensity of diffraction peaks changed withthe increase of annealing temperature. At lower annealing tem-perature ribbons consisted of mainly B19 phase with (1 1 1)preferential orientation. High temperature annealing weakenedthe texture in B19 phase and assisted the formation of B11-TiCuphase.

Texture development in the annealed ribbon was investigatedby measuring pole figures of {0 0 2}, {1 1 1}, {0 1 2} and {1 3 1}planes of B19 martensite.

In the ribbon annealed at 500 ◦C, only {1 1 1} pole figureshowed a clustered distribution around ND (shown in Fig. 2).The pole figures of {0 0 2}, {0 1 2} and {1 3 1} showed randomdistribution. Compared with the pole figures of the 450 ◦C-annealed ribbon, the position of the maximum pole density of{1 1 1} in Fig. 2 did not change. But the maximum relative inten-sity of {1 1 1} pole decreased from 31.7 to 9.5. This indicatesthat the texture component of the ribbon annealed at 500 ◦C wasstill a [2 1 1] fiber texture. But it was weaker than that of the450 ◦C-annealed ribbon.

Fig. 3 is the pole figures of {1 1 1} and {0 1 2} in the ribbonannealed at 550 ◦C. It can be seen that the clustered distributionof {1 1 1} is similar to that of the 500 ◦C-annealed sample. Inadfp

otationally around [2 1 1] direction, which is ND of the ribbonlane.

.2. Textures development in B19 phase as a function ofnnealing temperature

The evolution of crystal structure with annealing tempera-ure was reported in our previous study [7]. Samples annealed

ddition, the pole figure of {0 1 2} showed obvious clusteredensity distribution with one pole density peak located at 50◦rom ND along RD. The maximum relative intensity of {0 1 2}ole is 36.9. Qualitative analysis revealed that the texture compo-

Fig. 3. Pole figures of Ti50Ni25Cu25 ribbon annealed at 550 ◦C.

270 G.P. Cheng et al. / Materials Science and Engineering A 425 (2006) 268–271

nent in this ribbon was a (1 1 1)[7̄ 1 5] sheet texture. (111) planeof B19 was still nearly parallel to ribbon surface. Therefore,increasing annealing temperature did not change the orientationof (1 1 1) plane of B19 phase, but changed the texture componentfrom fiber to sheet texture.

The pole figures of {0 0 2}, {1 1 1}, {0 1 2} and {1 3 1} inthe ribbon annealed at 600, 650 and 700 ◦C were also measured.They shared similar distribution of maximum pole density tothose of the 550 ◦C-annealed ribbon. The maximum relativeintensity of {1 1 1} pole in the 600, 650 and 700 ◦C-annealedribbons was 5.3, 4.9 and 4.8, respectively. Therefore, the texturecomponent did not change with further annealing: (1 1 1)[7̄ 1 5]sheet texture, but its intensity decreased.

It can be seen that texture components and intensity changedwith the annealing temperature. Annealing below 550 ◦Cfavored [2 1 1] fiber texture, while higher temperature annealingbenefited (1 1 1)[7̄ 1 5] sheet texture. But {1 1 1} plane of B19phase is nearly parallel to the ribbon surface in all the annealedribbons.

Kim et al. [8] reported that the texture development inthe annealed cold-rolled Ti–Nb–Ta alloy was due to recrys-tallization. From the XRD results [7], it is known thatB11 TiCu precipitates formed above 550 ◦C. The changeof texture with annealing temperature may be due to therestrained orientation of martensite by the formation of thesep

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Fl

Fig. 5. Shape recovery strain of Ti50Ni25Cu25 ribbon annealed at different tem-peratures under different loads.

specimen started to elongate due to the martensitic transforma-tion at Ms and finished the deformation at Mf upon cooling.While it started to recover the deformation and shrink due to thereverse transformation at As and finished the recovery at Af uponheating.

The shape recovery strain was calculated as a differencebetween the elongation in martensitic and austenite phase. Theannealed ribbon showed a very small recovery strain under alower applied load (0.2% under 6.3 MPa). With the increase ofapplied load, the shape recovery strain increased. Under 45 MPa,a recoverable strain of 1.5% was obtained.

Fig. 5 provides the shape recovery strain as a function ofannealing temperatures under different biasing loads. Under alower load (6.3 MPa), recovery strain decreased slowly with theannealing temperature. It is due to the small volume of reori-ented martensite under low stress. When stressed under a higherload (45 MPa), there was a sharp decrease of recovery strainwith increasing annealing temperature from 500 to 550 ◦C. Forthe ribbon annealed at 450 ◦C, the recoverable strain was 1.8%.It decreased drastically to 0.38% for the sample annealed at750 ◦C.

As observed in the texture analysis, the 450 ◦C-annealed rib-bon had a pure sharp [2 1 1] fiber texture with strong intensity.At annealing temperature 550 ◦C and above, a sheet texturecomponent (1 1 1) [2̄ 1 1] existed and the intensity of tex-ture decreased. In addition, B11-TiCu precipitates formed intrcrte

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recipitates.

.3. Shape recovery strain

Thermal cycling tests on the annealed Ti50Ni25Cu25 ribbonsere carried out under different tensile biasing loads: 6.3, 11.3,5, 37.5 and 45 MPa. Strain–temperature curves were mea-ured under various constant stresses during a thermal cyclerom below Mf (−40 ◦C) to above Af (120 ◦C). Fig. 4 showshe strain–temperature curves of the 500 ◦C-annealed ribbonnder different loads. The elongation on cooling and shrink-ge on heating indicate that the annealed ribbon exhibited aell-defined shape memory effect. Under a constant stress, the

ig. 4. TMA curves of Ti50Ni25Cu25 ribbon annealed at 500 ◦C under differentoads.

he ribbons annealed above 550 ◦C act as an obstacle to theeorientation of martensite. The combination of texture and pre-ipitate determined the shape recovery strain of the annealedibbons. Our observations indicate B19 martensite with strongexture and no precipitates was beneficial to shape memoryffect.

The [2 1 1] fiber texture has its fiber axis perpendicular to theibbon surface. Crystalline orientation distribution is uniformn the ribbon surface so that the transformation anisotropy iseak. The isotropic transformation strain on the ribbon surface

s convenient for designing micro-actuators.

G.P. Cheng et al. / Materials Science and Engineering A 425 (2006) 268–271 271

4. Conclusions

Texture evolution in the annealed Ti50Ni25Cu25 ribbonswas observed. Qualitative analysis of the pole figures revealedthat texture components changed with the annealing temper-ature. Annealed at lower temperature (450 ◦C), a pure strong[2 1 1] fiber texture existed in the sample. Annealing at 550 ◦Cand above changed texture components to a (1 1 1)[7̄ 1 5]sheet texture. In this study, strong textured B19 marten-site without precipitates is beneficial to the shape memoryproperty.

References

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35A (2004) 761.[5] R. Santamarta, J. Pons, E. Cesari, J. Phys. IV 11 (2001) 351.[6] Y. Liu, Mater. Sci. Eng. A 354 (2003) 286.[7] G.P. Cheng, Z.L. Xie, Y. Liu, J. Alloys Compd., in press.[8] H.Y. Kim, T. Sasaki, K. Okutsu, J.I. Kim, T. Inamura, H. Hosoda, S.

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