Superelasticity in TiNi Alloys and Its Applications in Smart Systems. Wei Cai, Yufeng Zheng, Xianglong Meng and Liancheng Zhao

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1 Materials Science Forum Vols (200) pp online at Trans Tech Publications, Switzerland Superelasticity in TiNi Alloys and Its Applications in Smart Systems Wei Cai, Yufeng Zheng, Xianglong Meng and Liancheng Zhao Department of Materials Physics and Chemistry, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 10001, China, Keywords: TiNi alloy, Superelasticity, Application, Smart system Abstract. The superelasticity is one of the most important properties of TiNi alloy, which has been widely used in the smart systems of many fields, such as aerospace, aviation, biomedicine and energy etc. The current state of superelasticity of TiNi alloy has been reviewed with emphasis on the superelasticity, phase transformations, thermo-mechanical treatment and their relationship. The mechanisms of linear superelasticity and non-linear superelasticity have been revealed. Some successful applications based on the superelasticity of TiNi alloys in smart systems have been introduced. Introduction TiNi Shape memory alloys (SMAs) attracted many attentions in recent years due to the excellent shape memory effect (SME) and superelasticity caused by martensitic transformation and its reverse transformation. Superelasticity, which is a pseudoelasticity occurring at a temperature above A f, is caused by stress-induced martensitic transformation upon loading and by the subsequent reverse transformation upon unloading [1]. Superelasticity of SMA includes linear superelasticity and non-linear superelasticity. Up to now, the superelasticity of TiNi SMAs has been widely investigated and used in many systems [2-]. It has been proved that the superelasticity of TiNi alloys is strongly dependent on composite and thermal-mechanical treatment. In the present paper, the linear and non-linear superelasticity and the corresponding mechanisms and their applications of TiNi alloys have been revealed and discussed. Superelasticity in TiNi alloys 1. Linear superelasticity Linear superelasticity can be obtained in the neutron irradiated or cold deformed equiatomic TiNi alloys and solution-treated or low temperature aged Ni-riched TiNi alloys. For the cold worked TiNi alloys, the complete linear superelasticity only can be achieved after several stress-strain cycles. Figure 1 shows the stress strain curves of the 1% cold-drawn Ti 9.8at.%Ni alloy tensioned to.% at 20 o C under different cycling numbers. It could be seen that the residual plastic deformation exists after the first cycle. With increasing the number of cycles, the residual plastic deformation strain decreases gradually. After six stress-strain cycles, the residual plastic deformation strain is zero and the complete linear superelasticity is obtained. Figure 2 shows the effect of cold drawn strain on the complete linear superelasticity in the Ti 9.8at.%Ni alloy. It is found that the linear superelasticity reaches the maximum value when the cold drawn strain is 22%. Further increasing the cold drawn strain, the linear superelasticity keeps constant. Licensed to Yufeng Zheng (yfzheng@hit.edu.cn) - Peking University - China P.R. All rights reserved. No part of the contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, (ID: /01/0,10:09:11)

2 1916 PRICM- Deformation temperature has an obvious effect on the superelasticity of cold drawn TiNi shape memory alloy. Figure illustrates the influence of deformation temperature on the linear superelasticity of Ti 9.8at.%Ni alloy during tensile deformation. When the deformation temperature is in the range between 8 o C (M s ) and 12 o C (A f ), the linear superelasticity decreases with increasing the deformation temperature. Moreover, when the deformation temperature is lower than 8 o C or higher than 12 o C, the linear superelasticity keeps stable with changing the deformation temperature. Figure 1 Stress-strain curves of the 1% cold-drawn Ti 9.8at.%Ni alloy tensioned at 20 o C Cold Drawn Strain, % Figure 2 Effect of cold drawn strain on the complete linear superelasticity in the Ti 9.8at.%Ni alloy Deformation Temperature, o C Figure Effect of deformation temperature on the complete linear superelasticity in the Ti 9.8at.%Ni alloy In-situ TEM observation in tension has been carried out to illustrate the variation of the microstructure under different deformation conditions, as shown in Figure. The TEM micrograph after unloading is omitted since it is almost identical to Fig. (a). The microstructural change during loading and unloading sequentially can be deduced as follows. Before tension, few fill-in plates could be found inside the <011>Type II twinning substructure, as shown by Fig. (a,b). Under the action of the applied stress, new planar twinning plates nucleate and grow up inside the substructural bands in the favorable orientation, then these newly generated twinning strips broaden, associated with the formation of some other new thin plates with the increase of the applied stress, as shown by Fig. (c e). Fig. (f), the corresponding diffraction pattern taken from the newly-formed martensite plates, indicates the twinning relationship is (001) compound type. During the unloading, these needle-like (001) twinning martensite plates shrink back and the whole observed area returns to its initial condition, mostly when the load is released. These results suggest that the linear superelastic behavior of the moderately cold-drawn specimen is caused by the reversible motion of twin boundaries, which associates with the appearance and disappearance of (001) deformation microtwins upon loading and unloading, respectively. The formation of the microtwins is attributed to the fineness of the lamellar martensite crystals due to cold-drawing and the existence of the immobile boundaries between the lamellae.

3 Materials Science Forum Vols Figure TEM in-situ observations showing the microstructural changes with the applied stress in the specimen subjected to 1% area reduction at 20 C. (a) Bright-field image before tensile deformation. (b) EDPs taken from the framed area in (a), electron beam // [ 1 10] M // [101] T. (c) Bright-field image corresponding to a strain of approximately 0.8%. (d) Bright-field image corresponding to a strain of approximately 1.8%. (e). Bright-field image corresponding to a strain of approximately 2.%. (f) EDPs derived from area A in (e), electron beam//[ 1 10] M,T 2. Non-linear superelasticity It is well known that when TiNi SMAs are deformed in temperature range above A f the corresponding stress-strain curves exhibit stress plateau upon loading due to stress-induced martensitic transformation. The strain recoveries upon unloading due to the reverse martensitic transformation. This is so-called non-linear superelasticity. For TiNi alloys, the method to get excellent non-linear superelasticity is thermomechanical treatment (cold rolling + proper annealing). Figure shows the stress-strain curves of cold drawn Ti-9.8at.%Ni alloy deformed at different temperatures. It is seen that when the cold drawing reduction is less than 6.9%, no complete non-linear superelasticity is observed in the temperature range from 27 o C to 100 o C. However, when the cold drawing reduction exceeds 22%, complete non-linear superelasticity appears over a temperature range. It also can be found that, increasing the amount of cold drawing, the temperature range for the occurrence of complete non-linear superelasticity is widened. It is considered that the improvement of the non-linear superelasticity in TiNi alloys through the thermomechanical treatment is attributed to the increase of the resistance to slip deformation in the parent phase.

4 1918 PRICM- Figure Stress-strain curves of Ti-9.8at.%Ni alloy with different cold drawn strain (a) 6.9%, (b) 22%, (c) 1%, (d) 9% Figure 6 depicts the effect of stress-strain cycling and annealing temperature on the non-linear superelasticity in the Ti-9.8at.%Ni alloy with the cold drawing reduction of 8.%. Clearly, when the Ti-9.8at.%Ni alloy is annealed at a proper temperature (0), the non-linear superelasticity strain is high and show excellent stability during the stress-strain cycling. Figure 7 shows the effect of stress-strain cycling and annealing temperature on the non-linear superelasticity in the Ti-9.8at.%Ni alloy with the cold drawing reduction of 8.%. For the specimens with the same thermomechanical treatment, the number of stress-strain cycles for stable non-linear superelasticity increases when the deformation temperature increases. At the same time, the maximum stable non-linear superelasticity decreases. 7 6 annealing temperature 0 o C 0 o C 0 o C 6 deformation temperature o C 6 o C 7 o C Number of cycles, N Figure 6 Effect of stress-strain cycling and annealing temperature on the non-linear superelasticity in the Ti-9.8at.%Ni alloy with the cold drawing reduction of 8.% Number of Cycles, N Figure 7 Effect of stress-strain cycling and deformation temperature on the non-linear superelasticity in the Ti-9.8at.%Ni alloy with the cold drawing reduction of 8.%

5 Materials Science Forum Vols All in all, the characteristics of non-linear superelasticity in TiNi alloys can be influenced remarkably by thermomechanical treatment, which strengthens the parent phase and prohibits the slip of the dislocations. Applications of TiNi superelasticity in smart systems Up to now, more than 10,000 patents which include the SMAs have been proposed. Among them, more and more applications utilizing the superelasticity are presented in the recent years. In the following, we would introduce several new application of superelasticity in TiNi alloys. Figure 8 shows the pictures of TiNi torsion springs which can output the constant moment of forces. Compared with the steel torsion spring, the moment of the TiNi torsion spring exhibits the non-linear superelasticity characteristics when increasing the torsion angel due to the superelasticity of TiNi alloy. This indicates that the TiNi torsion spring would release less energy upon unloading than conventional steel torsion spring. TiNi torsion spring with constant elasticity can be used to open the door of satellite or to release the antenna to effectively reduce the impact and vibration in order to improve the safety of the whole system. Recently, a lot of non-metal based composites were used because of their light weight and high strength. But they are usually brittle and easy to crack during overloading. Therefore, it is required to connect the non-metal based composite components with constant force to make sure that the connection force keeps constant when the joint is loaded. TiNi superelastic components can be used to achieve this aim. Figure 9 is the schematic diagram to illustrate to connect the non-metal based composite components with TiNi superelastic rings and pipes. TiNi superelastic rings can distribute the stress uniformly to avoid stress concentration. Thus, even the stress concentration occurs, the superelasticity of TiNi rings would be induced to release the stress, resulting in protecting the non-metal based composite components. In addition, some superelastic TiNi bolts also can be used for constant force connections. SMA rings Composite pipes SMApipe Composite pipes Figure 8 Picture of TiNi superelastic torsion springs Figure 9 Schematic diagram of connection of non-metal based composite pipes with TiNi superelastic components Figure 10a) shows the illustration of superelastic TiNi oil-well packer. When the superelastic TiNi packer is used in the oil-well, a cone expand the packer under an axial load to achieve the seal. At the same time the packer is fixed by the friction between the packer and the pipe. When unsealing, the cone is taken out by some tools, and the superelastic TiNi packer recover to its original shape due to the superelasticity. Thus the packer and the pipe is separated each other. Compared with the conventional rubber packer for oil-well, the superelastic TiNi oil-well packer

6 1920 PRICM- has the following advantages: 1) long application life, 2) stable and reliable properties, ) wide application temperature range and ) large inner diameter. Fig. 10b) shows the picture of the superelastic TiNi oil-well packer. Cone TiNi packer pipe (a) (b) Figure 10 Application of TiNi superelastic components in oil-well (a) schematic diagram of TiNi superelastic packer for oil-well, (b) picture of TiNi superelastic packer for oil-well Superelastic TiNi alloy also can be used as high damp components to reduce the vibration. Upon loading, the stress-induced martensitic transformation occurs in the superelastic TiNi components and the energy of vibration is absorbed. Summary In the present paper, we revealed the mechanisms and the influence factors of linear and non-linear superelasticity in TiNi alloys. It is found that the excellent superelasticity must be obtained under some certain thermomechanical treatment. Besides, several successful applications of superelasticity of TiNi alloys have been introduced. References [1] K. Ostuka, X. Ren: Intermetallics Vol. 7 (1999), p.11 [2] L.C. Zhao, W. Cai and Y.F. Zheng: Shape Memory Effect and Superelasticity in Alloys (National Defence Industry Press, China 2002) [] Y.F. Zheng, B.M. Huang, J.X. Zhang and L.C. Zhao: Mater. Sci. Eng. A Vol. 279 (2000), p. 2 [] C.S. Zhang, Y.Q. Wang, J.X. Cheng and L.C. Zhao: Proceedings of the First International Conference on Shape Memory and Superelastic Technologies, Asilomar Conference Center, Pacific Grove, California, USA, (199), p.8