Microstructures and Shape Memory Behavior of a TiNi Alloy Tube ...

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ScienceDirect Materials Today: Proceedings 2S (2015) S775 – S778

International Conference on Martensitic Transformations, ICOMAT-2014

Microstructures and shape memory behavior of a TiNi alloy tube made from a Ni-coated Ti wire Y. Kishia,*, Z. Yajimaa, C. Shiomib, T. Matsumotob, T. Ohigashib and Y. Nishimotob a

AMS R and D Center, Kanazawa Institute of Technology, 3-1 Yatsukaho, Hakusan, Ishikawa 924-0838, JAPAN b Fuji Filter MFG. Co., Ltd., 2-3-4 Nihonbashi, Chuo-ku, Tokyo 103-8308, JAPAN.

Abstract We propose a new fabrication technique for a TiNi tube-like device via a Ni-coated Ti wire. The Ni-coated Ti wire was wound on to a cylindrical mandrel. A subsequent heat-treatment, which was heated at 1173 K for various times in vacuum, was carried out. Observations of X-ray diffractometry and scanning electron microscopy show that TiNi compounds formed in the heattreated devices. An endothermic peak and an exothermic peak in differential scanning calorimetry diagram were observed on the heating and cooling process, respectively. The device heat-treated at 1173K for 28.8ks demonstrates a good shape memory motion. © 2015 2014The TheAuthors. Authors.Published Published Elsevier © byby Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and under responsibility the BY-NC-ND chairs of the International Conference on Martensitic Transformations 2014. is Peer-review an open access article under theofCC license (http://creativecommons.org/licenses/by-nc-nd/3.0/). 2014. This Keywords: shape memory alloy; martensitic transformation; interdiffusion; alloying process

1. Introduction TiNi alloys are expected to be a promising material for actuator devices because their work output per volume exceeds that of other actuator materials. [1] Although several unique fabrication methods of TiNi bulk alloys were proposed [2-4], fundamental fabrication methods are a high induction melting, casting and plastic working. The ingot of TiNi alloys is forged and rolled into a bar or a slab. After that, commercial products such as rods and wires

* Corresponding author. Tel.: +81-76-274-9254; fax: +81-76-274-9251. E-mail address: [email protected]

2214-7853 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations 2014. doi:10.1016/j.matpr.2015.07.397

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Y. Kishi et al. / Materials Today: Proceedings 2S (2015) S775 – S778

are formed using a bar rolling mill. Although the TiNi alloys are unique intermetallic compounds which are workable in the cold state, actual cold-working is not so easy. Plastic workability of the TiNi alloys is not to be compared with pure metals and/or the other alloys. This is due to a high work hardening rate of TiNi alloys. Therefore production for wires, foils, tubes, etc. of the alloys requires complex procedures in combination with coldworking and annealing [5], and this causes a high cost of the materials. In this paper, a new fabrication method for a TiNi tube-like device via a Ni-coated Ti wire was presented. The Nicoated Ti wire, which was seamless and continuous one, was wound on to a cylindrical mandrel. A subsequent heattreatment, which was heated at 1173 K for various times in vacuum, was carried out. This heat-treatment leads to the formation of TiNi intermetallic compounds by the interdiffusion. X-ray diffractometry, differential scanning calorymetory and scanning electron microscopy were carried out in order to confirm the microstructural transition during the heat-treatment. The TiNi tube-like device, which was heat-treated on the appropriate condition, was excited by an electrical heater and its shape recovery motion was recorded. The effectiveness of our proposed technique was discussed based on the experimental results. 2. Experimental Procedure Figure 1 shows schematic illustration of production for a TiNi tube-like device made from a Ni-coated Ti wire. A Ni-coated Ti wire with 125 m in diameter was used. Ni layer was electroplated by using a sulfamate nickel bath on the Ti wire, which was cleaned via acid pickling. After the nickel plating, the wire was cold-drawn to the final diameter of 125 m. The thickness of the Ni layer for the cold-drawn wire was about 12 m. The cold-drawn wire was cold-rolled to a quadrilateral cross-section and wound on to the cylindrical mandrel. And then, the wire with the mandrel was heat-treated in vacuum at 1173 K for several times, in order to form a TiNi layer on the wire through the interdiffusion between nickel and titanium. In order to reveal the microstructural transition of the devices during the heat-treatment processes, X-ray diffraction measurement was carried out at room temperature with an X-ray diffractometer (Rigaku RAD-2B). The transformation behavior of the heat-treated devices was observed by a differential scanning calorimeter (Rigaku DSC8230) according to Japanese Industrial Standards JIS H7101-2002. [6] The samples for X-ray diffractometry and differential scanning calorimetry were the rolled wires which were heat-treated under the same conditions as the heat-treated devices. Cross-sectional microstructures of the heat-treated devices were observed by a scanning electron microscope with a field emission electron gun (Hitachi S-4200). Chemical compositions of the heat-treated devices were determined by a scanning electron microscope (Hitachi S-3400N) equipped with an energy dispersive X-ray spectrometer (Horiba EX-250). The shape recovery motion of the appropriate heat-treated device was recorded using a video camera, when the device was placed on an electric heating plate kept at 453 K.

Fig. 1. Producing procedure for a TiNi tube-like device via a Ni-coated Ti wire.


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3. Results and Discussion

Fig. 2. X-ray diffraction profiles of Ni-coated Ti wires heat-treated at 1173 K for 3.6 ks, 14.4 ks and 28.8 ks. Standard diffraction patterns [7] for -Ti, Ni, TiNi3, Ti2Ni, TiNi-R and TiNi-B19’ are also indicated.

Fig. 3. DSC curves of Ni-coated Ti wires heat-treated at 1173 K for 3.6 ks, 14.4 ks and 28.8 ks.

X-ray diffraction profiles of the wires heat-treated under the same conditions as the heat-treated devices are shown in Fig. 2. X-ray profile of non-heat-treated wire and standard diffraction patterns [7] of various TiNi intermetallic compounds are also indicated in Fig. 2. Diffraction peaks diffracted from -Ti cannot be detected. Peak intensity diffracted from Ni decreased with increasing the heat-treatment time. Diffraction peaks originated from TiNi-B19’ and/or TiNi-R were observed on the profiles of the heat-treated wires. These are results of diffusion of Ni to Ti and formation of TiNi alloys in the wire with progress of the heat-treatment. Figure 3 shows the DSC curves obtained for the wires heat-treated under the same conditions as the heat-treated devices. These DSC curves agree with the results of the X-ray diffractometry shown in Fig. 2. For all the heattreated wire, the exothermic and endothermic peaks, which correspond to the martensitic and its reverse transformations, are clearly observed in both the cooling and heating processes. Transformation temperatures were not influenced on heat-treatment times. The value of about 345 K is obtained for the martensitic transformation start temperature in all heat-treated devices. However, transformation heat, which was the integral value of the peak in the DSC curve, was increase with increasing heat-treatment times. This suggests that the volume fraction of TiNi phase in the device was increase with increasing the heat-treatment time. Figure 4 shows cross sectional SEM images of the tube-like device heat-treated at 1173 K for 28.8 ks. Energy dispersive X-ray spectroscopy analysis exhibited the formation of several TiNi compounds during the heat-treatment. Although it was expected from the results of X-ray diffractometry and differential scanning calorimetry, near stoichiometric TiNi phase, which are enclosed by dotted line in Fig. 4 (b), was formed in the heat-treated devices. The longer heat-treatment results an increase of a volume of the near stoichiometric TiNi phase. However homogeneous or completion structure was not yet obtained under this heat-treatment conditions. The shape memory motion of the device heat-treated at 1173K for 28.8ks was shown in Fig. 5. The device was stretched and deformed to the pseudo-shape at ambient temperature (about 298K). The deformed device was placed on the plate kept at 453K. The shape of the deformed device was fully restored at 40 sec after placing on the plate. Naturally, this restorable motion was obtained repeatedly. This phenomenon is a typical shape memory effect resulted from the thermo-elastic martensitic transformation. These results are expected to find extensive applications as a small actuator. This proposed process will contribute to producing the three dimensional TiNi device in much low cost. The internal structure, shape recovery force, and so on will be investigated in our future work.

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Fig. 4. Cross sectional SEM images taken of a tube-like device heattreated at 1173 K for 28.8 ks. (a) Macroscopic feature, (b) Enlarged image of framed area b in (a), (c) Enlarged image of framed area c in (b). Chemical compositions determined from energy dispersive X-ray spectrometry measurements are also indicated in (c).

Fig. 5. Shape memory motion of a tube-like device heat-treated at 1173 K for 28.8 ks. The device was set on an electric heating plate kept at 453 K.

4. Conclusion A new fabrication method for a TiNi tube-like device via a Ni-coated Ti wire was presented. The Ni-coated Ti wire, which was seamless and continuous one, was wound on to a cylindrical mandrel. A subsequent heat-treatment, which was heated at 1173 K for various times in vacuum, leads to the formation of TiNi intermetallic compounds by the interdiffusion. Scanning electron microscopy and X-ray diffractometry observations revealed that TiNi phase was formed in the heat-treated devices. An exothermic peak and an endothermic peak in the differential scanning calorimetry diagrams were clearly observed on the cooling and the heating process, respectively. These peaks were corresponding to martensitic and its reverse transformation. The device heat-treated at 1173K for 28.8ks shows a good shape memory motion. References [1] I. Ohkata, Y. Suzuki, in: K. Otsuka, C.M. Wayman (Eds.), Shape Memory Materials, Cambridge University Press, Cambridge, UK, 1999, Chapter 11, pp. 240-266. [2] M. Igharo, J.V. Wood, Powder Metallurgy 28 (1985) 131-139. [3] K. Kusaka, T. Shimizu, DENKI-SEIKO (ELECTRIC FURNACE STEEL) 58 (1987) 226-234. [4] K. Tsuchiya, D. Tomus, K. Dohman, M. Sasaki, D. Imai, M. Umemoto, Proceedings of The Fourth Pacific Rim International Conference on Advanced Materials and Processing (PRICM4) (2001) 1545-1547. [5] Y. Suzuki, in: K. Otsuka, C. M. Wayman (Eds.), Shape Memory Materials, Cambridge University Press, Cambridge, UK, 1999, Chapter 6, pp. 133-148. [6] Japanese Industrial Standards, Method for Determining the Transformation Temperature of Shape Memory Alloys, JIS H7101-2002, 2002. [7] Inorganic Material Database (AtomWork), http://crystdb.nims.go.jp/index_en.html.