Size Effects on the Martensitic Phase Transformation in Ti54.5Ni45.5 ...

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ScienceDirect Materials Today: Proceedings 2S (2015) S891 – S896

International Conference on Martensitic Transformations, ICOMAT-2014

Size effects on the martensitic phase transformation in Ti54.5Ni45.5 alloy thin films Z. Gao*, Y. Zhu, H. Wang, W. Cai, X. Meng Science and Technology on Materials Performance Evaluation in Space Environment Laboratory, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China

Abstract Ti54.5Ni45.5 alloy nonorganic thin film was prepared by direct current (DC) magnetron sputtering system and subsequent rapid annealing treatment. Electrical resistivity(ER), X-ray diffraction (XRD) and transmission electron microscope (TEM) were carried out to investigate martensitic phase transformation behaviour and microstructure. The results demonstrated that the grains of the thin film can be refined through increasing heating rate, the smaller grain size is, the lower martensitic phase transition temperature. The martensitic phase transformation and its reversible phase transition were observed as the grain size was 50 nm, which was smaller than the critical dimension (60 nm) of recent theoretical predictions. In addition, the martensitic variant mainly showed the single-pair morphologies with (001) compound twinning. © 2014 The Authors. Published by Elsevier Ltd. © 2015 Published by Elsevier 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/3.0/). an open access under the BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). 2014. This Selection andisPeer-review underarticle responsibility of CC the chairs of the International Conference on Martensitic Transformations 2014. Keywords: nanocrystal; Ti-Ni alloy thin films; martensitic transformation; size effect

1. Introduction

In the last about 20 years, Ti-Ni shape memory thin films have attracted increasing attentions for as preferred candidate of micro-electro-mechanical systems[MEMS][1,2].The shape memory effect of TiNi alloy thin film is derived from the reversible phase transition between martensitic in low temperature and austenite in high temperature, which have different crystal structures. It is noted that Ti-Ni thin films sputter-deposited at ambient temperature are often amorphous, and consequently require post-sputtering annealing to obtain the

* Corresponding author. Tel.: +86 451 86418649; fax: +86 451 86418649. E-mail address: [email protected]

2214-7853 © 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations 2014. doi:10.1016/j.matpr.2015.07.425

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Z. Gao et al. / Materials Today: Proceedings 2S (2015) S891 – S896

desired characteristics [3,4]. The crystallization process of amorphous thin film involves the nucleation and nucleus growth. Crystallization annealing process directly decides shape memory effect, mechanical properties, and structure of the Ti-Ni alloy thin film [5]. Therefore, the crystallization of Ti-Ni alloy thin film begins to become more and more attractive [6]. There are abundant phase transition behaviors in micron-grain size Ti-Ni alloy thin film, thermally induced martensitic phase transformation[7], stress induced martensitic phase transformation [8,9] and R phase transformation have been found until now[10,11]. But, the study of the martensitic phase transformation in the nanocrystalline Ti-Ni alloy thin film has few reported systematically. At the same time, due to the effect of grain size in nanocrystalline Ti-Ni thin film, the phase transition behavior must be different from that of micron-grain size inTi-Ni alloy thin film [12,13]. The effect of nanograin sizes on the nature of martensitic transformation has only received limited attention [14-16] due to the difficulty of fabricating consistently nano-crystalline Ti-Ni. Waitz et al have recently studied the effect of nanograin sizes on the martensitic transformation in HPT-processed and annealed Ni47.9Ti50.3 alloys resulting in small and thin samples. They found that the samples with a grain size of less than 15 nm contain B2 austenite at room temperature, those of sizes between 15 and 60 nm contain the mixture of B2 and R phases, the sizes between 60 and 150 nm contain R-phase and B19’ martensitic mixture, and the grains of a size above 150 nm contain only B19’ martensitic, they also suggested that the grains of a size less than about 60 nm do not transform to martensitic even upon large undercooling [14,15,17]. Other researchers have also found similar results. Some [12,13] have found nanocrystalline martensitic in Ti-Ni bulk alloy material by transmission electron microscope (TEM), determined it (001)composite twin was, and forecasted the martensitic phase state did not exist in nanograin of less than 60 nm. In other words, the minimum of grain sizes with martensitic phase theoretically was defined, but the corresponding experimental evidence has not been provided. Therefore, the study of martensitic phase transformation in nanograin is only a beginning, its detailed microscopic phase transformation mechanism and influencing factors need to be further studied In the present work, the effect of grain sizes on martensitic phase transformation in Ti-Ni alloy thin film was studied. Ti54.5Ni45.5 alloy thin film was prepared by magnetron sputtering. Electrical resistivity(ER) and X-ray diffraction (XRD) were carried out to investigate the martensitic phase transformation behavior and thermal stability, TEM was adopted to study the morphology, twin relationship and structure characteristics of nanograin martensite. 2. Experimental procedure

The Ti54.5Ni45.5 thin films were deposited on SiO2/Al foils substrates by high vacuum direct current (DC) magnetron sputtering system. The SiO2 thin films about 30 nm were deposited on heated Al foil by radio frequency (r.f.) reaction magnetron sputtering system. The middle buffer layers were effective to prevent Ti54.5Ni45.5 thin film from reacting with Al during the crystallization and to provide the well support. Asdeposited Ti-Ni thin films were rapid post-annealed to obtain crystalline specimens at various heating rate 1200 ºC/min and 4800 ºC/min in vacuum. The thickness of Ti54.5Ni45.5 thin films was about 90nm determined by observing cross section using Scanning electron microscopy (SEM). The chemical composition of the deposited films was 54.5 at.% Ti determined by using energy dispersive X-ray spectrometry (EDS). Phase identification was performed in a Rigaku D/max-rB X-ray diffractometer (XRD) with Cu Kα radiation. The martensitic phase transform temperature of Ti-Ni alloy thin films was measured by electrical resistivity(ER). The curves of electrical resistance dependent on temperature were obtained by Vb compiled program. The sample size was 1×1cm2, error of system measurement was ᇞTmax < ±2 ºC. The instrument structure and other properties can be found in the literature [18]. The size and lattice structure of thin films were analysed in a Philips CM 12TEM/STEM. The operating voltage is 120 kv, the rotation range of X, Y axis was ± 45 º using double tilting table. The grain size is determined by Nano Measurer.


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3. Results and discussion

Figure 1(a) shows the curve of resistivity dependent on temperature of Ti54.5Ni45.5 alloy thin film with the heating rate of 1200 ºC/min and 4800 ºC/min. Martensitic phase transformation all took place in the alloy thin film at the two kinds of heating rate, In addition, the martensite start temperature (Ms, 44 ºC) obtained by resistivity measurements of the film crystallized at a heating rate of 4800K was higher than the austenite start temperature (As, 35 ºC), this may be because that there are more surface after nanocrystallization, and the interface energy could provide extra energy for martensite phase transformation.

Fig.1 (a) Electrical resistance vs. temperature curves of Ti54.5Ni45.5 alloy thin film, (b) Effects of annealing temperature on the phase transform temperature for Ti54.5Ni45.5 alloy thin film.

Figure 1(b) demonstrated the effects of annealing temperature on the phase transform temperature for Ti54.5Ni45.5 alloy thin film, the annealing temperature was 650, 670, 700, and 750 ºC, respectively. The phase transform temperature (Ms, As) lower gradually with the increasing annealing temperature, this was because of the different residual stress of thin film. In addition, the range of phase transform temperature was from 20 to 130 ºC, indicating the heat treatment process was an effective means to adjust the nanocrystalline phase transform temperature. The curve of resistivity dependent on different heat preservation times of Ti54.5Ni45.5 alloy thin film at 600 ºC was shown in Fig. 2(a). There was only a step of martensitic phase transformation, and the temperature was all lower than that of austenite. Moreover, the incubation time of phase transformation increased gradually with the extension of heat preservation time, which showed that heat preservation crystallization of long time was conducive to the release of residual stress. Figure 2(b) shows the influences of heat preservation times on the phase transform temperature of Ti54.5Ni45.5 alloy thin film. The As increased with the increasing heat preservation time at the range of 2 min~600 min, but the growth of As temperature was limited at heat preservation times of 60 min and 600 min. the Ms temperature multiplied rapidly as the extension of heat preservation time, and the influence of the hysteresis of phase transformation by heat preservation time was great. The hysteresis temperature of phase transformation with the heat preservation time of 600 min reduced 20 ºC, compared with that of 2 min. Figure 3(a) shows XRD spectra of Ti54.5Ni45.5 alloy thin film with the thickness of 90 nm after rapid thermal annealing (RTA), the heating rate was 1200 ºC/min and 4800 ºC/min, respectively. The diffraction peak of matensite became broader by RTA, compared with that of conventional thermal annealing, this may be because of the refinement effect of grain. According to Scherrer-formula, the diameter of grain was 70 nm in the thin film with conventional thermal annealing, the grain size of 50 nm was obtained when the heating rate was 1200 ºC/min, while grain size could not be calculated when the heating rate was 4800 ºC/min because of

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the weak diffraction peak. It was concluded that the heating rate was the critical factor to affect the size of TiNi alloy nanograin thin film; the ideal nano-grain was obtained when the heating rate increased to 1200 ºC/min.

Fig. 2. (a) Electrical resistance vs. temperature curves of the phase Ti54.5Ni45.5 alloy thin film. (b) Effect of heat preservation times on transformation temperature for Ti54.5Ni45.5 alloy thin film.

(a)

(b)

(c)

103M -111M 221M

Fig. 3. (a) XRD patterns of Ti54.5Ni45.5 alloy thin film annealed by different heating treatment. (b) TEM image of Ti54.5Ni45.5 alloy thin film bright field image (c) enlargement and corresponding diffraction pattern.

In addition, martensitic phase transformation still happened when the grain size is 50 nm, which was smaller than the critical dimension (60 nm) with martensitic phase transformation in the TiNi alloy of recent theoretical predictions. As it is known, the martensite by thermoforming in TiNi ally and TiNi ally thin film are almost selfaccommodated morphology, the substructure of martensite is twin crystal, and the grain sizes are different in range from several hundred nanometers to several microns, which are related to preparation technology. Some scholars [19] have prepared nanocrystalline alloy that the grain size is about 60 nm by severe plastic deformation method and forecasted the martensitic phase state did not exist in nanograin of less than 60 nm theoretically. The results of TEM, XRD and Nano Measurer indicated that the grain size of nanocrystalline Ti54.5Ni45.5 ally thin film was in the range of 20 nm~80 nm by amorphous crystallization method, and there was nanocrystalline martensite with the grain size of 20 nm at room temperature. Figs. 3(b) and (c) shows the TEM image of Ti54.5Ni45.5 alloy thin film annealed by 600 {C for 1 h and its diffraction patterns, the grain size was

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about between 20~50nm, the diffraction patterns calibration indicated that the sample was mainly martensitic phase in Fig. 3(b), the interface between martensitic variants in internal crystalline was clear and straight, without impingement and cross. Hence, the microstructures between nanocrystalline martensite and coarse martensite had no obvious difference, but the grain size reduced. Figure 4 presents the TEM image of thermal martensite inTi54.5Ni45.5 alloy thin film annealed by 600{& for 1 h and its diffraction patterns, the diffraction pattern corresponds two sets of diffraction spots arising from twin related crystal lattices, which is related to residual stress by underlying substrate on membrane and restriction of the low thickness on the grain longitudinal growth [20,21].

(a)

(b)

(c)

0

(0 01 )

Fig. 4. TEM image of (001) compound twined thermal martensite in Ti54.5Ni45.5 alloy thin film (a,b) and (c) diffraction patterns of (001).

The results have proved that Τ twinning was the most common in the internal structure of coarse grains martensite of TiNi alloy bulk material, and it was recognized as the most important way of lattice constant shear martensite. In the TiNi alloy thin film, < 011 > Τ type twin was still the most common in the absence of precipitated phase, (001)composite twin predominated when the phase precipitation existed. While, compound twinning also presented in thermal induced nanocrystalline martensite of TiNi alloy. Figure 4(b) shows TEM image of thermal martensite and its FFT transformation inTi54.5Ni45.5 alloy thin film annealed by 600 ºC for 1 h. M1, M2 are martensitic variants (001) with twinning relationship, the interface of martensite was clear, integrated, distortionless and well coherent. The width of martensitic variants was about 2.4 nm, which was in accordance with the Waitz’s result [12,13] of TiNi alloy bulk material by HPT. (001) compound twinning has the characteristic of single variation, the variants width is far less than that of TiNi alloy thin film (a few tens to a few hundreds of nanometers) [22] and TiNi alloy bulk material (hundreds of nanometers to several micrometers) [23-25]. Krishnan et al. [26, 27] considered that the twinning width was codetermined by misfit strain and interfacial energy, misfit strain is proportional to the square of twinning width, while interfacial energy is inversely related to twinning width. In order to reduce the total energy of system, the twinning width must be tend to be more narrow direction in the process of martensitic variants evolving. The total interfacial energy increased, which was compensated by the reduction of strain energy. 4. Conclusions

In the paper, the influencing factors, phase transition behavior and microstructure of nanograin Ti-Ni alloy thin film were investigated, the conclusion follows: The grains of Ti54.5Ni45.5 alloy thin film can be refined by increasing the heating rate. The grains with the diameter of 50 nm were obtained in the thin films with the thickness of 90 nm as the heating rate was 1200 ºC/min, and thin film shows forward and reverse martensitic phase transformation, the grains size was smaller than the critical dimension (60 nm) of recent theoretical predictions.

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The nanograin Ti-Ni alloy thin film produced phase transformation. The phase transformation temperature all reduced with the increase of heating rate and annealing temperature. The hysteresis of phase transformation was strongly influenced by crystallization heat preservation time. The martensite varians mainly showed single-pair morphologies with (001) compound twinning. Residual stress and constraint condition of longitudinal direction are the main causes of formation. The width of martensite variants was about 2.4 nm, less than that of coarse grain an order of magnitude, which effectively balanced the energy in the process of variation in shear. Acknowledgements

This work was supported by the National Natural Science Foundation of China, Grant no. 51271066, 51322102 and Natural Science Foundation of Heilongjiang, Grant no. E201232. References [1] A.G. Ramirez, H. Ni, H.J. Lee, Mat. Sci. Eng. A 434-440 (2006) 703–709. [2] L. Zhang, C.Y. Xie, J.S. Wu, J. Alloy Compd. 434 (2007) 318–322. [3] Y.Q. Fu, H.J. Dua, W.M. Huang, Sensor Actuat A: Phys. 112 (2004) 395–408. [4] S. Miyazaki, A. Ishida, Mat. Sci. Eng A. (1999) 106–133. [5] H.J. Lee, A.G. Ramirez, Appl. Phys. Lett. 85 (2004) 1146–1148. [6] M. Peterlechner, J. Bokeloh, G. Wilde, T. Waitz, Acta Mater. 20 (2010) 6637–6648. [7] X. Wang, J.J. Vlassak. Scr. Mater. 54 (2006) 925–930. [8] A. Ishida, M. Sato, K. Ogawa, Mat. Sci. Eng. A. 25(2008) 91–94. [9] M. Wuttig, Y. Zheng, J.S. Slutsker, K. Mori, Q. Su, Scr. Mater.41(1999) 529–533. [10] R. Zarnetta, D. König, C. Zamponi, A. Aghajani, A. Ludwig, Acta Mater. 57(2009) 4169–4177. [11] C. Urbina, S. De la Flor, F. Ferrando. J. Alloy Compd. 490(2010) 499–507. [12] H.P. Karnthaler, T. Waitz, C. Rentenberger, Mat. Sci. Eng. A. (2004) 777–782. [13] T. Waitz, H.P. Karnthaler, Acta Mater. 19 (2004) 5461–5469. [14] T. Waitz, T. Antretter, F.D. Fischer, J. Mech. Phys. Solids 55 (2007) 419–444. [15] T. Waitz, V. Kazykhanov, H.P. Karnthaler, Acta Mater. 52 (2004) 137–147. [16] T. Waitz, Acta Mater. 53 (2005) 2273–2283. [17] B. Kockar, I. Karaman, J.I. Kim, Y.I. Chumlyakov, Acta Mater. 56 (2008) 3630–3646. [18] W. Cai. Masters dissertation of Harbin Institute of Technology.1990. [19] Y. Kudoh, M. Tokonami, S. Miyazaki, K. Otsuka, Acta Metall. 33(1985) 2049–2056. [20]M. Nishida, H. Ohgi, I. Itai, A. Chiba and K, Acta Metall. Mater. 43(1995) 1219–1227. [21] K. Yamauchi, M. Nishida, I. Itai, K. Kitamura, A. Chiba, Mater Trans. 37(1996) 210–217. [22] X.L. Meng, M. Sato, A. Ishida, Acta Mater. 56(2008) 3394–3402. [23] J.X. Zhang, M. Sato, A. Ishida, Acta Mater. 54(2006) 1185–1198 [24] T. Saburi, S. Nenno, T. Fukuda, J. Less Comm. Metal. 125(1986) 157–166. [25] W.J. Moberly, J.L. Proft, T.W. Duerig, R. Sinclair, Acta Metall. Mater. 38 (1990) 2601–2612. [26] M. Krishnan, J.B. Singh, Acta Mater. 48(2000)1325–1344. [27] M. Krishnan, Acta Mater. 46(1998) 1439–1457.