Crystallization and grain refinement of Ti30Ni20Cu (at%) alloy ribbons prepared by melt spinning

Crystallization and grain refinement of Ti30Ni20Cu (at%) alloy ribbons prepared by melt spinning

Journal of Alloys and Compounds 577S (2013) S179–S183 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: ww...

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Journal of Alloys and Compounds 577S (2013) S179–S183

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom

Crystallization and grain refinement of Ti 30Ni 20Cu (at%) alloy ribbons prepared by melt spinning Min-su Kim a , Jung-pil Noh a , Gyu-bong Cho a , Yeon-wook Kim b , Yinong Liu c , Hong Yang c , Tae-hyun Nam a,∗ a

School of Materials Science and Engineering, WCU & RIGET, Gyeongsang National University, 900 Gazwadong, Jinju, Gyeongnam 660-701, Republic of Korea Department of Material Engineering, Keimyung University, 1000 Shindang-dong, Dalseo-gu, Taegu 704-710, Republic of Korea c School of Mechanical Engineering, The University of Western Australia, Crawley, WA 6009, Australia b

a r t i c l e

i n f o

Article history: Received 19 September 2011 Received in revised form 31 October 2011 Accepted 29 December 2011 Available online 5 January 2012 Keywords: Shape memory alloys (SMA) Grain refining Crystallization Melt spinning Two-step annealing

a b s t r a c t Crystallization behavior of amorphous Ti 30Ni 20Cu (at%) alloy ribbons and martensitic transformation behavior, shape memory effect and superelasticity of crystallized ribbons were investigated by means of X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, thermal cycling tests under constant load and tensile tests. Crystallization occurred in the sequence of amorphous–B2–Ti(Ni,Cu)2 and the activation energy for crystallization of the B2 phase was 198.8 kJ/mol. Average grain size of the sample annealed in two-step, i.e., annealing at 773 K for 40 s followed by annealing at 748 K for 3.6 ks was 0.25 ␮m, which was very small comparing with the sample annealed in one-step, i.e., annealing at 823 K for 3.6 ks (1.20 ␮m). Grain refinement reduced the heat of transformation (H) associated with the B2–B19 transformation and increased the critical stress for slip deformation causing the perfect shape memory recovery under the applied stress of 400 MPa. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Grain size refinement of Ti Ni based shape memory alloys has been studied extensively because grain size affects both martensitic transformation behavior and mechanical properties such as the shape memory effect and superelasticity. Martensitic transformation start temperature (Ms) decreases with decreasing grain size because grain boundaries serve to stabilize the parent phase [1,2]. Transformation hysteresis decreases with decreasing grain size [3–5], which was attributed to a specific arrangement of martensitic variants (single pair of variants) in small grains [4]. Many attempts for grain refinement of Ti Ni based shape memory alloys have been made in order to investigate an effect of grain size on transformation behavior and mechanical properties [1–7]. Severe cold working and subsequent annealing, i.e., thermomechanical treatment (TMT) is the most frequently used for grain refinement. From TMT, very fine grains with an average size of 50–200 nm were obtained depending on the amount of cold working and annealing temperature [1,5,6]. Introduction of dislocations, however, always occurs by TMT. Since dislocations also affect both martensitic transformation behavior and mechanical properties of Ti Ni based alloys [8,9], TMT is not suitable for investigating an

∗ Corresponding author. Tel.: +82 55 751 5307; fax: +82 55 759 1745. E-mail address: [email protected] (T.-h. Nam). 0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2011.12.163

effect of grain size solely on transformation behavior and mechanical properties transformation. Crystallization of amorphous ribbons is known to be effective to control grain size by varying crystallization condition such as annealing temperature and time. An advantage of crystallization from amorphous ribbons is that it can exclude introduction of dislocations. In fact, from crystallization of amorphous ribbons, grains with an average size of 0.5–5.0 ␮m were obtained depending on annealing temperature and time [2,7]. Recently, the present authors report that a novel two-step annealing builds up very fine grains with an average grain size of 0.25 ␮m from amorphous Ti 30Ni 20Cu (at%) alloy ribbons prepared by melt spinning [10]. The two-step annealing is designed based on experimental results that activation energy required for the crystal growth is lower than that for the nucleation in Ti Ni alloys [11,12]. The first step annealing is made at a high temperature enough for supplying thermal energy for nucleation for short time with minimizing grain growth. The second step annealing is made at a low temperature enough for supplying thermal energy for growth of the crystals nucleated in the first step annealing. However, martensitic transformation behavior, shape memory effect and superelasticity of the two-step annealed ribbons are not reported yet. In the present study, amorphous Ti 30Ni 20Cu (at%) ribbons were prepared by melt spinning and then they were crystallized by two-step annealing for obtaining fine grains. Transformation behavior and mechanical properties such as shape memory effect

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Fig. 1. XRD patterns obtained from free side (a) and wheel side (b) of as-spun Ti 30Ni 20Cu alloy ribbons.

and superelasticity of the two-step annealed ribbons were investigated and results obtained were compared with the ribbons crystallized at a specific temperature (one-step annealing). 2. Experimental procedure A Ti 30Ni 20Cu (at%) pre-alloy was prepared by high frequency vacuum induction melting. Billet charges of about 15 g cut from the pre-alloys were placed into quartz crucibles and the chamber of the melt spinning system had been pumped down to less than 1 × 10−3 Pa before re-melting. After re-melting, it was ejected through the nozzle on the outer surface of the rotating quenching wheel made of copper. Melt spinning temperature and the linear velocity were 1873 K and 40 m/s, respectively. Average width and thickness of the ribbons obtained were found to be 3.1 mm and 21.3 ␮m, respectively. Two-step annealing for crystallization was made by annealing (the first step) at 773 K for 40 s followed by annealing (the second step) at 748 K for 3.6 ks. For comparison, some ribbons were annealed at 823 K for 3.6 ks (one-step annealing). Microstructures of the alloy ribbons were examined by field emission scanning electron microscopy (FE-SEM) after etching in a solution of H2 O:HCl:H2 O2 (3:2:1). The crystal structures of the ribbons were investigated by X-ray diffraction (XRD) using Cu K␣ radiation with successively changing experimental temperatures. For the study of crystallization behavior and martensitic transformation behaviors of the ribbons, differential scanning calorimetry (DSC) measurements were made at heating and cooling rate of 0.17 K/s using TA Instrument DSC-2010. Thermal cycling tests under the various applied stress with a cooling and heating rate of 0.017 K/s and tensile tests were made for investigating the shape memory effect and deformation behavior of the crystallized ribbons, respectively. Average sample size was 3.0 mm (width) × 19.5 ␮m (thickness) × 50.0 mm (length) and gage length was 30.0 mm.

3. Results and discussion Fig. 1(a) and (b) are XRD patterns obtained from free side and wheel side of as-spun Ti 30Ni 20Cu alloy ribbons, respectively. Any significant diffraction peaks corresponding to crystals are not

observed in both patterns, indicating that the as-spun ribbons are amorphous. In order to investigate crystallization behavior of amorphous Ti 30Ni 20Cu alloy ribbons, DSC measurements were made on the ribbons at various heating rates ranging from 0.08 K/s to 0.42 K/s and then DSC curves obtained are shown in Fig. 2(a). Two exothermic DSC peaks are observed in the curves. From the Ti Ni Cu alloy phase diagram [13] and Ref. [14], the DSC peak designated by single headed arrow is considered to be due to crystallization of the B2 phase from amorphous and that designated by double headed arrow is attributed to formation of Ti(Ni,Cu)2 . Measuring the peak shifts of the DSC curves with varying in heating rate in Fig. 2(a), the apparent activation energy for crystallization of the B2 phase from amorphous is obtained from the Kissinger plots. The Kissinger equation is as follows: ln

B T2

=−

E + constant RT

where B is the heating rate, E is the apparent activation energy, R is the gas constant and T is a specific absolute temperature such as peak temperature measured at selected heating rates B. By plotting ln(B/T2 ) vs. 1/(RT), as shown in Fig. 2(b), the activation energy is obtained from the slope of a straight line. The activation energy obtained is found to be 198.8 kJ/mol, which is very small comparing with 350–450 kJ/mol in Ti Ni alloy thin films [15,16]. Relatively small activation energy for crystallization comparing with Ti Ni alloys was also observed in Ti 25Ni 25Cu (at%) alloy ribbons [17]. Amorphous as-spun ribbons were annealed at various temperatures, as shown in DSC curves of Fig. 2(a), for crystallization. Annealing for crystallization was made in two ways; one-step annealing and two-step annealing. The one-step annealing is done

Fig. 2. (a) DSC curves of as-spun Ti 30Ni 20Cu alloy ribbons obtained at various heating rates (b) Kissinger plot obtained from (a).

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Fig. 3. XRD patterns obtained from the two-step annealed (a) and the one-step annealed (b) Ti 30N 20Cu (at%) alloy ribbons.

Fig. 4. FE-SEM images of the two-step annealed (a) and the one-step annealed (b) Ti 30N 20Cu (at%) alloy ribbons.

by annealing at 823 K for 3.6 ks which is higher than the exothermic peak temperature corresponding to formation of Ti(Ni,Cu)2 as indicated by “O” in Fig. 2(a). The two-step annealing is done by annealing at 773 K which is higher than the exothermic peak temperature corresponding to crystallization of B2 and lower than that corresponding to formation of Ti(Ni,Cu)2 indicated by “T − 1” for very short time, 40 s in order to suppress grain growth, then annealing at 748 K for 3.6 ks which is close to the on-set temperature of the exothermic peak corresponding to crystallization of B2 as indicated by “T − 2”. XRD patterns obtained from the two-step annealed (TA) and the one-step annealed (OA) samples are shown in Fig. 3(a) and (b), respectively. It is evident that the diffraction peaks corresponding to Ti(Ni,Cu)2 phase are observed on the OA sample, while they are not found in the TA sample. This is attributed to the fact that the OA sample was annealed at 823 K designated by “O” in Fig. 2(a) which is higher than the exothermic peak temperature corresponding to formation of Ti(Ni,Cu)2 , while the TA sample was annealed at 773 K and 748 K designated by “T − 1” and “T − 2” in Fig. 2(a) which are much lower than the exothermic peak temperature corresponding to formation of Ti(Ni,Cu)2 . Fig. 4(a) and (b) shows FE-SEM images of the TA and OA samples, respectively. It is clear that average grain size of the TA sample is found to be 0.25 ␮m, which is very small comparing with the OA sample (1.20 ␮m). The grain refinement by the two-step annealing is ascribed to the fact that the first step annealing at 773 K anneal-

ing enhances nucleation of crystals by supplying thermal energy enough for nucleation and the second step annealing at 748 K the low temperature (748 K) annealing for long time depresses grain growth since thermal energy for nucleation is much larger than that for grain growth in Ti Ni based alloys [11,12]. The distributions of grain size measured from Fig. 4(a) and (b) are shown in Fig. 5. It is found that the OA sample shows wide grain size distribution from 0.1 ␮m to 3.7 ␮m, while the TA sample shows

Fig. 5. Grain size distribution of Ti 30N 20Cu (at%) alloy ribbons annealed in onestep and two-step.

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Fig. 6. DSC curves of the two-step annealed (a) and the one-step annealed (b) Ti 30N 20Cu (at%) alloy ribbons.

narrow grain size distribution from 0.05 ␮m to 0.85 ␮m. In particular, many small grains less than 50 nm are observed in the TA sample as indicated by arrows, while those small grains are not observed in the OA sample. DSC curves of the TA and OA samples are shown in Fig. 6(a) and (b), respectively. One clear peak is observed in each cooling and heating curve in both samples, which is ascribed to the B2–B19 transformation. Comparing Fig. 6(a) and (b), it is found that Ms (the B2–B19 transformation start temperature) of the OA sample is lower than the TA sample. This is possibly ascribed to a hardening of the B2 phase due to formation of Ti(Ni,Cu)2 phase. It is also found that the heat of transformation (H) associated with the B2–B19 transformation of the OA sample (14.9 J/g) is larger than that of the TA sample (12.6 J/g), although Ti(Ni,Cu)2 phase exists in the OA sample. This may be ascribed to the fact that some very fine grains less than 50 nm are observed in the TA sample. According to Waitz et al. [18], very fine grains less than 50 nm do not show the B2–B19 transformation behavior in a Ti 49.7Ni (at%) alloy. The B2–B19 transformation may not occur in the fine grains in the TA sample, which causes a decrease in H. In order to investigate the shape memory effect of crystallized Ti 30Ni 20Cu ribbons, thermal cycling tests under various applied stresses from 160 MPa to 400 MPa were made and then elongation vs. temperature curves obtained from the TA and OA

samples are shown in Fig. 7(a) and (b), respectively. Transformation elongation associated with the B2–B19 transformation (εT ) occurs on cooling in both samples. Comparing Fig. 7(a) and (b), it is found that transformation elongation which occurred on cooling is recovered completely on heating in the TA sample even under the applied stress of 400 MPa, while it is not recovered completely in the OA sample and thus a residual elongation (εR ) of about 0.1% occurs. εR is known to be due to slip deformation [8]. Therefore, it is believed that the critical stress for slip deformation is improved by grain size refinement through the two-step annealing. It is also found that transformation elongation of the TA is smaller than that of the OA, which may be ascribed to the very fine grains less than 50 nm which may not show martensitic transformation and thus do not contribute to the transformation elongation. In order to investigate the superelasticity of crystallized Ti 30Ni 20Cu ribbons, tensile tests were made at 342 K which is 7 K higher than Af (the B19–B2 reverse transformation finish temperature) and then stress vs. strain curves obtained from the TA and OA samples are shown in Fig. 8(a) and (b), respectively. Both specimens show stress plateau associated with the stress induced B2–B19 martensitic transformation. The TA sample shows the perfect superelasticity, while only the partial superelasticity is observed in the OA sample. The strain remained after unloading is not recovered after heating up to 373 K, meaning that

Fig. 7. Elongation vs. temperature curves of the two-step annealed (a) and the one-step annealed (b) Ti 30N 20Cu (at%) alloy ribbons.

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Fig. 8. Stress vs. strain curves of the two-step annealed (a) and the one-step annealed (b) Ti 30N 20Cu (at%) alloy ribbons.

plastic deformation occurs in the OA sample. The perfect superelasticity in the TA sample is ascribed to the increase in the critical stress for slip deformation due to grain refinement. 4. Conclusions Crystallization behavior of amorphous Ti 30Ni 20Cu (at%) alloy ribbons was investigated and then martensitic transformation behavior, shape memory effect and superelasticity of crystallized ribbons were examined. Results obtained are as follows. (1) Crystallization occurred in the sequence of amorphous–B2–Ti(Ni,Cu)2 and the activation energy for crystallization of the B2 phase was 198.8 kJ/mol. (2) Average grain size of the sample annealed in two-step, i.e., annealing at 773 K for 40 s followed by annealing at 748 K for 3.6 ks was 0.25 ␮m, which was very small comparing with the sample annealed in one-step, i.e., annealing at 823 K for 3.6 ks (1.20 ␮m). (3) Heat of transformation (H) associated with the B2–B19 transformation of the two-step annealed sample (12.6 J/g) was smaller than that of the one-step annealed sample (14.9 J/g), which was ascribed to the fact that some very fine grains less than 50 nm which is too small to undergo martensitic transformation were observed in the two-step annealed sample. (4) Grain refinement by the two-step annealing increased the critical stress for slip deformation and thus caused the perfect shape memory recovery under the applied stress of 400 MPa.

Acknowledgments This research was supported by WCU (World Class University) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (Grant Number: R32-2008-000-20093-0). This work was also supported by the Korea Research Foundation Grant funded by the Korean Government (KRF-2008-220-D00061). This work was also supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. References [1] T. Waitz, H.P. Karnthaler, Acta Mater. 52 (2004) 5461. [2] S.W. Kang, Y.H. Lee, Y.M. Lim, J.M. Nam, T.H. Nam, Y.W. Kim, Scr. Mater. 59 (2008) 1186. [3] W.C. Crone, A.N. Yahya, J.H. Perepezko, Mater. Sci. Forum 386–388 (2002) 597. [4] Y. Liu, Mater. Sci. Eng. A 354 (2003) 286. [5] Y.H. Kim, G.B. Cho, Y.J. Lee, T.H. Nam, Mater. Sci. Eng. A 438–440 (2006) 531. [6] V. Demers, V. Brailovski, S. Prokoshkin, K. Inaekyan, Mater. Sci. Eng. A 513–514 (2009) 185. [7] R. Santamarta, D. Schryvers, Mater. Trans. 44 (2003) 1760. [8] T. Todoroki, H. Tamura, Trans. JIM 28 (1978) 83. [9] T.H. Nam, T. Saburi, K. Shimizu, Trans. JIM 33 (1991) 814. [10] M.S. Kim, G.B. Cho, J.P. Noh, Y.M. Jeon, Y.W. Kim, S. Miyazaki, T.H. Nam, Scr. Mater. 63 (2010) 1001. [11] X. Wang, J.J. Vlassak, Scr. Mater. 54 (2006) 925. [12] Y. Xu, X. Huang, A.G. Ramirez, J. Alloy Compd. 480 (2009) L13. [13] F.J.J. Van Lo, G.F. Bastin, A.J.H. Leenen, J. Less-Common Met. 57 (1978) 111. [14] A. Ishida, M. Sato, K. Ogawa, Philos. Mag. 88 (2008) 2427. [15] M.J. Vestel, D.S. Grummon, R. Gronsky, A.P. Pisano, Acta Mater. 51 (2003) 5309. [16] L. Zhang, C.Y. Xie, J.S. Wu, Scr. Mater. 55 (2006) 609. [17] T.H. Nam, S.M. Park, T.Y. Kim, Y.W. Kim, Smart Mater. Struct. 14 (2005) 239. [18] T. Waitz, T. Antretter, F.D. Fischer, N.K. Simha, H.P. Karnthaler, J. Mech. Phys. Solids 55 (2007) 419.