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Effect of electrothermal annealing on the transformation behavior of TiNi shape memory alloy and two-way shape memory spring actuated by direct electrical current
ARTICLE IN PRESS
Physica B 349 (2004) 365–370
Effect of electrothermal annealing on the transformation behavior of TiNi shape memory alloy and two-way shape memory spring actuated by direct electrical current Z.G. Wanga, X.T. Zua,d,*, X.D. Fengb, S. Zhuc, J. Dengb, L.M. Wangc a
Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu 610054, People’s Republic of China b Department of Physics, Sichuan University, Chengdu 610064, People’s Republic of China c Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor 48109, USA d National Key Laboratory of High-Power Vacuum Electronics, University of Electronic Science and Technology of China, Chengdu, Sichuan 610054, People’s Republic of China Received 20 January 2004; received in revised form 8 April 2004; accepted 15 April 2004
Abstract In this work, the effect of electrothermal annealing on the transformation characterization of TiNi shape memory alloy and the electrothermal actuating characteristics of a two-way shape memory effect (TWSME) extension spring were investigated with direct electrical current. The results showed that with increasing direct electrical current density, the B2-R-phase transformation shifts to a lower temperature and R-phase-B190 shifts to a higher temperature in the cooling process. When annealing electrical current density reached 12.2 A/mm2, the R-phase disappeared and austenite transformed into martensite directly. The electrothermal annealing was an effective method of heat treatment in a selected part of shape memory alloy device. The electrothermal actuating characteristics of a TWSME spring showed that the time response and the maximum elongation greatly depended on the magnitude of the electrical current. r 2004 Elsevier B.V. All rights reserved. PACS: 62.50.Ks; 61.66.Dk; 62.20.Fe; 62.40.+i; 64.70.Kb; 81.40.J Keywords: TiNi shape memory alloy; Electrothermal annealing; Phase transformation; Two-way shape memory effect
1. Introduction TiNi shape memory alloy (SMA) of nearequiatomic composition is a technologically im*Corresponding author. Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu 610054, People’s Republic of China. Tel./fax: 86-2883201939. E-mail address: [email protected] (X.T. Zu).
portant shape memory material because the TiNi SMA combines good functional and structural properties [1–3]. There is also an interest in Ni-rich NiTi alloys, because, the phase transition temperatures can be controlled through heat treatment [4,5]. Depending on the heat treatment, TiNi alloy exhibits either a one-step martensitic transformation (MT) from the high-temperature B2 to the B190 (monoclinic) phase, or a two-step MT
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.04.064
ARTICLE IN PRESS 366
Z.G. Wang et al. / Physica B 349 (2004) 365–370
from the B2 to the rhombohedral R-phase to the B190 . The heat treatments are conventially carried out in a furnace. SMA wires are most easily actuated by electrical current via Joule heating, which is an easy and efficient way [6]. In the same way, with a higher current, the SMA can be heated up to an annealing temperature. So the SMA also can be annealed with electrical current. The electrothermal annealing will be more useful in local annealing, i.e. we can anneal the part of our device by controlling where the electrical current passes through. To our knowledge, there are few reports in the literature on the effect of electrothermal annealing on the transformation characteristics of shape memory alloy. In this work the effects of electrothermal annealing on the transformation characteristics of TiNi SMA as well as the electrothermal actuating characteristics of a two-way shape memory effect (TWSME) of an extension spring were investigated.
2. Experiment A Ti-49.8 at%Ni wire with diameter of 0.55 mm, provided by the Northwest Institute of NonFerrous Metal of China, was used for this study. The shape memory alloy was electrothermal annealed with 8.4, 9.3, 10.5 and 12.2 A/mm2 direct electrical current for 1 h. In order to make a comparison between the electrothermal heat treatment and the conventional heat treatment, some samples were annealed with conventional furnace at temperatures 400 C and 450 C for 1 h. The transformation temperatures of the samples were measured using differential scanning calorimetry (DSC) with a scanning rate of 10 C/min under nitrogen atmosphere. The TWSME springs were prepared as the method that was described in Ref. [7]. The asreceived TiNi wire was wound on a cylindrical jig. They were annealed at 500 C with a regular furnace for 1 h followed by air cooling. The mean diameter of the spring is 4.5 mm and the pitch of the spring is 0.1 mm after annealing heat treatment. Then the springs were extended till the pitch reached 12 mm and constrained at the extension
state and annealed at 550 C with a regular furnace for 1 h followed by air cooling. The mean diameter of the spring is 4.5 mm and the pitch of the spring is 12 mm after the above treatment. And the springs were thermal–mechanical trained for 150 cycles. The two-way shape elongation was about 65%. The two-way shape memory spring was actuated by Joule heating and air convection cooling. In this experiment, 2.7–8.4 A/mm2 current densities were used to drive the two-way shape memory spring.
3. Results and discussion 3.1. Effect of electrothermal annealing on the transformation behavior of TiNi SMA Fig. 1 shows the DSC curves of both the asreceived sample and the samples annealed with different electrical currents. It can be seen from the figure that there is only one-step phase transformation occurring in the as received wire in the temperature range between 0 C and 140 C. In the case of the samples annealed with 8.4 and 9.3 A/ mm2 for 1 h, two-step transformation from austenite to R-phase and further to martensite have taken place upon cooling, one step reverse transformation happens upon heating. For the samples annealed with 10.5 A/mm2 for 1 h, the B2-R-phase and R-phase-B190 transformation almost merged into one peak, the R-phase-B190 transformation appears as a shoulder on the low-temperature side of B2-R-phase transformation. When annealing electrical current reaches 12.2 A/mm2, as shown in Fig. 1(e), the R-phase disappears and austenite transforms into martensite directly. The transformation temperatures for the samples annealed with different electrical current is shown in Table 1. The transformation temperatures are acquired by determining the onset point of the slope change in the DSC curves. In the cooling process, the B2-R-phase transformation shifts to lower temperature and R-phaseB190 shifts to higher temperature with increasing direct current density, these results are consistent with the previous results obtained with conventional heat treatments [8]. The values of latent heat
ARTICLE IN PRESS Z.G. Wang et al. / Physica B 349 (2004) 365–370
of transformation for the heating curves are shown in Fig. 2. The figure shows that the latent heat of transformation increases with increasing annealing electrical current. (a)
(b) ,
(c)
,
(d)
,
(e)
,
0
20
40
60
80
100 120 140
Fig. 1. DSC curves of Ti-49.8 at%Ni shape memory alloy (a) as-received sample and electrothermal annealed with (b) 8.4 A/ mm2; (c) 9.3 A/mm2; (d) 10.5 A/mm2 and (e) 12.2 A/mm2 for 1 h.
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Fig. 3 shows the DSC curves of both the asreceived sample and the samples annealed at 400 C and 450 C for 1 h. Upon cooling, two-step transformation among austenite, R-phase and martensite can be observed, while upon heating, only one-step transformation between martensite and austenite can be detected for the sample annealed at 400 C. When the sample annealed at 450 C as shown in Fig. 1(c), the R-phase disappeared and austenite transforms into martensite directly. The transformation temperatures for the samples annealed with different temperatures are shown in Table 1. The latent heats of transformation upon heating are 16.12 and 16.91 J/g for the sample annealed at 400 C and 450 C. The latent heat increases with increasing annealing temperature, too. The transformation changes with different direct current density can be explained by the reason that the recovery and recrystallization process of the shape memory alloy [9]. When the current passes through the TiNi alloy and electrical energy is converted into thermal energy according to the Joule’s law. Annealing happens which can modify the microstructure of the SMA. So the electrothermal annealing is a very effective method of the heat treatment applicable easily to SMA wires, especially in a selected part of shape memory alloy device. It is well known that severe cold work inhibits the martensitic transformation by introduction of defects, which are essentially dislocations [10]. The SMAs are in cold drawn state in the present investigation, so there are many dislocations in the TiNi specimen. Annealing induces annihilation of the dislocation and recrystallization of the cold worked specimen. At low annealing electrical
Table 1 Transformation temperatures of the Ti-49.8 at%Ni samples annealed with different direct current density or temperature Conditions of annealing Direct current density Direct current density Direct current density Direct current density Temperature 400 C Temperature 450 C
8.4 A/mm2 9.3 A/mm2 10.5 A/mm2 12.2 A/mm2
As ( C)
Af ( C)
Rs ( C)
Rf ( C)
Ms ( C)
Mf ( C)
69 66 69 69 68 69
76 75 79 86 74 82
68 54 48 — 51 —
45 49 — — 47 —
22 26 — 56 36 49
20 22 36 44 28 42
ARTICLE IN PRESS Z.G. Wang et al. / Physica B 349 (2004) 365–370
368
Heat of transformation(J/g)
18
17
16
15
14
2.0
2.2
2.4
2.6
2.8
3.0
Electrical current (A)
Fig. 2. Effect of electrical current on the latent heat of transformation upon heating of the TiNi wire.
(a)
(b)
(c)
0
current or temperature, the dislocations density reduces and the internal stress reduces, too, which leads to the increase of Ms and Mf : The latent heat of transformation increases with increasing annealing electrical current or temperature is consistent with previous results [11], which show that the dislocations associated with high levels of plastic deformation generate an internal stress, which restricts the martensite from transforming into austenite. This martensitic phase remains ‘‘pinned’’ in the microstructure until the dislocations are removed through an annealing process. Therefore, with increasing annealing electrical current temperature, more dislocations are removed and more martensite transforms into austenite, so the latent heat increases. Several studies have reported that the appearance of the R-phase could be due to dislocations [12,13]. The R-phase nucleates preferentially in the internal stresses concentrated regions [13,14]. With increasing annealing temperature, the dislocations density reduces and the internal stress reduces, too, thus the Rs and Rf decreases. The disappearance of the R-phase transformation can be attributed to the disappearance of the internal stresses in recrystallized grains, which has been verified by Khelfaoui et al. [10]. So the parent to R-phase transformation disappears when the internal stresses disappear from the grains. 3.2. Characterization of the TWSME spring actuated by direct electrical current
20
40
60
80
100 120
140
Fig. 3. DSC curves of Ti-49.8 at%Ni shape memory alloy (a) as-received sample and annealed at (b) 400 C and (c) 450 C for 1 h.
current or temperature, the dislocations density is high, and the associated internal stress generated by these dislocations restricts the specimen from austenite transforming into martensite. So the martensitic transformation happens at low temperature. With increasing the annealing electrical
Fig. 4 shows the elongation–temperature curve of the TiNi extension spring showing that the maximum elongation is about 65%. The transformation temperatures evaluated from the curve were: As ¼ 68 C; Af ¼ 71 C; Ms ¼ 48 C and Mf ¼ 39:5 C: Fig. 5 shows the elongation–time curves for different actuating direct electrical current densities. The time interval between the start and the end of the spring’s shape change (termed time response) and the maximum elongation at different electrical current deduced from Fig. 5 are shown in Fig. 6. As can be observed from Fig. 6, the spring shows no shape change at 2.7 A/mm2. When the
ARTICLE IN PRESS Z.G. Wang et al. / Physica B 349 (2004) 365–370
Cooling Time response (s)
60
Elongation /%
50 40 30 20 10 0 -10 20
Heating 30
40
20
80
18
70
16
60
14
50
12
60
70
80
90
100
Temperature /oC Fig. 4. Elongation–temperature curve for TWSME TiNi spring heat treated by a conventional furnace.
Fig. 5. Elongation of TWSME extension spring vs. time at different electrical current 2.7–8.4 A/mm2.
electrical current is lower than 6.3 A/mm2, the maximum elongation increases and the time response decreases with increasing the electrical current. The maximum elongation increases from 0% to 65% and time response decreases from 17 to 7 s. When the electrical current is higher than 6.3 A/mm2, the time response continually decreases with increasing current, but the maximum elongation decreases. When the current passes through the TiNi alloy, the electrical energy is converted into thermal energy according to the Joule’s law. The magnitude of the thermal energy produced depends on the magnitude of the electrical current and time. For a small electrical current, a long time is needed to lead to the transformation of the TiNi alloy spring, and vice versa. As the experiment was done
30
8 6
20
4
10
2
0
0
50
40
10
2
3
4
5
6
7
8
9
Maximum elongation(%)
70
369
-10
Electrical current density (A/mm2)
Fig. 6. Curves for the maximum elongation and the time response for different actuating electrical current densities.
in air, Joule heating and the air convection cooling happened at the same time. So at a lower electrical current, the heat generated by Joule heating dissipates, leading to only a part martensite transforms to austenite in the TiNi alloy, which leads to a small maximum elongation. By increasing the electrical current, the heating is accelerated, the dissipated heat reduced, and more martensite transformed to austenite, thereby increasing the maximum elongation. It is commonly accepted that the two-way shape memory effect was obtained by the anisotropic dislocation structure generated during thermomechanical training [15–17]. This dislocation structure creates an anisotropic stress field in the matrix, which guides the formation of martensite into variants of preferential orientations in relation to the deformation adopted in the training procedure, thus resulting in a macroscopic shape change during subsequent thermal transformation cycles. By martensitic transformation in an untrained specimen, and when free of external stress, all martensitic variants are crystallographically and thermodynamically equivalent. However, the dislocation structures generated by training cause a free energy decrease in one or in a group of variants and they become no longer thermodynamically equivalent. Then upon cooling, a larger amount of these variants is formed, leading to a macroscopic form change as observed in the trained samples. When the electrical current is too high, annealing heat treatment will happen as
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shown in Section 3.1, the dislocation pattern will be changed by the electrothermal heating. Thus, higher electrical current leads to a decrease in the maximum elongation.
Foundation 03ZQ026-059 and by the Projectsponsored by SRF for ROCS, SEM.
References 4. Conclusions The electrothermal annealing is a very effective method of heat treating in a selected part of a shape memory alloy device. After the shape memory alloy annealed by electrical current, the B2-R-phase transformation shifts to lower temperature and R-phase-B190 shifts to higher temperature in the cooling process with increasing direct current density. When annealing electrical current reaches 12.2 A/mm2, the R-phase disappears and austenite transforms into martensite directly. The electrothermal actuating characteristics of a TWSME spring shows that the time response and the maximum elongation greatly depend on the magnitude of the electrical current. The time response continually decreases with increasing current very quickly at first stage then slowed down. The maximum elongation increases with increasing current first and then decreased.
Acknowledgements This study was financially supported by the National Natural Science Foundation of China (10175042), by the Sichuan Young Scientists
[1] J.V. Humbeeck, Mater. Sci. Eng. A 273–275 (1999) 134. [2] T. Duerig, A. Pelton, D. Stoeckel, Mater. Sci. Eng. A 273–275 (1999) 149. [3] Jafar Khalil-Allafi, Antonin Dlouhy, Gunther Eggeler, Acta Mater. 50 (2002) 4255. [4] K. Otsuka, C.M. Wayman, Shape Memory Materials, Cambridge Unversity Press, Cambridge, London, 1998, p. 49. [5] W. Tang, B. Sundmann, R. Sandstr.om, C. Quiu. Acta Mater. 47 (1999) 3457. [6] J. Abadie, N. Chaillet, C. Lexcellent, A. Bourjault, Proc. SPIE 3667 (1999) 326. [7] Z.G. Wang, X.T. Zu, X.D. Feng, L.B. Lin, S. Zhu, L.M. Wang, Mater. Sci. Eng. A 345 (2003) 249. [8] X. Huang, Y. Liu, Scr. Mater. 45 (2001) 153. [9] F. Khelfaoui, G. Gu!enin, Mater. Sci. Eng. A 355 (2003) 292. [10] F. Khelfaoui, G. Thollet, G. Gu!enin, Mater. Sci. Eng. A 338 (2002) 305. [11] D.A. Miller, D.C. Lagoudas, Mater. Sci. Eng. A 308 (2001) 161. [12] S.K. Wu, H.C. Lin, Y.C. Yen, Mater. Sci. Eng. A 215 (1996) 113. [13] T. Fukuda, T. Saburi, K. Dio, S. Nenno, Mater. Trans. JIM 33 (1992) 271. [14] L. Bataillard, J.-E Bidaux, R. Gotthardt, Philos. Mag. 78 (1998) 327. [15] H. Scherngell, A.C. Kneissl, Scr. Metall. 39 (1998) 205. [16] H. Scherngell, A.C. Kneissl, Mater. Sci. Eng. A 273–275 (1999) 400. [17] Y. Liu, P.G. McCormick, Acta Metall. Mater. 38 (1990) 1321.
Physica B 349 (2004) 365–370
Effect of electrothermal annealing on the transformation behavior of TiNi shape memory alloy and two-way shape memory spring actuated by direct electrical current Z.G. Wanga, X.T. Zua,d,*, X.D. Fengb, S. Zhuc, J. Dengb, L.M. Wangc a
Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu 610054, People’s Republic of China b Department of Physics, Sichuan University, Chengdu 610064, People’s Republic of China c Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor 48109, USA d National Key Laboratory of High-Power Vacuum Electronics, University of Electronic Science and Technology of China, Chengdu, Sichuan 610054, People’s Republic of China Received 20 January 2004; received in revised form 8 April 2004; accepted 15 April 2004
Abstract In this work, the effect of electrothermal annealing on the transformation characterization of TiNi shape memory alloy and the electrothermal actuating characteristics of a two-way shape memory effect (TWSME) extension spring were investigated with direct electrical current. The results showed that with increasing direct electrical current density, the B2-R-phase transformation shifts to a lower temperature and R-phase-B190 shifts to a higher temperature in the cooling process. When annealing electrical current density reached 12.2 A/mm2, the R-phase disappeared and austenite transformed into martensite directly. The electrothermal annealing was an effective method of heat treatment in a selected part of shape memory alloy device. The electrothermal actuating characteristics of a TWSME spring showed that the time response and the maximum elongation greatly depended on the magnitude of the electrical current. r 2004 Elsevier B.V. All rights reserved. PACS: 62.50.Ks; 61.66.Dk; 62.20.Fe; 62.40.+i; 64.70.Kb; 81.40.J Keywords: TiNi shape memory alloy; Electrothermal annealing; Phase transformation; Two-way shape memory effect
1. Introduction TiNi shape memory alloy (SMA) of nearequiatomic composition is a technologically im*Corresponding author. Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu 610054, People’s Republic of China. Tel./fax: 86-2883201939. E-mail address: [email protected] (X.T. Zu).
portant shape memory material because the TiNi SMA combines good functional and structural properties [1–3]. There is also an interest in Ni-rich NiTi alloys, because, the phase transition temperatures can be controlled through heat treatment [4,5]. Depending on the heat treatment, TiNi alloy exhibits either a one-step martensitic transformation (MT) from the high-temperature B2 to the B190 (monoclinic) phase, or a two-step MT
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.04.064
ARTICLE IN PRESS 366
Z.G. Wang et al. / Physica B 349 (2004) 365–370
from the B2 to the rhombohedral R-phase to the B190 . The heat treatments are conventially carried out in a furnace. SMA wires are most easily actuated by electrical current via Joule heating, which is an easy and efficient way [6]. In the same way, with a higher current, the SMA can be heated up to an annealing temperature. So the SMA also can be annealed with electrical current. The electrothermal annealing will be more useful in local annealing, i.e. we can anneal the part of our device by controlling where the electrical current passes through. To our knowledge, there are few reports in the literature on the effect of electrothermal annealing on the transformation characteristics of shape memory alloy. In this work the effects of electrothermal annealing on the transformation characteristics of TiNi SMA as well as the electrothermal actuating characteristics of a two-way shape memory effect (TWSME) of an extension spring were investigated.
2. Experiment A Ti-49.8 at%Ni wire with diameter of 0.55 mm, provided by the Northwest Institute of NonFerrous Metal of China, was used for this study. The shape memory alloy was electrothermal annealed with 8.4, 9.3, 10.5 and 12.2 A/mm2 direct electrical current for 1 h. In order to make a comparison between the electrothermal heat treatment and the conventional heat treatment, some samples were annealed with conventional furnace at temperatures 400 C and 450 C for 1 h. The transformation temperatures of the samples were measured using differential scanning calorimetry (DSC) with a scanning rate of 10 C/min under nitrogen atmosphere. The TWSME springs were prepared as the method that was described in Ref. [7]. The asreceived TiNi wire was wound on a cylindrical jig. They were annealed at 500 C with a regular furnace for 1 h followed by air cooling. The mean diameter of the spring is 4.5 mm and the pitch of the spring is 0.1 mm after annealing heat treatment. Then the springs were extended till the pitch reached 12 mm and constrained at the extension
state and annealed at 550 C with a regular furnace for 1 h followed by air cooling. The mean diameter of the spring is 4.5 mm and the pitch of the spring is 12 mm after the above treatment. And the springs were thermal–mechanical trained for 150 cycles. The two-way shape elongation was about 65%. The two-way shape memory spring was actuated by Joule heating and air convection cooling. In this experiment, 2.7–8.4 A/mm2 current densities were used to drive the two-way shape memory spring.
3. Results and discussion 3.1. Effect of electrothermal annealing on the transformation behavior of TiNi SMA Fig. 1 shows the DSC curves of both the asreceived sample and the samples annealed with different electrical currents. It can be seen from the figure that there is only one-step phase transformation occurring in the as received wire in the temperature range between 0 C and 140 C. In the case of the samples annealed with 8.4 and 9.3 A/ mm2 for 1 h, two-step transformation from austenite to R-phase and further to martensite have taken place upon cooling, one step reverse transformation happens upon heating. For the samples annealed with 10.5 A/mm2 for 1 h, the B2-R-phase and R-phase-B190 transformation almost merged into one peak, the R-phase-B190 transformation appears as a shoulder on the low-temperature side of B2-R-phase transformation. When annealing electrical current reaches 12.2 A/mm2, as shown in Fig. 1(e), the R-phase disappears and austenite transforms into martensite directly. The transformation temperatures for the samples annealed with different electrical current is shown in Table 1. The transformation temperatures are acquired by determining the onset point of the slope change in the DSC curves. In the cooling process, the B2-R-phase transformation shifts to lower temperature and R-phaseB190 shifts to higher temperature with increasing direct current density, these results are consistent with the previous results obtained with conventional heat treatments [8]. The values of latent heat
ARTICLE IN PRESS Z.G. Wang et al. / Physica B 349 (2004) 365–370
of transformation for the heating curves are shown in Fig. 2. The figure shows that the latent heat of transformation increases with increasing annealing electrical current. (a)
(b) ,
(c)
,
(d)
,
(e)
,
0
20
40
60
80
100 120 140
Fig. 1. DSC curves of Ti-49.8 at%Ni shape memory alloy (a) as-received sample and electrothermal annealed with (b) 8.4 A/ mm2; (c) 9.3 A/mm2; (d) 10.5 A/mm2 and (e) 12.2 A/mm2 for 1 h.
367
Fig. 3 shows the DSC curves of both the asreceived sample and the samples annealed at 400 C and 450 C for 1 h. Upon cooling, two-step transformation among austenite, R-phase and martensite can be observed, while upon heating, only one-step transformation between martensite and austenite can be detected for the sample annealed at 400 C. When the sample annealed at 450 C as shown in Fig. 1(c), the R-phase disappeared and austenite transforms into martensite directly. The transformation temperatures for the samples annealed with different temperatures are shown in Table 1. The latent heats of transformation upon heating are 16.12 and 16.91 J/g for the sample annealed at 400 C and 450 C. The latent heat increases with increasing annealing temperature, too. The transformation changes with different direct current density can be explained by the reason that the recovery and recrystallization process of the shape memory alloy [9]. When the current passes through the TiNi alloy and electrical energy is converted into thermal energy according to the Joule’s law. Annealing happens which can modify the microstructure of the SMA. So the electrothermal annealing is a very effective method of the heat treatment applicable easily to SMA wires, especially in a selected part of shape memory alloy device. It is well known that severe cold work inhibits the martensitic transformation by introduction of defects, which are essentially dislocations [10]. The SMAs are in cold drawn state in the present investigation, so there are many dislocations in the TiNi specimen. Annealing induces annihilation of the dislocation and recrystallization of the cold worked specimen. At low annealing electrical
Table 1 Transformation temperatures of the Ti-49.8 at%Ni samples annealed with different direct current density or temperature Conditions of annealing Direct current density Direct current density Direct current density Direct current density Temperature 400 C Temperature 450 C
8.4 A/mm2 9.3 A/mm2 10.5 A/mm2 12.2 A/mm2
As ( C)
Af ( C)
Rs ( C)
Rf ( C)
Ms ( C)
Mf ( C)
69 66 69 69 68 69
76 75 79 86 74 82
68 54 48 — 51 —
45 49 — — 47 —
22 26 — 56 36 49
20 22 36 44 28 42
ARTICLE IN PRESS Z.G. Wang et al. / Physica B 349 (2004) 365–370
368
Heat of transformation(J/g)
18
17
16
15
14
2.0
2.2
2.4
2.6
2.8
3.0
Electrical current (A)
Fig. 2. Effect of electrical current on the latent heat of transformation upon heating of the TiNi wire.
(a)
(b)
(c)
0
current or temperature, the dislocations density reduces and the internal stress reduces, too, which leads to the increase of Ms and Mf : The latent heat of transformation increases with increasing annealing electrical current or temperature is consistent with previous results [11], which show that the dislocations associated with high levels of plastic deformation generate an internal stress, which restricts the martensite from transforming into austenite. This martensitic phase remains ‘‘pinned’’ in the microstructure until the dislocations are removed through an annealing process. Therefore, with increasing annealing electrical current temperature, more dislocations are removed and more martensite transforms into austenite, so the latent heat increases. Several studies have reported that the appearance of the R-phase could be due to dislocations [12,13]. The R-phase nucleates preferentially in the internal stresses concentrated regions [13,14]. With increasing annealing temperature, the dislocations density reduces and the internal stress reduces, too, thus the Rs and Rf decreases. The disappearance of the R-phase transformation can be attributed to the disappearance of the internal stresses in recrystallized grains, which has been verified by Khelfaoui et al. [10]. So the parent to R-phase transformation disappears when the internal stresses disappear from the grains. 3.2. Characterization of the TWSME spring actuated by direct electrical current
20
40
60
80
100 120
140
Fig. 3. DSC curves of Ti-49.8 at%Ni shape memory alloy (a) as-received sample and annealed at (b) 400 C and (c) 450 C for 1 h.
current or temperature, the dislocations density is high, and the associated internal stress generated by these dislocations restricts the specimen from austenite transforming into martensite. So the martensitic transformation happens at low temperature. With increasing the annealing electrical
Fig. 4 shows the elongation–temperature curve of the TiNi extension spring showing that the maximum elongation is about 65%. The transformation temperatures evaluated from the curve were: As ¼ 68 C; Af ¼ 71 C; Ms ¼ 48 C and Mf ¼ 39:5 C: Fig. 5 shows the elongation–time curves for different actuating direct electrical current densities. The time interval between the start and the end of the spring’s shape change (termed time response) and the maximum elongation at different electrical current deduced from Fig. 5 are shown in Fig. 6. As can be observed from Fig. 6, the spring shows no shape change at 2.7 A/mm2. When the
ARTICLE IN PRESS Z.G. Wang et al. / Physica B 349 (2004) 365–370
Cooling Time response (s)
60
Elongation /%
50 40 30 20 10 0 -10 20
Heating 30
40
20
80
18
70
16
60
14
50
12
60
70
80
90
100
Temperature /oC Fig. 4. Elongation–temperature curve for TWSME TiNi spring heat treated by a conventional furnace.
Fig. 5. Elongation of TWSME extension spring vs. time at different electrical current 2.7–8.4 A/mm2.
electrical current is lower than 6.3 A/mm2, the maximum elongation increases and the time response decreases with increasing the electrical current. The maximum elongation increases from 0% to 65% and time response decreases from 17 to 7 s. When the electrical current is higher than 6.3 A/mm2, the time response continually decreases with increasing current, but the maximum elongation decreases. When the current passes through the TiNi alloy, the electrical energy is converted into thermal energy according to the Joule’s law. The magnitude of the thermal energy produced depends on the magnitude of the electrical current and time. For a small electrical current, a long time is needed to lead to the transformation of the TiNi alloy spring, and vice versa. As the experiment was done
30
8 6
20
4
10
2
0
0
50
40
10
2
3
4
5
6
7
8
9
Maximum elongation(%)
70
369
-10
Electrical current density (A/mm2)
Fig. 6. Curves for the maximum elongation and the time response for different actuating electrical current densities.
in air, Joule heating and the air convection cooling happened at the same time. So at a lower electrical current, the heat generated by Joule heating dissipates, leading to only a part martensite transforms to austenite in the TiNi alloy, which leads to a small maximum elongation. By increasing the electrical current, the heating is accelerated, the dissipated heat reduced, and more martensite transformed to austenite, thereby increasing the maximum elongation. It is commonly accepted that the two-way shape memory effect was obtained by the anisotropic dislocation structure generated during thermomechanical training [15–17]. This dislocation structure creates an anisotropic stress field in the matrix, which guides the formation of martensite into variants of preferential orientations in relation to the deformation adopted in the training procedure, thus resulting in a macroscopic shape change during subsequent thermal transformation cycles. By martensitic transformation in an untrained specimen, and when free of external stress, all martensitic variants are crystallographically and thermodynamically equivalent. However, the dislocation structures generated by training cause a free energy decrease in one or in a group of variants and they become no longer thermodynamically equivalent. Then upon cooling, a larger amount of these variants is formed, leading to a macroscopic form change as observed in the trained samples. When the electrical current is too high, annealing heat treatment will happen as
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shown in Section 3.1, the dislocation pattern will be changed by the electrothermal heating. Thus, higher electrical current leads to a decrease in the maximum elongation.
Foundation 03ZQ026-059 and by the Projectsponsored by SRF for ROCS, SEM.
References 4. Conclusions The electrothermal annealing is a very effective method of heat treating in a selected part of a shape memory alloy device. After the shape memory alloy annealed by electrical current, the B2-R-phase transformation shifts to lower temperature and R-phase-B190 shifts to higher temperature in the cooling process with increasing direct current density. When annealing electrical current reaches 12.2 A/mm2, the R-phase disappears and austenite transforms into martensite directly. The electrothermal actuating characteristics of a TWSME spring shows that the time response and the maximum elongation greatly depend on the magnitude of the electrical current. The time response continually decreases with increasing current very quickly at first stage then slowed down. The maximum elongation increases with increasing current first and then decreased.
Acknowledgements This study was financially supported by the National Natural Science Foundation of China (10175042), by the Sichuan Young Scientists
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