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Damping behavior of TiNi-based shape memory alloys
Materials Science and Engineering A 394 (2005) 78–82
Damping behavior of TiNi-based shape memory alloys W. Cai, X.L. Lu∗ , L.C. Zhao School of Materials Science and Engineering, Harbin Institute of Technology, P.O. Box 405, Harbin 150001, China Received 16 August 2004; received in revised form 1 November 2004; accepted 15 November 2004
Abstract The damping behavior of TiNi-based alloys has been investigated by dynamic mechanical analyzer (DMA) instruments. It shows that an appropriate cold-rolling deformation on the martensitic Ti50 Ni50 alloy can enhance its damping capacity. The damping behavior of the Ti49.2 Ni50.8 alloy is aging condition dependent and the damping capacity peak value corresponding to the phase transformation increases due to the increase of the amount of boundaries between the martensite and parent phase as a result of the existence of the Ti3 Ni4 particles. The Ti44 Ni47 Nb9 alloy possesses high damping capacity either during phase transformation or in the parent phase, which is very important for the engineering application. © 2004 Elsevier B.V. All rights reserved. Keywords: Damping; Cold rolling; Secondary particles; TiNi-based alloys
1. Introduction In recent years, TiNi-based shape memory alloys have started to attract increasing attention due to their damping properties. It has also been found that TiNi-based alloys can exhibit high damping capacity during phase transformation or in the martensitic state, which open a new application field in engineering [1–4]. In the past, there have been many applications to utilize the damping properties in civil constructions, especially in buildings and bridges against earthquake damage [5,6]. From the application point of view, the investigations about the influence factors on damping capacity are of importance. Nowadays, the effects of the temperature changing rate, the frequency and the strain amplitude on the damping capacity have been systematically investigated [7,8], and several researches have been conducted on TiNi-based alloys having different composition, processing histories, etc. [9,10]. But, the results about the effects of cold-rolling deformation in martensite state and the secondary particles on the damping capacity are very limited. In this paper, the damping capacity of the deformed martensite, the aged Ni-rich TiNi alloys which have Ti3 Ni4 ∗
Corresponding author. Tel.: +86 451 86412163; fax: +86 451 86413922. E-mail address: [email protected] (X.L. Lu).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.11.030
hard particles and the damping capacity of TiNiNb alloys which have -Nb soft particles have been investigated by using dynamical mechanical analyzer (DMA) measurements. The effects of cold-rolling deformation and secondary particles on the damping capacity are discussed and the significance on engineering applications is also illustrated.
2. Experimental The materials used in these experiments are a Ti50 Ni50 alloy, a Ni-rich Ti49.2 Ni50.8 alloy and a Ti44 Ni47 Nb9 alloy. The plates of Ti50 Ni50 alloy were solution treated and then were cold-rolled at room temperature to 8, 15, 21% reductions in thickness after having been quenched into liquid nitrogen to achieve full martensitic structure at the beginning. Samples with the dimensions of 48 mm × 6 mm × 1 mm were spark cut. The specimens of Ti49.2 Ni50.8 alloy were solution treated then aged in the 500 ◦ C for 0, 1, 2, 4, 8, 16 h, respectively, in quartz capsules and then quenched into water. The specimens of Ti44 Ni47 Nb9 alloy were annealed at 850 ◦ C for 1 h in quartz capsules and furnace cooled. The Perkin-Elmer Diamond DSC was used to determine the transformation temperatures Mf , Ms , As , Af . The weights of samples for DSC measurement were 10–20 mg, and the heating and cooling rates
W. Cai et al. / Materials Science and Engineering A 394 (2005) 78–82
79
Table 1 The transformation temperature of the experimental alloys Composition
Heat treatment
Mf (◦ C)
Ms (◦ C)
Ti50 Ni50 Ti49.2 Ni50.8 Ti44 Ni47 Nb9
850 ◦ C × 2 h, water cooled 850 ◦ C × 2 h, water cooled 850 ◦ C × 1 h, furnace cooled
21 −123 −125
68 −105 −88
were 10 ◦ C/min. During measurements, the samples were first quickly heated to 150 ◦ C and then cooled to −150 ◦ C at constant cooling rate. When the preset low temperature was reached, the specimen was again heated to 150 ◦ C at 10 ◦ C/min (see Table 1). Damping tests were performed either in temperature variation or at room temperature using a dynamic mechanical analyzer (Rheometric Scientific DMA IV) instrument. During temperature variation measurements, the samples were at first quickly heated to 60 ◦ C, which was well above the Af temperature, and then cooled to −140 ◦ C at a cooling rate of 5 ◦ C/min. The measuring frequency was 1 Hz and the strain amplitude was 3 × 10−5 . At room temperature measurement, the frequency of oscillation was in the range of 0.1–10 Hz, while the strain amplitude was in the range of 1 × 10−5 –1 × 10−4 .
3. Results and discussion 3.1. The damping behavior of martensite in Ti50 Ni50 alloy with different cold-rolling deformation The damping capacity versus frequency curves of martensite in Ti50 Ni50 alloy with different cold-rolling deformation are shown in Fig. 1. It can be observed that the evolution of the damping capacity with the frequency is quite different. The 8 and 15% deformed martensite specimens show the similar behavior: with the increase of frequency, the damping capacity decreases drastically at the beginning, and then
Fig. 1. The damping capacity vs. frequency curves of deformed martensite for Ti50 Ni50 alloy. The strain amplitude was 3 × 10−5 and the testing temperature was room temperature.
As (◦ C)
Af (◦ C)
47 −79 −56
79 −57 −15
decreases slightly. However, the damping capacity of the thermal martensite and the 21% deformed martensite decreases slightly with frequency increasing in the whole range. The damping capacity has been plotted as a function of the cold-rolling degree in Fig. 2. The results show that the damping capacity of martensite goes up with an increase of the cold-rolling degree, and then drops with further increasing the cold-rolling degree when the frequency and amplitude are constant. As we know, the damping capacity of martensite is closely related to the movement of twin interfaces. It is also well known that dislocations can be induced through the coldrolling deformation and the movement of dislocations also contribute to the damping capacity of materials. Thus, it is reasonable that the damping capacity of the deformed martensite arises from two aspects, i.e. interface damping and dislocation damping. The relationship between damping capacity and frequency observed in the 8 and 15% deformed martensite attributed either to the motion of the dislocations or the motion of the interfaces. Furthermore, the experimental results mentioned in Fig. 2 can be explained as follows: on the one hand, the interface damping is determined by the mobility of the interfaces and the occurrence of dislocations can lower their mobility, in consequence, the interface damping decreases when the cold-rolling degree increases. On the other hand, according to the Granato-L¨ucke model the dislocation damping is proportional to the dislocation density and also related to the mobility of dislocations. At the beginning, with the increase of cold-rolling degree, the dislocation damping increases due to the increasing of dislocation density. However, the mobility of dislocation decreases as a result
Fig. 2. The damping capacity as a function of cold-rolling degree for Ti50 Ni50 alloy. The strain amplitude was 3 × 10−5 , the frequency was 0.1 Hz and the testing temperature was room temperature.
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Fig. 3. The damping capacity vs. temperature curves of Ti49.2 Ni50.8 alloy with different aging conditions: 500 ◦ C aging (a) 0 h; (b) 1 h; (c) 2 h; (d) 4 h; (e) 8 h; (f) 16 h. The strain amplitude was 3 × 10−5 , the frequency was 1 Hz and the cooling rate was 5 ◦ C/min.
of increasing dislocation density. The combination of the two aspects can explain the variation of damping capacity with the cold-rolling degree in the martensite. 3.2. The damping behavior of Ti49.2 Ni50.8 alloy with different aging conditions Fig. 3(a)–(f) gives the damping capacity as a function of temperature during cooling for the Ti49.2 Ni50.8 alloy with several different aging conditions. It can be seen from Fig. 3(a)
that there is one damping capacity peak which is associated with the martensitic transformation. Fig. 3(b) demonstrates that there are two peaks observed on cooling. Peak P1 is obviously associated with the R-phase transformation, and peak P2 corresponds to the martensitic transformation [3]. Otherwise, a broad peak, or rather a hump, observed in the temperature range about from −60 to −100 ◦ C. Fig. 3(c) shows that three damping capacity peaks occur apparently and among them P2 is considered to correspond to martensitic transformation. Fig. 3(d) exhibits the same behavior as Fig. 3(c), but
W. Cai et al. / Materials Science and Engineering A 394 (2005) 78–82
the peak values and the temperature located are different. In Fig. 3(e), there exist two peaks again which are associated with R-phase transformation and martensitic transformation, respectively. Fig. 3(f) is very similar to Fig. 3(e). It is worth noticing that in Fig. 3(e) and (f), the damping capacity keeps an almost constant value at a relatively wide temperature range about from the 0 to 18 ◦ C, which may provide potential engineering applications. Aging treatment in Ni-rich alloy will induce R-phase transformation, which corresponds to the new damping capacity peak in the aging specimens. In Fig. 3(c) and (d), the three damping capacity peaks are considered to be related to the multi-step transformation [11]. Choosing the damping capacity peak corresponding to the martensitic transformation from the Ti49.2 Ni50.8 alloy with different aging conditions, it can be seen that in general, the damping capacity peak value of the aging treatment specimens is higher than that of unaged specimen. Moreover, the damping capacity peak value increases with increasing aging time below 2 h and decreases when above it. There could be two reasons for the observed phenomena. First, in the early aging state, the Ti3 Ni4 precipitates in the specimens are fine, disperse and good coherence with the matrix [12], which induced internal strain fields around them. It is suggested that the improvement of damping capacity peak value maybe arise from the internal strain fields. With the aging time prolong, the Ti3 Ni4 precipitates are coarsened and the internal stress fields become weak, as a result of the decreasing of damping capacity peak, which is in accordance with the previous suggestion. Second, the interfaces between the precipitates and the matrix also contribute to the damping capacity during phase transformation and the reason is explained below. However, the details of how the Ti3 Ni4 precipitates affect the damping capacity are under further studies. 3.3. The damping behavior of the Ti44 Ni47 Nb9 alloy The damping capacity of the Ti44 Ni47 Nb9 alloy as a function of temperature in the cooling process is shown in Fig. 4. It shows that one damping capacity peak appears due to the martensitic transformation, however, above the Ms temperature, the damping capacity remains nearly constant in a rather wide temperature range. Comparing the result of Figs. 3(a) and 4, it is worth noticing that the damping capacity peak value corresponding to martensitic transformation of Ti44 Ni47 Nb9 alloy is much higher than that of the Ti49.2 Ni50.8 alloy. As we know, during the martensitic transformation, most of the energy is dissipated due to the movement of martensite/parent interfaces, which causes the peak of the damping capacity to appear in the transformation region. That is to say, the dissipated energy is due to the creation or the motion of the interphase boundaries. Therefore, the experimental results can be explained as follows: the microstructure of the Ti44 Ni47 Nb9 alloy is characterized by an ordered TiNi matrix phase containing a fine dispersion of -Nb particles [13]. The interfaces between the -Nb particles and matrix pro-
81
Fig. 4. The damping capacity vs. temperature curves for Ti44 Ni47 Nb9 alloy. The strain amplitude was 3 × 10−5 , the frequency was 1 Hz and the cooling rate was 5 ◦ C/min.
vided the nucleus sites for the martensite. As a consequence, the amount of boundaries between the martensite and parent phase increases, which may be greatly beneficial to the damping capacity during the phase transformation. The reason that the aged Ti49.2 Ni50.8 alloys exhibit the high damping capacity during phase transformation is the same with it. Fig. 5 shows the damping capacity as a function of frequency for the Ti44 Ni47 Nb9 alloy in the parent phase state. It indicates that the damping capacity decreases drastically at the beginning and then slightly decreases when the vibration frequency is larger than 0.15 Hz. It is of great interest that the damping capacity of parent phase reaches a very high value at 0.1 Hz, which is even higher than the result of thermal martensite in Fig. 1. Fig. 6 demonstrates the evolution of the damping capacity as a function of strain amplitude in the parent phase for the Ti44 Ni47 Nb9 alloy. It can be seen that the damping capacity
Fig. 5. The damping capacity tan δ vs. frequency curve for Ti44 Ni47 Nb9 alloy. The strain amplitude was 3 × 10−5 and the testing temperature was room temperature.
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Fig. 6. The damping capacity vs. strain amplitude curve for Ti44 Ni47 Nb9 alloy. The frequency was 0.1 Hz and the testing temperature was room temperature.
of the parent phase decreases rapidly with an increase of strain amplitude at first, and then decreases slowly after the amplitude reaches a certain value of about 3 × 10−5 . It is well known that -Nb particles existing in TiNiNb alloys are soft and easily deformed by external stress. Thus, there are dislocations existing in the vicinity of interfaces between -Nb particles and the matrix as a result of lattice mismatching. These dislocations maybe move under a certain external stress due to the low stiffness of the -Nb particles, which can contribute to the high damping capacity in the parent phase of the Ti44 Ni47 Nb9 alloy. Additionally, we suggest that the high damping capacity of the parent phase for the Ti44 Ni47 Nb9 alloy is closely related to the low strength of soft -Nb particles existing in the Ti44 Ni47 Nb9 alloy, which will be studied further. As we know, even though the martensite of TiNi alloys has high damping capacity, it has relatively low strength compared with the parent phase. According to the experimental result, the parent phase of the Ti44 Ni47 Nb9 alloy possesses high damping capacity at low strain amplitude and low frequency conditions, which is very significant for engineering applications because the materials which possess not only enough damping capacity but also enough strength should be pursued.
4. Conclusions The damping capacity of martensite in the Ti50 Ni50 alloy first increases with the increasing cold-rolling deforma-
tion, and then decreases with further increase of the coldrolling degree. The specimen with 8% deformation degree has the highest damping capacities at the frequency range of 0.1–10 Hz, which are obviously higher than that of undeformed martensite. The dislocations introduced by the coldrolling deformation are responsible for these high damping capacities. Aging treatment for Ti49.2 Ni50.8 alloy can apparently enhance the damping capacity during phase transformation. The highest damping capacity arises from the increase of the amount of boundaries between the martensite and parent phase due to the existence of the Ti3 Ni4 particles because the interfaces between the Ti3 Ni4 particles and matrix provided the nucleus sites for the martensite. The Ti44 Ni47 Nb9 alloy possesses high damping capacity during martensitic transformation as well as in parent phase and martensite, which is closely related to the -Nb soft particles existing in the Ti44 Ni47 Nb9 alloy and the motion of dislocations in the vicinity of interfaces between -Nb particles and matrix. The high damping capacity of parent phase for the Ti44 Ni47 Nb9 alloy provides potential applications for engineering because of enough damping capacity and strength.
References [1] J. van Humbeeck, J. Stoibeer, L. Delaey, R. Gotthardt, Z. Metallkd. 86 (1995) 176–183. [2] I. Yoshida, D. Monma, K. Iinoa, T. Ono, K. Otsuka, M. Asai, Mater. Sci. Eng. A 370 (2004) 444–448. [3] S.K. Wu, H.C. Lin, J. Alloy Compd. 355 (2003) 90–96. [4] O. Mercier, K.N. Melton, Y. de Preville, Acta Mater. 27 (1979) 1467–1475. [5] M. Dolce, D. Cardone, Int. J. Mech. Sci. 43 (2001) 2631–2656. [6] D. Cardone, M. Dolce, R. Marnetto, SMST-99: Proceedings of the First European Conference on Shape Memory and Superelastic Technologies, Belgium, 1999, pp. 345–352. [7] F. Ddeborde, V. Pelosin, A. Rivi´ere, Scripta Mater. 33 (1995) 1993–1998. [8] S. Golyandin, S. Kustov, K. Sapozhnikov, M. Parlinska, R. Gotthardt, J. van Humbeeck, J. Alloy Compd. 310 (2000) 312–317. [9] B. Coluzzi, A. Biscarini, R. Campanella, G. Mazzolai, L. Trotta, F.M. Mazzolai, J. Alloy Compd. 310 (2000) 300–305. [10] I. Yoshida, T. Ono, M. Asai, J. Alloy Compd. 310 (2000) 339– 343. [11] J.K. Allafi, R. de Batist, L. Delaey, Acta Mater. 50 (2002) 793– 803. [12] C.Y. Xie, L.C. Zhao, T.C. Lei, Scripta Mater. 24 (1990) 1753–1758. [13] C.S. Zhang, L.C. Zhao, T.W. Duerig, C.M. Wayman, Scripta Mater. 24 (1990) 1807–1812.
Damping behavior of TiNi-based shape memory alloys W. Cai, X.L. Lu∗ , L.C. Zhao School of Materials Science and Engineering, Harbin Institute of Technology, P.O. Box 405, Harbin 150001, China Received 16 August 2004; received in revised form 1 November 2004; accepted 15 November 2004
Abstract The damping behavior of TiNi-based alloys has been investigated by dynamic mechanical analyzer (DMA) instruments. It shows that an appropriate cold-rolling deformation on the martensitic Ti50 Ni50 alloy can enhance its damping capacity. The damping behavior of the Ti49.2 Ni50.8 alloy is aging condition dependent and the damping capacity peak value corresponding to the phase transformation increases due to the increase of the amount of boundaries between the martensite and parent phase as a result of the existence of the Ti3 Ni4 particles. The Ti44 Ni47 Nb9 alloy possesses high damping capacity either during phase transformation or in the parent phase, which is very important for the engineering application. © 2004 Elsevier B.V. All rights reserved. Keywords: Damping; Cold rolling; Secondary particles; TiNi-based alloys
1. Introduction In recent years, TiNi-based shape memory alloys have started to attract increasing attention due to their damping properties. It has also been found that TiNi-based alloys can exhibit high damping capacity during phase transformation or in the martensitic state, which open a new application field in engineering [1–4]. In the past, there have been many applications to utilize the damping properties in civil constructions, especially in buildings and bridges against earthquake damage [5,6]. From the application point of view, the investigations about the influence factors on damping capacity are of importance. Nowadays, the effects of the temperature changing rate, the frequency and the strain amplitude on the damping capacity have been systematically investigated [7,8], and several researches have been conducted on TiNi-based alloys having different composition, processing histories, etc. [9,10]. But, the results about the effects of cold-rolling deformation in martensite state and the secondary particles on the damping capacity are very limited. In this paper, the damping capacity of the deformed martensite, the aged Ni-rich TiNi alloys which have Ti3 Ni4 ∗
Corresponding author. Tel.: +86 451 86412163; fax: +86 451 86413922. E-mail address: [email protected] (X.L. Lu).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.11.030
hard particles and the damping capacity of TiNiNb alloys which have -Nb soft particles have been investigated by using dynamical mechanical analyzer (DMA) measurements. The effects of cold-rolling deformation and secondary particles on the damping capacity are discussed and the significance on engineering applications is also illustrated.
2. Experimental The materials used in these experiments are a Ti50 Ni50 alloy, a Ni-rich Ti49.2 Ni50.8 alloy and a Ti44 Ni47 Nb9 alloy. The plates of Ti50 Ni50 alloy were solution treated and then were cold-rolled at room temperature to 8, 15, 21% reductions in thickness after having been quenched into liquid nitrogen to achieve full martensitic structure at the beginning. Samples with the dimensions of 48 mm × 6 mm × 1 mm were spark cut. The specimens of Ti49.2 Ni50.8 alloy were solution treated then aged in the 500 ◦ C for 0, 1, 2, 4, 8, 16 h, respectively, in quartz capsules and then quenched into water. The specimens of Ti44 Ni47 Nb9 alloy were annealed at 850 ◦ C for 1 h in quartz capsules and furnace cooled. The Perkin-Elmer Diamond DSC was used to determine the transformation temperatures Mf , Ms , As , Af . The weights of samples for DSC measurement were 10–20 mg, and the heating and cooling rates
W. Cai et al. / Materials Science and Engineering A 394 (2005) 78–82
79
Table 1 The transformation temperature of the experimental alloys Composition
Heat treatment
Mf (◦ C)
Ms (◦ C)
Ti50 Ni50 Ti49.2 Ni50.8 Ti44 Ni47 Nb9
850 ◦ C × 2 h, water cooled 850 ◦ C × 2 h, water cooled 850 ◦ C × 1 h, furnace cooled
21 −123 −125
68 −105 −88
were 10 ◦ C/min. During measurements, the samples were first quickly heated to 150 ◦ C and then cooled to −150 ◦ C at constant cooling rate. When the preset low temperature was reached, the specimen was again heated to 150 ◦ C at 10 ◦ C/min (see Table 1). Damping tests were performed either in temperature variation or at room temperature using a dynamic mechanical analyzer (Rheometric Scientific DMA IV) instrument. During temperature variation measurements, the samples were at first quickly heated to 60 ◦ C, which was well above the Af temperature, and then cooled to −140 ◦ C at a cooling rate of 5 ◦ C/min. The measuring frequency was 1 Hz and the strain amplitude was 3 × 10−5 . At room temperature measurement, the frequency of oscillation was in the range of 0.1–10 Hz, while the strain amplitude was in the range of 1 × 10−5 –1 × 10−4 .
3. Results and discussion 3.1. The damping behavior of martensite in Ti50 Ni50 alloy with different cold-rolling deformation The damping capacity versus frequency curves of martensite in Ti50 Ni50 alloy with different cold-rolling deformation are shown in Fig. 1. It can be observed that the evolution of the damping capacity with the frequency is quite different. The 8 and 15% deformed martensite specimens show the similar behavior: with the increase of frequency, the damping capacity decreases drastically at the beginning, and then
Fig. 1. The damping capacity vs. frequency curves of deformed martensite for Ti50 Ni50 alloy. The strain amplitude was 3 × 10−5 and the testing temperature was room temperature.
As (◦ C)
Af (◦ C)
47 −79 −56
79 −57 −15
decreases slightly. However, the damping capacity of the thermal martensite and the 21% deformed martensite decreases slightly with frequency increasing in the whole range. The damping capacity has been plotted as a function of the cold-rolling degree in Fig. 2. The results show that the damping capacity of martensite goes up with an increase of the cold-rolling degree, and then drops with further increasing the cold-rolling degree when the frequency and amplitude are constant. As we know, the damping capacity of martensite is closely related to the movement of twin interfaces. It is also well known that dislocations can be induced through the coldrolling deformation and the movement of dislocations also contribute to the damping capacity of materials. Thus, it is reasonable that the damping capacity of the deformed martensite arises from two aspects, i.e. interface damping and dislocation damping. The relationship between damping capacity and frequency observed in the 8 and 15% deformed martensite attributed either to the motion of the dislocations or the motion of the interfaces. Furthermore, the experimental results mentioned in Fig. 2 can be explained as follows: on the one hand, the interface damping is determined by the mobility of the interfaces and the occurrence of dislocations can lower their mobility, in consequence, the interface damping decreases when the cold-rolling degree increases. On the other hand, according to the Granato-L¨ucke model the dislocation damping is proportional to the dislocation density and also related to the mobility of dislocations. At the beginning, with the increase of cold-rolling degree, the dislocation damping increases due to the increasing of dislocation density. However, the mobility of dislocation decreases as a result
Fig. 2. The damping capacity as a function of cold-rolling degree for Ti50 Ni50 alloy. The strain amplitude was 3 × 10−5 , the frequency was 0.1 Hz and the testing temperature was room temperature.
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W. Cai et al. / Materials Science and Engineering A 394 (2005) 78–82
Fig. 3. The damping capacity vs. temperature curves of Ti49.2 Ni50.8 alloy with different aging conditions: 500 ◦ C aging (a) 0 h; (b) 1 h; (c) 2 h; (d) 4 h; (e) 8 h; (f) 16 h. The strain amplitude was 3 × 10−5 , the frequency was 1 Hz and the cooling rate was 5 ◦ C/min.
of increasing dislocation density. The combination of the two aspects can explain the variation of damping capacity with the cold-rolling degree in the martensite. 3.2. The damping behavior of Ti49.2 Ni50.8 alloy with different aging conditions Fig. 3(a)–(f) gives the damping capacity as a function of temperature during cooling for the Ti49.2 Ni50.8 alloy with several different aging conditions. It can be seen from Fig. 3(a)
that there is one damping capacity peak which is associated with the martensitic transformation. Fig. 3(b) demonstrates that there are two peaks observed on cooling. Peak P1 is obviously associated with the R-phase transformation, and peak P2 corresponds to the martensitic transformation [3]. Otherwise, a broad peak, or rather a hump, observed in the temperature range about from −60 to −100 ◦ C. Fig. 3(c) shows that three damping capacity peaks occur apparently and among them P2 is considered to correspond to martensitic transformation. Fig. 3(d) exhibits the same behavior as Fig. 3(c), but
W. Cai et al. / Materials Science and Engineering A 394 (2005) 78–82
the peak values and the temperature located are different. In Fig. 3(e), there exist two peaks again which are associated with R-phase transformation and martensitic transformation, respectively. Fig. 3(f) is very similar to Fig. 3(e). It is worth noticing that in Fig. 3(e) and (f), the damping capacity keeps an almost constant value at a relatively wide temperature range about from the 0 to 18 ◦ C, which may provide potential engineering applications. Aging treatment in Ni-rich alloy will induce R-phase transformation, which corresponds to the new damping capacity peak in the aging specimens. In Fig. 3(c) and (d), the three damping capacity peaks are considered to be related to the multi-step transformation [11]. Choosing the damping capacity peak corresponding to the martensitic transformation from the Ti49.2 Ni50.8 alloy with different aging conditions, it can be seen that in general, the damping capacity peak value of the aging treatment specimens is higher than that of unaged specimen. Moreover, the damping capacity peak value increases with increasing aging time below 2 h and decreases when above it. There could be two reasons for the observed phenomena. First, in the early aging state, the Ti3 Ni4 precipitates in the specimens are fine, disperse and good coherence with the matrix [12], which induced internal strain fields around them. It is suggested that the improvement of damping capacity peak value maybe arise from the internal strain fields. With the aging time prolong, the Ti3 Ni4 precipitates are coarsened and the internal stress fields become weak, as a result of the decreasing of damping capacity peak, which is in accordance with the previous suggestion. Second, the interfaces between the precipitates and the matrix also contribute to the damping capacity during phase transformation and the reason is explained below. However, the details of how the Ti3 Ni4 precipitates affect the damping capacity are under further studies. 3.3. The damping behavior of the Ti44 Ni47 Nb9 alloy The damping capacity of the Ti44 Ni47 Nb9 alloy as a function of temperature in the cooling process is shown in Fig. 4. It shows that one damping capacity peak appears due to the martensitic transformation, however, above the Ms temperature, the damping capacity remains nearly constant in a rather wide temperature range. Comparing the result of Figs. 3(a) and 4, it is worth noticing that the damping capacity peak value corresponding to martensitic transformation of Ti44 Ni47 Nb9 alloy is much higher than that of the Ti49.2 Ni50.8 alloy. As we know, during the martensitic transformation, most of the energy is dissipated due to the movement of martensite/parent interfaces, which causes the peak of the damping capacity to appear in the transformation region. That is to say, the dissipated energy is due to the creation or the motion of the interphase boundaries. Therefore, the experimental results can be explained as follows: the microstructure of the Ti44 Ni47 Nb9 alloy is characterized by an ordered TiNi matrix phase containing a fine dispersion of -Nb particles [13]. The interfaces between the -Nb particles and matrix pro-
81
Fig. 4. The damping capacity vs. temperature curves for Ti44 Ni47 Nb9 alloy. The strain amplitude was 3 × 10−5 , the frequency was 1 Hz and the cooling rate was 5 ◦ C/min.
vided the nucleus sites for the martensite. As a consequence, the amount of boundaries between the martensite and parent phase increases, which may be greatly beneficial to the damping capacity during the phase transformation. The reason that the aged Ti49.2 Ni50.8 alloys exhibit the high damping capacity during phase transformation is the same with it. Fig. 5 shows the damping capacity as a function of frequency for the Ti44 Ni47 Nb9 alloy in the parent phase state. It indicates that the damping capacity decreases drastically at the beginning and then slightly decreases when the vibration frequency is larger than 0.15 Hz. It is of great interest that the damping capacity of parent phase reaches a very high value at 0.1 Hz, which is even higher than the result of thermal martensite in Fig. 1. Fig. 6 demonstrates the evolution of the damping capacity as a function of strain amplitude in the parent phase for the Ti44 Ni47 Nb9 alloy. It can be seen that the damping capacity
Fig. 5. The damping capacity tan δ vs. frequency curve for Ti44 Ni47 Nb9 alloy. The strain amplitude was 3 × 10−5 and the testing temperature was room temperature.
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Fig. 6. The damping capacity vs. strain amplitude curve for Ti44 Ni47 Nb9 alloy. The frequency was 0.1 Hz and the testing temperature was room temperature.
of the parent phase decreases rapidly with an increase of strain amplitude at first, and then decreases slowly after the amplitude reaches a certain value of about 3 × 10−5 . It is well known that -Nb particles existing in TiNiNb alloys are soft and easily deformed by external stress. Thus, there are dislocations existing in the vicinity of interfaces between -Nb particles and the matrix as a result of lattice mismatching. These dislocations maybe move under a certain external stress due to the low stiffness of the -Nb particles, which can contribute to the high damping capacity in the parent phase of the Ti44 Ni47 Nb9 alloy. Additionally, we suggest that the high damping capacity of the parent phase for the Ti44 Ni47 Nb9 alloy is closely related to the low strength of soft -Nb particles existing in the Ti44 Ni47 Nb9 alloy, which will be studied further. As we know, even though the martensite of TiNi alloys has high damping capacity, it has relatively low strength compared with the parent phase. According to the experimental result, the parent phase of the Ti44 Ni47 Nb9 alloy possesses high damping capacity at low strain amplitude and low frequency conditions, which is very significant for engineering applications because the materials which possess not only enough damping capacity but also enough strength should be pursued.
4. Conclusions The damping capacity of martensite in the Ti50 Ni50 alloy first increases with the increasing cold-rolling deforma-
tion, and then decreases with further increase of the coldrolling degree. The specimen with 8% deformation degree has the highest damping capacities at the frequency range of 0.1–10 Hz, which are obviously higher than that of undeformed martensite. The dislocations introduced by the coldrolling deformation are responsible for these high damping capacities. Aging treatment for Ti49.2 Ni50.8 alloy can apparently enhance the damping capacity during phase transformation. The highest damping capacity arises from the increase of the amount of boundaries between the martensite and parent phase due to the existence of the Ti3 Ni4 particles because the interfaces between the Ti3 Ni4 particles and matrix provided the nucleus sites for the martensite. The Ti44 Ni47 Nb9 alloy possesses high damping capacity during martensitic transformation as well as in parent phase and martensite, which is closely related to the -Nb soft particles existing in the Ti44 Ni47 Nb9 alloy and the motion of dislocations in the vicinity of interfaces between -Nb particles and matrix. The high damping capacity of parent phase for the Ti44 Ni47 Nb9 alloy provides potential applications for engineering because of enough damping capacity and strength.
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