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Constrained martensitic transformations in TiNiCu films
Thin Solid Films 434 (2003) 271–275
Constrained martensitic transformations in TiNiCu films Corneliu M. Craciunescu, Jian Li, Manfred Wuttig* Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD 20742-2115, USA Received 15 May 2002; received in revised form 31 January 2003; accepted 14 March 2003
Abstract TiNiCu shape memory alloy films were deposited at different temperatures on heated (1 0 0) Si cantilever-type substrates. The films are crystalline when deposited above 200 8C. The stress relief observed in the TiNiCuySi bimorphs depends on the composition and the deposition temperature. For lower Cu contents a B2™R-phase™B199 sequence of transformations was observed for films that start to transform under a low thermoelastic stress and a B2™B199 sequence when the stress at the onset of the transformation is high. For higher Cu content the sequence of transformation observed was B2™B19™B199 and B2™ B199 for low and high stress, respectively. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Phase transitions; Shape memory; Internal friction; Thermoelastic stress
1. Introduction NiTi shape memory alloys (SMAs) are widely used as bulk or thin films because of their wide spectra of properties. However, their properties can be better adapted to specific requirements by adding a third element to the basic composition w1–3x. For instance, the addition of Pd increases the transformation temperatures w4–6x and Nb is effective for increasing the temperature hysteresis of the transformation w7–9x, while Cu decreases it w1,10,11x. NiTi-based individual films have been systematically investigated during the past few years w1x. For films attached to substrates the martensitic transformation occurs under constraint, as shown for NiTi w12–16x and TiNiPd w5,6,17x. In most of the investigated films attached to substrates, the transformation takes place between the cubic B2 high temperature phase and the monoclinic B199 (for NiTi) or the orthorhombic B19 (for TiNiPd) martensites. An R-phase transition is suggested to occur in Ni-rich NiTi films deposited in the vicinity of the transformation temperature. *Corresponding author. Department of Materials and Nuclear Engineering, Stadium Drive, Bldg. 090, Room 1110, University of Maryland, College Park, MD 20742-2115, USA. Tel.: q1-301-4055212; fax: q1-301-314-2029.. E-mail address: [email protected] (M. Wuttig).
TiNiCu films investigated under constant stress show sequences of transformation similar to the ones observed in bulk alloys w18x. TiNiCu filmysubstrate bimorphs have also been investigated w17,19x, but less work has been done on films deposited on heated substrates. This paper shows experimental results obtained using the mechanical spectroscopy technique w16,20x for TiNiCu films, deposited on heated silicon substrates. Compared to NiTi films deposited under the same conditions, it was observed that NiTiCu films crystallize at lower temperatures. Their martensitic transformation under constraint is significantly influenced by the stress that is built into the films on cooling from the deposition temperatures. The features of the stress relief as a function of the temperature curve are influenced by the Cu content and the stress in the film at the onset of the transformation. The displacement associated with the stress relief for films with high Cu is up to 200% higher than the one observed for NiTi films deposited under the same conditions, making TiNiCu films an important candidate for the MEMS applications. 2. Experimental details Nominally Ti50Ni30Cu20, Ti50Ni42.5Cu7.5 and Ti50Ni50 targets were used to sputter deposit films on (1 0 0)oriented thermally oxidized silicon cantilever substrates w21x, using a d.c. magnetron sputtering system. The
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Fig. 1. Modulus defect vs. temperature for Ti51.7Ni34.1Cu14..2ySi bimorphs deposited at 250 8C (h), 300 8C (s) and 385 8C (n). The 385 8C deposition shows distinctively two slopes.
deposition parameters were: 266=10y6 Pa vacuum, 133=10y3 MPa Ar gas pressure, 100 W power, 3.6 ks deposition time. A heating plate, mechanically attached to the back of the cantilever, was used during the deposition to maintain the substrate at a constant temperature. The temperature of the cantilever was calibrated prior to the deposition. The films were deposited on substrates heated at various temperatures between room temperature (RT) and 385 8C, using the parameters previously described. The deposition rate was approximately 30 nmys and the total thickness, measured with a stylus profiler, was in the range of 2 mm"10% for each film deposited on a 100 mm thick substrate. The structure of the films was investigated using a Rigaku X-ray diffractometer. The internal friction (IF) and modulus defect (DMyM) of the cantilevers were measured as a function of the temperature, using the clamped free-reed vibration method w16x. Measurements were individually performed on each filmycantilever composite using the same parameters (heatingycooling rate 2 8Cymin, vacuum 133 10y6 Pa). The displacement of each cantilever was analyzed on heating and on cooling. It was measured capacitively through the resonance frequency of the composite w16x, and was used to calculate the stress in the films applying the Stoney formula w22x. The composition of the films determined by wavelength dispersive spectroscopy in a JEOL 8900 Electron Microprobe was: Ti51.7Ni34.1Cu14.2, Ti50.6Ni46.1Cu3.3 and Ni51.9Ti48.1, respectively. 3. Results and discussion Figs. 1 and 2 show the mechanical spectroscopy data. The modulus defect DMyMswM(T)yM(T1 )x yM(T) vs.
temperature curves for the Ti51.7Ni34.1Cu14.2 films deposited between 250 and 385 8C displayed in Fig. 1 show the development of a typical martensitic transformation for SMA filmySi bimorphs w14–16x. Films deposited on the substrate heated at 250 8C already show the inflexion characteristic for crystalline SMAs. In fact, by comparison to NiTi films deposited under the same conditions it was observed that the NiTiCu films crystallize at lower temperatures. The amplitude of the change in the modulus defect gradually increases as the deposition temperature increases. The films deposited at 385 8C show two inflexions, suggesting that a sequence of two transformations is present. Fig. 2 presents the IF results for the Ti51.7Ni34.1Cu14.2 films. Films deposited up to 300 8C show an IF peak centered at approximately 0 8C, followed by a stabilization of the IF at values higher that those corresponding to the high temperature phase. The IF peak is shifted toward higher temperatures for films deposited at 300 8C and higher. Accordingly, two slopes can be observed on the IF curve of films deposited at high temperatures. A steep slope at approximately 100 8C, and a gentle slope that can be observed between 75 to 0 8C. TiNiCu bulk alloys show a B2™B19™B199 transition in the temperature range of our investigation w10x. A similar transition occurs in thin films as well w1x. However, the transition in thin films is influenced by the grain size. It has been shown that in NiTi films deposited on heated substrates, the grain size grows and the shape becomes more irregular as the deposition
Fig. 2. Change in the IF as a function of the temperature in the Ti51.7Ni34.1Cu14.2 films. Films deposited between 250 and 300 8C (n) show the development of a peak in the transition range, while films deposited above 325 up to 385 8C (h) show a two-slope type transition.
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spectrum taken at RT reveals the (1 1 1) and (0 2 0) reflections of the orthorhombic B19 martensite and the ¯ peak of the monoclinic B19 martensite. Such (1 1 1) orthorhombic peaks are known to accompany the (1 1 0) peak of the B2 parent phase w25x. The B19 peaks disappear and the (1 1 0)B2 peak grows as the film is heated to 100 8C. On cooling back to RT the B19 peaks reappear. Fig. 4a illustrates the stress relief observed during the phase transition in Ti51.7Ni34.1Cu14.2 films deposited at several temperatures on the Si substrate. The stress in the bimorph was calculated using the simplified Stoney formula for cases in which the deflection of the cantilever (af300–800 mm) is much smaller than the length of the cantilever (Ls20 mm) w24x: ss Fig. 3. X-ray spectra of the Ti51.7Ni34.1Cu14.2 filmsySi substrates deposited at 385 8C. The RT spectra shows the peaks for the ¯ B199 and (1 1 1)q(0 2 0)B19 martensites, while at higher tem(1 1 1) peratures the B19 peaks disappear.
temperature is increased. The peak observed for the 250 8C deposition can be tentatively attributed to the intermediary B2™B19 transition and to the small grain size observed in the vicinity of the deposition temperatures at which films become crystalline. The X-ray diffraction spectra of the Ti51.7Ni34.1Cu14.2 film deposited at 385 8C are shown in Fig. 3. The
aEsts 3L Ž1yns.tf 2
(1)
In Eq. (1) ts and tf are the thickness of the film and substrate, respectively, E represents the elastic modulus of the substrate and ns the Poisson ratio of the substrate. All films are at ‘zero stress’ at the deposition temperature; the experimentally determined bimorph stress (in Fig. 4a) should thus extrapolate to zero at the deposition temperature. In the present work extrapolations of ‘zero stress’ temperatures in Fig. 4a are approximately 50–70 8C higher than the nominal temperature. This difference is attributed to substrate’s heating by the deposition process. As the film is cooled from the deposition temperature the thermoelastic stress increases linearly
Fig. 4. Stress-relief vs. temperature curves associated with the phase transformation in Ti51.7 Ni34.1 Cu14..2 ySi (a) and NiTiySi (b) bimorphs. The ‘zero stress state’ for the TiNiCu films deposited at 250 8C (h), 300 8C (s) and 385 8C (m) is shown by the intersection between the dotted lines and the corresponding horizontals for each deposition temperature (e.g. 0250). The NiTi film was deposited at 385 8C.
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Fig. 5. Stress-relief vs. temperature curves associated with the phase transformation in the Ti50.6Ni46.1Cu3.3 ySi bimorphs. The ‘zero stress state’ for the films deposited at 200 8C (h), 250 8C (j), 325 8C (no symbol) and 385 8C (n) is shown by the intersection between the dotted lines and the corresponding horizontals. The films deposited at 200 8C are partially crystalline, films deposited between 250 and 325 8C show a B2™R-phase™B199 transition, while for the 385 8C deposition only the B2™B199 transition is observed.
until the phase transformation occurs. The stress relief of the films deposited at higher temperatures (300 and 385 8C) reveals a two-step transition similar to the one observed in individual stressed films not attached to substrates w23x. The Ti51.7Ni34.1Cu14.2 film deposited at 385 8C shows a very high stress relief that occurs as a result of two transitions. A B2™B19 transition occurs between q100 and q50 8C, where the film relieves the major part of the stress. A second B19™B199 transition occurs between q50 and y50 8C, accompanied by a smaller stress relief. By comparison to NiTi films deposited at the same temperature and under the same conditions (Fig. 4b), the stress relief associated with B2™ B19 transition in the Ti51.7Ni34.1Cu14.2 film is up to 50% higher than the corresponding one observed during the B2™B199 transition in the NiTi film. It can also be observed that the stress relief for the Ti51.7Ni34.1Cu14.2 films is almost complete (close to the ‘zero stress state’) and that the stress reliefs during the B199 transition are comparable in both films NiTi and Ti51.7Ni34.1Cu14.2 films deposited at 385 8C. By comparison to Ti51.7Ni34.1Cu14.2 films, the films with lower Cu contents and deposited at lower temperatures show a rhombohedrical R-phase transition. Fig. 5 shows a discernable R-phase transition for the films deposited at 250 and 325 8C, similar to the one reported for NiTiyMo bimorphs w19x. It is not observed in films deposited at higher temperatures. According to Fig. 4, as the films become crystalline, a B2™R™B199 sequence of transformation occurs when the films trans-
Fig. 6. Changes in the modulus defect as a function of temperature for Ti50.6Ni46.1Cu3.3 films deposited at 200 8C (h), 250 8C (j), 325 8C (no symbol) and 385 8C (n).
form under an initial thermoelastic stress of approximately 200–300 MPa, generated on cooling from the corresponding deposition temperature by the mismatch between the thermal expansion coefficients of the films and the substrate. For higher stresses, approximately 400 MPa, the only sequence observed was B2™B199. The changes in the modulus defect shown in Fig. 6 also allow the detection of the changes in the sequence of transformation as a function of the deposition temperatures. The inflection related to the sequence of transformations and their dependence of the deposition temperatures are summarized in Fig. 7. This figure
Fig. 7. The influence of the deposition temperature on the sequence of transformation in Ti50.6Ni46.1Cu3.3 films subject to thermoelastic stresses. The B2™R-phase (n), R-phase™B199 (h) and B2™B199 (s) data determined from the corresponding modulus defect vs. temperature curves.
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resembles known results reported for bulk Ni-rich NiTi alloys w25x and thin films. The occurrence of the Rphase transition for lower Pd contents in TiNiPd based alloys is also known w2x. In Ni-rich NiTi thin films it was further shown that sequence of transformation might be related to the small grains observed in the films deposited at low temperature. Therefore it is suggested that the stress developed in the film at the onset of the transformation and the grain size are the main factors contributing to the changes in the sequence of transformation. Stresses resulting from precipitates or changes of the lattice parameter can also favor the formation of R-phase martensite. In view of the general agreement of the film results presented here with the bulk data those two effects appear to be minor. 4. Conclusions The behavior of the TiNiCu films deposited on heated substrates is strongly influenced by two main factors: (1) the chemical composition of the films and (2) the deposition temperature. Films deposited on substrates at a low temperature transform under a low stress, and their sequence of transformation is affected by the crystallinity and the shape and size of the grains. Films deposited at a high temperature transform under a high stress and the stress relief associated with the transformation is high. For the Ti51.7Ni34.1Cu14.2 films the stress vs. temperature dependence shows distinctively the B2™B19™B199 sequence of transformation. By comparison to NiTi films deposited under the same conditions the total stress relief is significantly higher in the Ti51.7Ni34.1Cu14.2 films investigated. Ti50.6Ni46.1Cu3.3 films deposited on substrates heated up to 350 8C show a B2™R™B199 transition, while for higher temperatures, the sequence observed is B2™ B199. It is suggested that stress and grains size are influencing the behavior observed in the TiNiCu films. The actuation of a bimorph cantilever is proportional to the stress relief, as it results from the Stoney equation. Our data suggest that the highest displacement for bimorph architectures subject to thermoelastic stresses is obtained for films undergoing a B2™B19™B199 sequence of transformations. This information can be useful for designing two-way SMA actuators. Acknowledgments The work was supported by the National Science Foundation, grant DMR0095166, and the Office of
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Naval Research, contracts No. MURI N000140110761, N000149910837 and N000140010849. References w1x S. Miyazaki, A. Ishida, Mater. Sci. Eng. A 273–575 (1999) 106. w2x K. Otsuka, C.M. Wayman, Shape Memory Materials, Cambridge University Press, Cambridge, New York, 1998. w3x K. Otsuka, Proceedings of the International Symposium on Shape Memory Materials, SMM-94, Beijing, China, 1994, p. 129. w4x K. Enami, T. Hoshiya, Mater. Tech’90, vol. 1, 1992. w5x S. Mathews, J. Li, Q. Su, M. Wuttig, Phil. Mag. Lett. 79 (5) (1999) 265. w6x K.N. Melton, J.L. Proft, T.W. Duering, Proceedings of the ICOMAT-86 Nara, 1986, p. 1053. w7x P. Min, K. Otsuka, S. Miyazaki, H. Horikawa, Mater. Trans. JIM 34 (1993) 919. w8x P. Min, S. Miyazaki, K. Otsuka, Mater. Trans. JIM 33 (1992) 337. w9x S. Miyazaki, K. Komatsubara, K. Otsuka, Collected Abstracts of Annual Fall Meeting of Japan Institute of Metals Sapporo, 1989, p. 408. w10x O. Mercier, K.N. Melton, Met. Trans. Part A 10 (1979) 1053. w11x J. Zhang, D.S. Grummon. in: E.P. George, R. Gotthardt, K. Otsuka, S. Troiler-McKinstry, M. Wun-Fogle (Eds.), MRS Proceeding of the Materials for Smart System II, vol. 459, E.P. MRS Pittsburg, 1997, p. 451. w12x Q. Su, S. Hua, M. Wuttig. in: Proceedings of the ‘Shape Memory Alloys’ Trans. Mater. Res. Soc. Jpn. 18B (1994) 1057. w13x A.L. Roytburd, Q. Su, T.S. Kim, J. Slutsker, M. Wuttig, Acta Mater. 46 (14) (1998) 5095. w14x T. Kim, Q. Su, M. Wuttig, Mater. Res. Soc. Symp. Proc. 360 (1995) 375. w15x M. Wuttig, Y. Zheng, J.S. Slutsker, K. Mori, Q. Su, Scripta Mater. 41 (5) (1999) 529. w16x M. Wuttig, Q.M. Su. in: B. Bhagat (Ed.), Proceedings of the Symp. on Damping in Multiphase Inorganic Materials, AMS Metals Park OH, 1993, p. 159. w17x B. Winzek, E. Quandt, Zeitschrift fhr Metallkunde 90 (10) (1999) 796. w18x T. Hashinaga, S. Miyazaki, T. Ueki, H. Horikawa, J. Phys. (France) IV 5 (1995) C8-689. w19x B. Winzek, E. Quandt, H. Holleck. in: Borgmann H (Ed.), ACTUATOR 2000. Seventh International Conference on New Actuators and International Exhibition on Smart Actuators and Drive Systems. Conference Proceedings; Bremen Germany: MESSE BREMEN GMBH, 2000, p. 172. w20x Q. Su, Y. Wen, M. Wuttig, J. Phys. Paris 6 (1996) C8-757. w21x M. Wuttig, C. Craciunescu, J. Li, Mater. Trans. JIM 41 (8) (2000) 933. w22x G.G. Stoney, Proc. R. Soc. A 82 (1909) 172. w23x T. Fukuda, T. Saburi, T. Chihara, Y. Tsuzuki, Mater. Trans. JIM 36 (10) (1995) 1244. w24x T. Kim, Ph. D. Thesis, University of Maryland at College Park, 1994. w25x G.B. Stachowiak, P.G. McCormick, Acta Metall 36 (1988) 291.
Constrained martensitic transformations in TiNiCu films Corneliu M. Craciunescu, Jian Li, Manfred Wuttig* Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD 20742-2115, USA Received 15 May 2002; received in revised form 31 January 2003; accepted 14 March 2003
Abstract TiNiCu shape memory alloy films were deposited at different temperatures on heated (1 0 0) Si cantilever-type substrates. The films are crystalline when deposited above 200 8C. The stress relief observed in the TiNiCuySi bimorphs depends on the composition and the deposition temperature. For lower Cu contents a B2™R-phase™B199 sequence of transformations was observed for films that start to transform under a low thermoelastic stress and a B2™B199 sequence when the stress at the onset of the transformation is high. For higher Cu content the sequence of transformation observed was B2™B19™B199 and B2™ B199 for low and high stress, respectively. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Phase transitions; Shape memory; Internal friction; Thermoelastic stress
1. Introduction NiTi shape memory alloys (SMAs) are widely used as bulk or thin films because of their wide spectra of properties. However, their properties can be better adapted to specific requirements by adding a third element to the basic composition w1–3x. For instance, the addition of Pd increases the transformation temperatures w4–6x and Nb is effective for increasing the temperature hysteresis of the transformation w7–9x, while Cu decreases it w1,10,11x. NiTi-based individual films have been systematically investigated during the past few years w1x. For films attached to substrates the martensitic transformation occurs under constraint, as shown for NiTi w12–16x and TiNiPd w5,6,17x. In most of the investigated films attached to substrates, the transformation takes place between the cubic B2 high temperature phase and the monoclinic B199 (for NiTi) or the orthorhombic B19 (for TiNiPd) martensites. An R-phase transition is suggested to occur in Ni-rich NiTi films deposited in the vicinity of the transformation temperature. *Corresponding author. Department of Materials and Nuclear Engineering, Stadium Drive, Bldg. 090, Room 1110, University of Maryland, College Park, MD 20742-2115, USA. Tel.: q1-301-4055212; fax: q1-301-314-2029.. E-mail address: [email protected] (M. Wuttig).
TiNiCu films investigated under constant stress show sequences of transformation similar to the ones observed in bulk alloys w18x. TiNiCu filmysubstrate bimorphs have also been investigated w17,19x, but less work has been done on films deposited on heated substrates. This paper shows experimental results obtained using the mechanical spectroscopy technique w16,20x for TiNiCu films, deposited on heated silicon substrates. Compared to NiTi films deposited under the same conditions, it was observed that NiTiCu films crystallize at lower temperatures. Their martensitic transformation under constraint is significantly influenced by the stress that is built into the films on cooling from the deposition temperatures. The features of the stress relief as a function of the temperature curve are influenced by the Cu content and the stress in the film at the onset of the transformation. The displacement associated with the stress relief for films with high Cu is up to 200% higher than the one observed for NiTi films deposited under the same conditions, making TiNiCu films an important candidate for the MEMS applications. 2. Experimental details Nominally Ti50Ni30Cu20, Ti50Ni42.5Cu7.5 and Ti50Ni50 targets were used to sputter deposit films on (1 0 0)oriented thermally oxidized silicon cantilever substrates w21x, using a d.c. magnetron sputtering system. The
0040-6090/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-6090(03)00531-5
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Fig. 1. Modulus defect vs. temperature for Ti51.7Ni34.1Cu14..2ySi bimorphs deposited at 250 8C (h), 300 8C (s) and 385 8C (n). The 385 8C deposition shows distinctively two slopes.
deposition parameters were: 266=10y6 Pa vacuum, 133=10y3 MPa Ar gas pressure, 100 W power, 3.6 ks deposition time. A heating plate, mechanically attached to the back of the cantilever, was used during the deposition to maintain the substrate at a constant temperature. The temperature of the cantilever was calibrated prior to the deposition. The films were deposited on substrates heated at various temperatures between room temperature (RT) and 385 8C, using the parameters previously described. The deposition rate was approximately 30 nmys and the total thickness, measured with a stylus profiler, was in the range of 2 mm"10% for each film deposited on a 100 mm thick substrate. The structure of the films was investigated using a Rigaku X-ray diffractometer. The internal friction (IF) and modulus defect (DMyM) of the cantilevers were measured as a function of the temperature, using the clamped free-reed vibration method w16x. Measurements were individually performed on each filmycantilever composite using the same parameters (heatingycooling rate 2 8Cymin, vacuum 133 10y6 Pa). The displacement of each cantilever was analyzed on heating and on cooling. It was measured capacitively through the resonance frequency of the composite w16x, and was used to calculate the stress in the films applying the Stoney formula w22x. The composition of the films determined by wavelength dispersive spectroscopy in a JEOL 8900 Electron Microprobe was: Ti51.7Ni34.1Cu14.2, Ti50.6Ni46.1Cu3.3 and Ni51.9Ti48.1, respectively. 3. Results and discussion Figs. 1 and 2 show the mechanical spectroscopy data. The modulus defect DMyMswM(T)yM(T1 )x yM(T) vs.
temperature curves for the Ti51.7Ni34.1Cu14.2 films deposited between 250 and 385 8C displayed in Fig. 1 show the development of a typical martensitic transformation for SMA filmySi bimorphs w14–16x. Films deposited on the substrate heated at 250 8C already show the inflexion characteristic for crystalline SMAs. In fact, by comparison to NiTi films deposited under the same conditions it was observed that the NiTiCu films crystallize at lower temperatures. The amplitude of the change in the modulus defect gradually increases as the deposition temperature increases. The films deposited at 385 8C show two inflexions, suggesting that a sequence of two transformations is present. Fig. 2 presents the IF results for the Ti51.7Ni34.1Cu14.2 films. Films deposited up to 300 8C show an IF peak centered at approximately 0 8C, followed by a stabilization of the IF at values higher that those corresponding to the high temperature phase. The IF peak is shifted toward higher temperatures for films deposited at 300 8C and higher. Accordingly, two slopes can be observed on the IF curve of films deposited at high temperatures. A steep slope at approximately 100 8C, and a gentle slope that can be observed between 75 to 0 8C. TiNiCu bulk alloys show a B2™B19™B199 transition in the temperature range of our investigation w10x. A similar transition occurs in thin films as well w1x. However, the transition in thin films is influenced by the grain size. It has been shown that in NiTi films deposited on heated substrates, the grain size grows and the shape becomes more irregular as the deposition
Fig. 2. Change in the IF as a function of the temperature in the Ti51.7Ni34.1Cu14.2 films. Films deposited between 250 and 300 8C (n) show the development of a peak in the transition range, while films deposited above 325 up to 385 8C (h) show a two-slope type transition.
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spectrum taken at RT reveals the (1 1 1) and (0 2 0) reflections of the orthorhombic B19 martensite and the ¯ peak of the monoclinic B19 martensite. Such (1 1 1) orthorhombic peaks are known to accompany the (1 1 0) peak of the B2 parent phase w25x. The B19 peaks disappear and the (1 1 0)B2 peak grows as the film is heated to 100 8C. On cooling back to RT the B19 peaks reappear. Fig. 4a illustrates the stress relief observed during the phase transition in Ti51.7Ni34.1Cu14.2 films deposited at several temperatures on the Si substrate. The stress in the bimorph was calculated using the simplified Stoney formula for cases in which the deflection of the cantilever (af300–800 mm) is much smaller than the length of the cantilever (Ls20 mm) w24x: ss Fig. 3. X-ray spectra of the Ti51.7Ni34.1Cu14.2 filmsySi substrates deposited at 385 8C. The RT spectra shows the peaks for the ¯ B199 and (1 1 1)q(0 2 0)B19 martensites, while at higher tem(1 1 1) peratures the B19 peaks disappear.
temperature is increased. The peak observed for the 250 8C deposition can be tentatively attributed to the intermediary B2™B19 transition and to the small grain size observed in the vicinity of the deposition temperatures at which films become crystalline. The X-ray diffraction spectra of the Ti51.7Ni34.1Cu14.2 film deposited at 385 8C are shown in Fig. 3. The
aEsts 3L Ž1yns.tf 2
(1)
In Eq. (1) ts and tf are the thickness of the film and substrate, respectively, E represents the elastic modulus of the substrate and ns the Poisson ratio of the substrate. All films are at ‘zero stress’ at the deposition temperature; the experimentally determined bimorph stress (in Fig. 4a) should thus extrapolate to zero at the deposition temperature. In the present work extrapolations of ‘zero stress’ temperatures in Fig. 4a are approximately 50–70 8C higher than the nominal temperature. This difference is attributed to substrate’s heating by the deposition process. As the film is cooled from the deposition temperature the thermoelastic stress increases linearly
Fig. 4. Stress-relief vs. temperature curves associated with the phase transformation in Ti51.7 Ni34.1 Cu14..2 ySi (a) and NiTiySi (b) bimorphs. The ‘zero stress state’ for the TiNiCu films deposited at 250 8C (h), 300 8C (s) and 385 8C (m) is shown by the intersection between the dotted lines and the corresponding horizontals for each deposition temperature (e.g. 0250). The NiTi film was deposited at 385 8C.
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Fig. 5. Stress-relief vs. temperature curves associated with the phase transformation in the Ti50.6Ni46.1Cu3.3 ySi bimorphs. The ‘zero stress state’ for the films deposited at 200 8C (h), 250 8C (j), 325 8C (no symbol) and 385 8C (n) is shown by the intersection between the dotted lines and the corresponding horizontals. The films deposited at 200 8C are partially crystalline, films deposited between 250 and 325 8C show a B2™R-phase™B199 transition, while for the 385 8C deposition only the B2™B199 transition is observed.
until the phase transformation occurs. The stress relief of the films deposited at higher temperatures (300 and 385 8C) reveals a two-step transition similar to the one observed in individual stressed films not attached to substrates w23x. The Ti51.7Ni34.1Cu14.2 film deposited at 385 8C shows a very high stress relief that occurs as a result of two transitions. A B2™B19 transition occurs between q100 and q50 8C, where the film relieves the major part of the stress. A second B19™B199 transition occurs between q50 and y50 8C, accompanied by a smaller stress relief. By comparison to NiTi films deposited at the same temperature and under the same conditions (Fig. 4b), the stress relief associated with B2™ B19 transition in the Ti51.7Ni34.1Cu14.2 film is up to 50% higher than the corresponding one observed during the B2™B199 transition in the NiTi film. It can also be observed that the stress relief for the Ti51.7Ni34.1Cu14.2 films is almost complete (close to the ‘zero stress state’) and that the stress reliefs during the B199 transition are comparable in both films NiTi and Ti51.7Ni34.1Cu14.2 films deposited at 385 8C. By comparison to Ti51.7Ni34.1Cu14.2 films, the films with lower Cu contents and deposited at lower temperatures show a rhombohedrical R-phase transition. Fig. 5 shows a discernable R-phase transition for the films deposited at 250 and 325 8C, similar to the one reported for NiTiyMo bimorphs w19x. It is not observed in films deposited at higher temperatures. According to Fig. 4, as the films become crystalline, a B2™R™B199 sequence of transformation occurs when the films trans-
Fig. 6. Changes in the modulus defect as a function of temperature for Ti50.6Ni46.1Cu3.3 films deposited at 200 8C (h), 250 8C (j), 325 8C (no symbol) and 385 8C (n).
form under an initial thermoelastic stress of approximately 200–300 MPa, generated on cooling from the corresponding deposition temperature by the mismatch between the thermal expansion coefficients of the films and the substrate. For higher stresses, approximately 400 MPa, the only sequence observed was B2™B199. The changes in the modulus defect shown in Fig. 6 also allow the detection of the changes in the sequence of transformation as a function of the deposition temperatures. The inflection related to the sequence of transformations and their dependence of the deposition temperatures are summarized in Fig. 7. This figure
Fig. 7. The influence of the deposition temperature on the sequence of transformation in Ti50.6Ni46.1Cu3.3 films subject to thermoelastic stresses. The B2™R-phase (n), R-phase™B199 (h) and B2™B199 (s) data determined from the corresponding modulus defect vs. temperature curves.
C.M. Craciunescu et al. / Thin Solid Films 434 (2003) 271–275
resembles known results reported for bulk Ni-rich NiTi alloys w25x and thin films. The occurrence of the Rphase transition for lower Pd contents in TiNiPd based alloys is also known w2x. In Ni-rich NiTi thin films it was further shown that sequence of transformation might be related to the small grains observed in the films deposited at low temperature. Therefore it is suggested that the stress developed in the film at the onset of the transformation and the grain size are the main factors contributing to the changes in the sequence of transformation. Stresses resulting from precipitates or changes of the lattice parameter can also favor the formation of R-phase martensite. In view of the general agreement of the film results presented here with the bulk data those two effects appear to be minor. 4. Conclusions The behavior of the TiNiCu films deposited on heated substrates is strongly influenced by two main factors: (1) the chemical composition of the films and (2) the deposition temperature. Films deposited on substrates at a low temperature transform under a low stress, and their sequence of transformation is affected by the crystallinity and the shape and size of the grains. Films deposited at a high temperature transform under a high stress and the stress relief associated with the transformation is high. For the Ti51.7Ni34.1Cu14.2 films the stress vs. temperature dependence shows distinctively the B2™B19™B199 sequence of transformation. By comparison to NiTi films deposited under the same conditions the total stress relief is significantly higher in the Ti51.7Ni34.1Cu14.2 films investigated. Ti50.6Ni46.1Cu3.3 films deposited on substrates heated up to 350 8C show a B2™R™B199 transition, while for higher temperatures, the sequence observed is B2™ B199. It is suggested that stress and grains size are influencing the behavior observed in the TiNiCu films. The actuation of a bimorph cantilever is proportional to the stress relief, as it results from the Stoney equation. Our data suggest that the highest displacement for bimorph architectures subject to thermoelastic stresses is obtained for films undergoing a B2™B19™B199 sequence of transformations. This information can be useful for designing two-way SMA actuators. Acknowledgments The work was supported by the National Science Foundation, grant DMR0095166, and the Office of
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