Texture and Microstructure of Ti-Ni Melt-Spun Shape Memory Alloy Ribbons

Transcription

1 Materials Transactions, Vol. 45, No. 2 (2004) pp. 214 to 218 Special Issue on Materials and Devices for Intelligent/Smart Systems #2004 The Japan Institute of Metals Texture and Microstructure of Ti-Ni Melt-Spun Shape Memory Alloy Ribbons Anak Khantachawana, Hiroshi Mizubayashi and Shuichi Miyazaki Institute of Materials Science, University of Tsukuba, Tsukuba , Japan The shape memory behavior, texture and microstructure were studied for Ti-Ni ribbons fabricated by a melt-spinning method where the Ni contents were designed to be 49.0 at%, 50.0 at% and 51.0 at%. The texture of the parent B2 phase was determined by X-ray diffraction pole figures. A strong h100i fiber texture was found in both pole figures and orientation distribution functions (ODF). TEM observation revealed that all the ribbons are fully crystallized and that disk-type precipitates of about 10 nm in length locate on {100} of B2 phase uniformly. The thermal cyclic tests under various constant stresses showed shape recoverable strains exceeding 5% and critical stresses for plastic deformation being higher than 400 MPa for Ti-49.0 at%ni and Ti-50.0 at%ni as-spun ribbons. These excellent shape memory characteristics of the melt-spun ribbons are due to the formation of these disk-type precipitates. In addition, Ti 2 Ni precipitates of 25 nm in diameter appeared along grain boundaries of Ti-51.0 at%ni as-spun ribbon. Since the Ni content of the matrix is condensed due to the formation of Ti 2 Ni precipitates, no shape memory effect was observed in Ti-51.0 at%ni as-spun ribbon under the experimental conditions. (Received December 1, 2003; Accepted December 26, 2003) Keywords: titanium-nickel shape memory alloy, melt-spinning, texture, pole figure, orientation distribution functions (ODF), transmission electron micrscopy (TEM), precipitates, Ti 2 Ni, B2 1. Introduction Near equiatomic Ti-Ni shape memory alloys (SMAs) have been used for actuators with various kinds of shapes such as wire and plate. 1 5) Recently Ti-Ni SMA plates of 100 micrometers or less in thickness could be fabricated by the cold-rolling method. Thus, actuators with two-dimensionalshape such as diaphragms and multi-directional beam-cantilever type actuators have been fabricated. 5 10) The coldrolling process is industrially advantageous to fabricate binary Ti-Ni alloy thin plates in a limited compositional range. On the other hand, by using the melt-spinning technique, thinner Ti-Ni ribbons such as 15 micrometers in thickness can be fabricated in a wide compositional range as well as ternary Ti-Ni base alloys. The process of rapid solidification provide very fine grains and microscopically homogeneous substructure with minimum processing steps, thereby reducing the cost and improving the shape memory characteristics ) Although many reports have been published on rapid solidification process of shape memory alloys in recent years, most of the authors have been devoted to Cu-based alloys. 13,15,16) Very few reports devoted to Ti-Ni alloys are available, 11,12) so that the texture and microstructure of Ti-Ni ribbons have not been well clarified. Generally, melt-spun ribbons are known to have unique texture different from bulk materials. It is important to know the effect of the texture on the anisotropy of transformation strain in order to apply such melt-spun ribbons for actuators. The anisotropy of transformation strain must be taken into account when designing the actuators. In the present paper, pole figures and crystallite orientation distribution functions (ODF) were investigated utilizing a X-ray diffraction method and the shape memory characteristics were correlated with the texture information in addition to the microstructure. 2. Experimental Procedure Ingots with Ti-49.0, 50.0 and 51.0 at%ni were made by Ar arc-melting method using high purity elemental Ti and Ni. The melting was repeatedly done for 6 times for homogenization. The ingots obtained were supplied to a meltspinning machine. Each ingot was induction-melted in Ar in a quartz crucible and ejected with a pressurized Ar gas out of 0.4 mm orifice onto a copper roller rotating with the velocity of 42 m/s at the roller surface. The size of melt-spun ribbons was 15 mm in thickness and 1 mm in width. The melt-spun ribbons was cut, pasted to a Teflon plate and supplied to X- ray diffraction (XRD) pole figure measurements. The specimens of 0.6 mm in width and 7.0 mm in length were prepared for thermal cyclic tests under constant stresses to investigate the shape memory behavior. Strain was measured under constant stresses during the thermal cycling and then socalled strain-temperature curves were obtained. Heating and cooling rates were 10 K/min and the temperature ranged from 123 to 393 K, approximately. The transformation temperatures of each specimen were determined by the differential scanning calorimetry (DSC) with a similar temperature range to the thermal cycling tests. XRD measurements were carried out for the Ti-51.0 at%ni specimen only at room temperature (RT). It should be noted that Ti-51.0 at%ni alloy is B2 phase at RT. Distribution of diffraction intensities reflected from three crystal planes {110}, {211} and {200} was measured and three corresponding pole figures were obtained. Then, orientation distribution functions (ODFs) were derived in order to evaluate texture. Microstructural observation and chemical analysis were done by TEM (transmission electron microscopy) and EDS (energy dispersive system), respectively. In this study, R s, M s, M f, A s and A f are abbreviations expressing the temperatures for R-phase transformation start, martensitic transformation start, martensitic transformation finish, reverse martensitic transformation start and reverse martensitic transformation finish, respectively. Symbols " R, " M, " A and " P express the strains due to R-phase transformation, martensitic transformation, reverse martensitic transformation and plastic deformation, respectively.

2 Texture and Microstructure of Ti-Ni Melt-Spun Shape Memory Alloy Ribbons 215 Fig. 1 {110}, {211} and {200} pole figures measured from Ti-51.0 at%ni as-spun ribbon. 3. Results and Discussion 3.1 Texture Figure 1 shows {110}, {211} and {200} pole figures of the Ti-51.0 at%ni as-spun ribbon. The center of the pole figure corresponds to the direction normal to the specimen surface (ND). The top and the right of pole figure correspond to the spinning direction (SD) and the transverse direction (TD), respectively. Each pole figure shows axis density distribution in the specimen coordinate system SD-TD-ND. As seen in Fig. 1, the h110i axis density distribution shows a dense ring region 45 apart from ND. This suggests that the crystal planes parallel to the specimen surface locate 45 from {110}. The {211} pole figure shows that an axis density ring peak is located at around 65 from ND and the second peak is located at around 35 from ND. The {200} pole figure shows that the high h200i axis density peak is completely located at ND. Therefore, {100} planes seem to locate preferentially on the top surface of the melt-spun specimen. Utilizing the above three pole figures, the ODF of the Ti-51.0 at%ni meltspun ribbon was derived and the section of 2 ¼ 90 is shown in Fig. 2. This result suggests a typical fiber texture with an axis around h100i. The maximum orientation density (I max )in this case is 7.5. It can be concluded that h100i fiber texture is preferentially formed in Ti-Ni alloys fabricated by the meltspinning method. 3.2 Shape memory behavior In order to clarify the deformation behavior of the meltspun ribbons, the thermal cycling tests under various constant stresses were carried out. Figure 3 shows the strain vs. temperature curves of Ti-49.0 at%ni and Ti-50.0 at%ni asspun ribbons. These strain-temperature curves represent shape memory behavior associated with the R-phase transformation (between B2 and R-phase) and the martensitic transformation (between R-phase and the monoclinic B19 phase). The strains were measured upon cooling (solid lines) and heating (dashed lines) under various constant stresses stepwisely increased in each thermal cycle. The results obtained from the Ti-51.0 at%ni ribbon are not drawn here since the Ti-51.0 at%ni ribbon did not show any shape change associated with the transformations even though the alloy was cooled down to 120 K. Since it is known that a small increase in Ni content causes a large decrease in Fig. 2 A section ( 2 ¼ 45 ) of the crystallite orientation distribution function for Ti-51.0 at%ni ribbon. transformation temperatures, 17) the phase transformation temperatures are judged to be lower than 120 K. The details will be discussed later. Figure 4 shows the recoverable strain " A and plastic strain " P as a function of applied stress for Ti-49.0 at%ni and Ti at%ni as-spun ribbons. In the case of the Ti-49.0 at%ni ribbon, it can be seen in this figure that the recoverable strain " A increases up to 5.5% with increasing applied stress up to 400 MPa, and that plastic strain is not appreciable within the stress range less than 400 MPa. This recoverable strain, " A, did not exceed about 5.5%. Appling the stress above 400 MPa, the plastic strain " p was introduced and it gradually increased with increasing applied stress. However, it should be mentioned that a recoverable strain of about 5% was still obtained even though the plastic deformation was largely recognized. Therefore, it can be said that melt-spun ribbons possess excellent shape memory properties. As seen in the strain-temperature curves of Fig. 3, the Ti at%ni ribbon exhibited the R-phase transformation under a stress less than 300 MPa. The recoverable strain " A increases with increasing applied stress and shows a

3 216 A. Khantachawana, H. Mizubayashi and S. Miyazaki Fig. 3 Strain-temperature curves under constant stresses for (a) Ti-49.0 at%ni and (b) Ti-50.0 at%ni as-spun ribbons. Fig. 5 Bright field image of Ti-51.0 at%ni as-spun ribbon. Fig. 4 Effect of applied stress on the transformation strain and plastic strain for Ti-49.0 at%ni and Ti-50.0 at%ni ribbons. maximum value of 6.0% under 500 MPa without exhibiting appreciable plastic strain as shown in Fig. 4. The critical stress for plastic deformation ( s ) was obtained over 600 MPa. Besides, M s of the Ti-50.0 at%ni ribbon is relatively low comparing with the Ti-49.0 at%ni, i.e. M s under the stress of 25 MPa is lower than 200 K. It is concluded that these melt-spun ribbons possess large transformation strains and excellent stable shape memory characteristics usable for practical applications. These stable shape memory behaviors must be due to the microstructures developed during the melt-spinning process. 3.3 Microstructure Figure 5 shows a bright field image of the Ti-51.0 at%ni ribbon. The crystallized B2 phase was confirmed in all areas in Fig. 5. Similarly to the result, crystallization was completely finished for the other two ribbons of Ti at%ni and Ti-50.0%Ni. These crystallized grains fabricated by the melt-spinning method show a texture: the h100i directions of most grains are near the direction normal to the specimen surface. The results of TEM are consistent with the results of XRD pole figures described already. Figure 6 is an enlarged micrograph near the grain Fig. 6 Enlarged micrograph near grain boundaries of Ti-51.0 at%ni asspun ribbon.

4 Texture and Microstructure of Ti-Ni Melt-Spun Shape Memory Alloy Ribbons 217 Fig. 7 X-ray profile of Ti-51.0 at%ni as-spun ribbon. boundaries of Ti-51.0 at%ni ribbon: some precipitates with a size of nm were found along the grain boundaries. The chemical composition of the area containing the grain boundaries was determined by EDS to be Ti-rich Ti at%ni. X-ray diffraction patterns are shown in Fig. 7 where the diffraction peaks from Ti 2 Ni were confirmed. Therefore, the precipitates at the grain boundaries are judged to be Ti 2 Ni. The mechanism of Ti 2 Ni formation in the Ni-rich Ti-Ni alloy is not sufficiently understood yet and further investigation is needed. The formation of Ti 2 Ni at grain boundaries results in a condensation of Ni in the matrix, causing for the decrease in the transformation temperatures. On the other hand, no precipitate could be found at grain boundaries of the Ti-49.0 at%ni and Ti-50.0 at%ni ribbons. Figure 8 shows bright field images obtained from the (a) Ti at%ni, (b) Ti-50.0 at%ni and (c) Ti-51.0 at%ni ribbons. The electron beams are parallel to [001]. Nonequilibrium disk precipitates with a radius of about 10 nm located uniformly on {100} of B2 phase for Ti-50.0 at%ni and Ti at%ni ribbons and similar precipitates with 6 nm in radius were also found in Ti-49.0 at%ni ribbon. Besides fairly strong streaks are seen along [100] and [010] in the [100] diffraction patterns, though they are not shown here. These strong streaks must come from the thin disks lying on {100} planes. It is noted that for the specimens having high transformation temperatures such as the Ti-49.0 at%ni ribbon, microstructure in the B2 parent phase was observed by TEM using a heating stage in order to obtain B2 phase. The superior shape memory characteristics can be attributed to the nonequilibrium disk precipitates formed by the process of melt-spinning. 4. Conclusions (1) According to three {110}, {211} and {200} pole figures of a Ti-51.0 at%ni ribbon, a strong h100i fiber texture is observed. This specific texture is also confirmed from the crystallite orientation distribution functions. (2) The maximum shape recoverable strain exceeds 5.5% for a Ti-49.0 at%ni ribbon and 6.0% for a Ti-50.0 at%- Ni ribbon. These large shape recoverable strains are still obtained under the high stress over 400 MPa. (3) TEM observation shows that the ribbons are fully crystallized for all melt-spun specimens. Nonequilibrium disk precipitates with the diameter of about 10 nm Fig. 8 Transmission electron micrographs showing the disk precipitates on {100} B2 inside grains for (a) Ti-49.0 at%ni, (b) Ti-50.0 at%ni and (c) Ti at%ni ribbons. located on {100} of B2 phase uniformly inside grains. These disk precipitates must be a cause of excellent shape memory characteristics with large shape recoverable strains and high critical stresses for slip deformation. (4) Not only disk-like precipitates but also Ti 2 Ni precipitates of 25 nm in diameter were also observed along grain boundaries of Ti-51.0 at%ni ribbon. The forma-

5 218 A. Khantachawana, H. Mizubayashi and S. Miyazaki tion of the Ti 2 Ni results in lowering of the martensitic transformation temperatures through the condensation of Ni content in the matrix. Acknowledgements This work was partially supported by the Grants-in-Aids for Fundamental Scientific Research (Kiban A ( ) and Kiban A ( ) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. REFERENCES 1) S. Miyazaki and A. Ishida: Mater. Sci. Eng. A (1999) ) S. Miyazaki, Y. Ohmi, K. Otsuka and Y. Suzuki: J. Phys (France) 43 (1982) C ) S. Miyazaki, in: T. W. Duerig, K. N. Melton, D. Stockel and C. M. Wayman (Eds.): Engineering Aspects of Shape Memory Alloys, (Butterworth-Heinemann, Guildford, UK, 1990) pp ) S. Miyazaki and K. Otsuka: ISIJ Int. 29 (1989) ) M. Kohl, E. Just, W. Pfleging and S. Miyazaki: Sensors and Actuators 83 (2000) ) M. Kohl, K. D. Skrobanek and S. Miyazaki: Proc. the 5th Int. Conf. on New Actuators (Actuator-96), Bremen (1996) pp ) M. Kohl, D. Dittmann, E. Quandt and B. Winzek: Sensors and Actuators 83 (2000) ) K. D. Skrobanek, M. Kohl and S. Miyazaki: Proc. the IEEE Micro- Electro-Mechanical Systems (MEMS-97), Nagoya, Japan (1997) pp ) M. Kohl, D. Dittmann, E. Quandt, B. Winzek, S. Miyazaki and D. M. Allen: Mater. Sci. Eng. A (1999) ) M. Kohl, K. D. Skrobanek and S. Miyazaki: Sensors and Actuators 72 (1999) ) M. Igharo and V. Wood: Mater. Sci. Eng. 98 (1988) ) A. Khantachawana, K. Yamazaki, H. Hosoda and S. Miyazaki: Trans MRS-J 26[1] (2001) ) Y. Furuya, M. Matsumoto, H. Kimura, K. Aoki and T. Masumoto: Mater. Trans., JIM 31 (1990) ) Z. L. Xie, J. Van Humbeeck, Y. Liu and L. Delaey: Scr. Mater. 37 (1997) ) H. Rösner, A. V. Shelyakov, A. M. Glezer, K. Feit and P. Schlo macher: Mater. Sci. Eng. A (1999) ) A. Khantachawana and S. Miyazaki: Mater. Sci. Forum (2002) ) W. Tang, B. Sudman, R. Sandström and C. Qiu: Acta Mater. 47 (1999)