- Email: [email protected]
Irreversibility in transformation behavior of equiatomic nickel–titanium alloy by electrical resistivity measurement
Journal of Alloys and Compounds 368 (2004) 182–186
Irreversibility in transformation behavior of equiatomic nickel–titanium alloy by electrical resistivity measurement Hitoshi Matsumoto∗ Department of Materials Science and Engineering, National Defense Academy, Hashirimizu, Yokosuka 239-8686, Japan Received 18 July 2003; received in revised form 19 August 2003; accepted 19 August 2003
Abstract Measurements of the electrical resistivity were precisely performed on shape memory Ni50 Ti50 alloy in order to reveal the irreversible behavior of the thermoelastic martensitic transformation with thermal cycling. The hump in the electrical resistivity during cooling is enhanced with increasing the number of complete thermal cycles to result in a peak, although no peak in the electrical resistivity is observed on the reverse transformation during heating. The electrical resistivity in the low-temperature phase, of which the temperature dependence is linear, increases with increasing the number of complete thermal cycles. The temperature coefficient of the electrical resistivity in the temperature region of the high-temperature phase increases with elevating the temperature. The transformation is strongly influenced by incomplete thermal cycles to result in a peak in the resistivity even on the reverse transformation after incomplete thermal cycling. It is thought that the anomalous behavior such as enhancement of a resistivity-peak, the increase in the electrical resistivity of the low-temperature phase, and the nonlinear relation between the resistivity and the temperature in the high-temperature phase are attributable to the appearance of an intermediate phase stabilized by transformation-induced defects, the accumulation of the transformation-induced defects, and the electron scattering due to the softening of a phonon mode in the high-temperature phase, respectively. It proved useful to make more accurate measurements of the electrical resistivity in order to investigate the intrinsic behavior of the transformation in NiTi. © 2003 Elsevier B.V. All rights reserved. Keywords: NiTi; Phase transition; Thermal cycle; Electrical resistivity; Pre-martensitic phenomenon; Intermediate phase; R-phase
1. Introduction The near equiatomic NiTi alloy has the unique property of a shape memory attributable to the thermoelastic martensitic transformation near room temperature [1–3]. The transformation has often been investigated by measurements of electrical resistivity [4–7] and ultrasonic properties [8–11], and by X-ray diffraction [12,13]. An R-phase, that is, an intermediate (rhombohedral) phase appears when the high-temperature phase (CsCl structure) is martensitically transformed into the low-temperature (monoclinic) phase. The appearance of the intermediate phase or the pre-martensitic phenomenon is characterized by an increase in the electrical resistivity. On the successive transformation from the intermediate phase to the low-temperature phase, the electrical resistivity decreases drastically to bring about ∗
Tel.: +81-46-841-3810; fax: +81-46-844-5910. E-mail address: [email protected] (H. Matsumoto).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.08.067
a peak in the electrical resistivity near the start temperature of the transformation to the low-temperature phase [4,7]. However, so far the intermediate phase has often not been observed for NiTi. The investigation on the generation of the intermediate phase is important in understanding the transformation behavior of NiTi. However, the behavior concerning the transformation to the intermediate phase has not yet been satisfactorily elucidated because precise measurements have scarcely been carried out with high accuracy, although the transformation is complicated, depending on the composition [14–23] and the transformation cycles [24–29]. It is known that a measurement of the electrical resistivity is effective in detecting a trace of the intermediate phase of NiTi in comparison with the measurements of ultrasonic and thermal properties. Therefore, in the present paper, a more precise study on the electrical resistivity of the equiatomic NiTi is performed in order to reveal the intrinsic behavior of the transformation.
H. Matsumoto / Journal of Alloys and Compounds 368 (2004) 182–186
183
2. Experimental procedure The Ni50 Ti50 (in at.%) alloy was prepared from nickel (purity, 99.99%) and titanium (purity, 99.9%) by electron-beam melting on a water-cooled copper hearth in a vacuum of 10−2 Pa [30,31]. After several remelts, the alloy ingot was annealed at 1000 ◦ C for an hour in a vacuum of 10−4 Pa for homogenization. The compositional change caused by the melting procedure was negligible, as estimated from the difference between the charged and the final weights. The specimen was cut off the alloy ingot and re-annealed subsequently under the above-mentioned annealing condition. The electrical resistivity was measured precisely by a four-probe potentiometric method in the temperature range of −120 to +150 ◦ C at a thermal cycle rate of ∼ 7 × 10−3 ◦ C s−1 . The relative accuracy of the resistivity measurement was determined by the accuracy of the temperature measurement (relative accuracy of 0.1 ◦ C) and was estimated to be within 10−4 . Details for the sample preparation and the measurement are shown elsewhere [4,24]. The temperature range of the thermal cycling is much wider than the temperature region of the transformation in the Ni50 Ti50 . The complete repetition of the transformation is carried out with such complete thermal cycling. An incomplete repetition of the transformation with an incomplete thermal cycling has also been studied in order to reveal precise variations in the transformation behavior due to thermal cycles.
Fig. 1. Electrical resistivity as a function of temperature during cooling. The numbers correspond to the number of complete thermal cycles.
3. Experimental results and discussion 3.1. Effect of complete thermal cycling The electrical resistivity versus temperature curves during cooling and heating are shown in Figs. 1 and 2, respectively. The numbers correspond to the number of complete thermal cycles. The dependence of the electrical resistivity upon the temperature shows a thermal hysteresis between the transformation and the reverse one and tends to shift to the lower temperature side with increasing the number of the thermal cycles. A faint hump in the electrical resistivity during the first cooling is observed near room temperature, which is enhanced with increasing the number of the thermal cycles to result in a clear peak [25], although no peak is detected during heating. The temperature at such a peak in the electrical resistivity, which is often regarded as the start temperature of the martensitic transformation to the low-temperature phase (Ms), shifts to a lower temperature side with increasing the number of the thermal cycles, as shown in Fig. 1. The titanium-rich NiTi prepared by the same method has a higher transformation temperature and no intermediate phase. Therefore, the appearance of an intermediate phase is intimately connected with the composition, that is, the transformation temperature. The electrical resistivity in the low-temperature phase increases with the thermal cycling, although the increase
Fig. 2. Electrical resistivity as a function of temperature during heating. The numbers correspond to the number of complete thermal cycles.
184
H. Matsumoto / Journal of Alloys and Compounds 368 (2004) 182–186
in the electrical resistivity is negligibly small in the high-temperature phase, as shown in Figs. 1 and 2. Such increase in the electrical resistivity of the low-temperature phase is attributable to the electron scattering due to structural defects such as dislocations induced during the transformation, because it is independent of the temperature. The alterations in the transformation behavior during cooling occur drastically within about six thermal cycles and tend to decrease with the subsequent thermal cycles. The dependence of the electrical resistivity versus temperature curve upon the complete thermal cycle agrees with previous results on quenched specimens, qualitatively, although the decrease in the Ms and the resistivity-shift in the low-temperature phase with increasing the number of the thermal cycles are smaller than those on quenched specimens [28]. It may be thought that the quenched-in vacancies influence the generation of transformation-induced defects and the stability of each phase with the thermal cycling because of no precipitation in the equiatomic NiTi. For a better understanding of the transformation behavior, the temperature coefficient of the electrical resistivity as a function of temperature is shown in Fig. 3, which is calculated from the results shown in Figs. 1 and 2. The
numbers correspond to the number of the thermal cycles. A drastic change of the temperature coefficient during cooling is observed in the transformation region and is progressively more pronounced with increasing the number of the thermal cycles, whereas the temperature coefficient during heating is nearly constant up to about room temperature and then drastically decreases. With increasing the number of the thermal cycles, a progressive decrease in the minimum value of the temperature coefficient during heating is observed in Fig. 3, which can be regarded as the indication of a peak in the electrical resistivity, though no peak appears during heating. The temperature coefficient in the temperature range above about +50 ◦ C gradually increases from 14.8 n cm ◦ C−1 (at +50 ◦ C) to 41.6 n cm ◦ C−1 (at +150 ◦ C) which is independent of the thermal cycles. Therefore, the relation of the electrical resistivity in the high-temperature phase to the temperature is nonlinear, and the temperature region showing the pre-martensitic phenomenon is broadened to the high temperature side. The anomaly of this nonlinear behavior is taken to be attributable to the electron scattering due to the softening of a phonon mode in relation to the transformation from the high-temperature phase to the intermediate phase. On the other hand, the temperature
Fig. 3. Temperature coefficient of electrical resistivity as a function of temperature, which is calculated from the results shown in Figs. 1 and 2. Open and closed circles show heating and cooling, respectively. The numbers correspond to the number of complete thermal cycles.
Fig. 4. Electrical resistivity as a function of temperature for incomplete thermal cycling. Open and closed circles show heating and cooling, respectively. The arrow corresponds to the As. The peak in the electrical resistivity is more enhanced with the incomplete thermal cycling up to the higher limited temperature.
H. Matsumoto / Journal of Alloys and Compounds 368 (2004) 182–186
coefficient below −50 ◦ C is of 151 n cm ◦ C−1 , which is independent of the temperature and the transformation cycles. 3.2. Effect of incomplete thermal cycling After the effect of the complete thermal cycling was diminished markedly by 15 thermal cycles and the transformation behavior had become practically reversible with the thermal cycling, the cooling was stopped at a temperature of −15 ◦ C followed by incomplete thermal cycles between −15 ◦ C and a higher temperature. The upper limit of the temperature during the incomplete thermal cycling was increased up to +15, +21, and +27 ◦ C, successively. The electrical resistivity versus temperature curve during the incomplete thermal cycles is shown in Fig. 4. The electrical resistivity during heating increases linearly, of which the slope changes at about +2 ◦ C, corresponding to the start temperature of the reverse transformation (As). The peak in the electrical resistivity during cooling becomes enhanced when heating to the higher temperature above As, and a thermal hysteresis is shown depending on the temperature range of the incomplete thermal cycle. The hysteresis of the electrical resistivity versus temperature curve is irreversible for the incomplete thermal cycling within the temperature
185
region of the transformation, even after becoming reversible with many complete thermal cycles. The electrical resistivity versus temperature curve during heating after the incomplete thermal cycles is shown in Fig. 5. A peak in the resistivity is observed, although it is rather small in comparison with the peak during cooling in Fig. 1. It is assumed that the appearance of the peak on the reverse transformation is also attributable to the stabilization of the intermediate phase due to defects such as dislocations induced by the incomplete thermal cycles. Therefore, the incomplete thermal cycle is effective in forming a peak in the electrical resistivity to result in the enhancement of the irreversible behavior of the transformation.
4. Conclusion In order to reveal the intrinsic behavior of the thermoelastic martensitic transformation of NiTi, more precise measurements of the electrical resistivity on Ni50 Ti50 were performed by a four-probe potentiometric method. A hump in the electrical resistivity is detected near room temperature during the first cooling after annealing and is enhanced with increasing the number of complete thermal cycles to result in a peak. The electrical resistivity in the low-temperature phase increases with the complete thermal cycles, which is caused by the electron scattering due to transformation-induced defects. Therefore, it is thought that the defects influence the formation of the intermediate phase. On the reverse transformation during heating, no peak in the electrical resistivity appears, although the indication of a resistivity-peak is observed in the temperature coefficient of the electrical resistivity versus temperature curve. The temperature dependence of the electrical resistivity in the high-temperature phase is nonlinear, which implies that the temperature region of the transformation is broadened to the high-temperature side. Such a pre-martensitic phenomenon is taken to be attributable to the enhancement of the electron scattering due to the softening of a phonon mode in the high-temperature phase. The incomplete thermal cycling induces a peak in the resistivity not only during cooling but also during heating, and is effective for the stability of the intermediate phase in comparison with the complete thermal cycling.
References
Fig. 5. Electrical resistivity as a function of temperature during heating after the incomplete thermal cycles. A peak in the resistivity is observed on the reverse transformation.
[1] T. Saburi, in: K. Otsuka, C.M. Wayman (Eds.), Shape Memory Materials, Cambridge University Press, Cambridge, 1998, p. 49. [2] K. Shimizu, Bull. Jpn. Inst. Met. 17 (1978) 5. [3] T. Honma, Bull. Jpn. Inst. Met. 19 (1980) 366. [4] H. Matsumoto, J. Jpn. Inst. Met. 66 (2002) 1350. [5] H. Matsumoto, J. Alloys Compd. 178 (1992) L1. [6] T. Honma, H. Takei, J. Jpn. Inst. Met. 39 (1975) 175. [7] G.D. Sandrock, A.J. Perkins, R.F. Hehemann, Metall. Trans. 2 (1971) 2769.
186
H. Matsumoto / Journal of Alloys and Compounds 368 (2004) 182–186
[8] H. Matsumoto, H. Ishiguro, J. Less-Common Met. 144 (1988) L39. [9] Y.N. Wang, J.M. Guei, K.F. Sun, Y. Yuan, in: Proceedings of the International Conference on Martensitic Transformations, Japan Institute of Metals, 1987, p. 127. [10] N.G. Pace, G.A. Saunders, Solid State Commun. 9 (1971) 331. [11] N.G. Pace, G.A. Saunders, Philos. Mag. 22 (1970) 73. [12] G.M. Michal, R. Sinclair, Acta Cryst. B37 (1981) 1803. [13] K. Otsuka, T. Sawamura, K. Shimizu, Phys. Status Solidi (a) 5 (1971) 457. [14] H. Matsumoto, H. Abe, Y. Morikawa, Physica B 322 (2002) 24. [15] N. Ishii, H. Matsumoto, J. Mater. Sci. Lett. 18 (1999) 1853. [16] H. Matsumoto, Solid State Commun. 86 (1993) 755. [17] H. Matsumoto, Physica B 160 (1989) 138. [18] H. Matsumoto, H. Ishiguro, J. Less-Common Met. 153 (1989) 57. [19] S.A. Shabalovskaya, Phys. Status Solidi (b) 132 (1985) 327. [20] C.M. Hwang, M. Meichle, M.B. Salamon, C.M. Wayman, Philos. Mag. A 47 (1983) 9. [21] H. Matsumoto, K. Ojima, J. Mater. Sci. Lett. 8 (1989) 727.
[22] M. Matsumoto, T. Honma, in: Proceedings of the First JIM Institute Symposium on New Aspects of Martensitic Transformation, Japan Institute of Metals, 1976, p. 199. [23] K. Sugimoto, K. Kamei, T. Sugimoto, T. Sodeoka, in: Proceedings of the International Conference on Martensitic Transformations, Japan Institute of Metals, 1987, p. 729. [24] H. Matsumoto, J. Alloys Compd. 350 (2003) 213. [25] H. Matsumoto, Physica B 190 (1993) 115. [26] H. Matsumoto, J. Mater. Sci. Lett. 10 (1991) 408. [27] G. Airoldi, B. Rivolta, C. Turco, in: Proceedings of the International Conference on Martensitic Transformations, Japan Institute of Metals, 1987, p. 691. [28] C.M. Wayman, I. Cornelis, K. Shimizu, Scr. Metall. 6 (1972) 115. [29] F.E. Wange, B.F. DeSavage, W.J. Buehler, W.R. Hosler, J. Appl. Phys. 39 (1968) 2166. [30] H. Matsumoto, J. Mater. Sci. Lett. 10 (1991) 596. [31] H. Matsumoto, J. Mater. Sci. Lett. 10 (1991) 417.
Irreversibility in transformation behavior of equiatomic nickel–titanium alloy by electrical resistivity measurement Hitoshi Matsumoto∗ Department of Materials Science and Engineering, National Defense Academy, Hashirimizu, Yokosuka 239-8686, Japan Received 18 July 2003; received in revised form 19 August 2003; accepted 19 August 2003
Abstract Measurements of the electrical resistivity were precisely performed on shape memory Ni50 Ti50 alloy in order to reveal the irreversible behavior of the thermoelastic martensitic transformation with thermal cycling. The hump in the electrical resistivity during cooling is enhanced with increasing the number of complete thermal cycles to result in a peak, although no peak in the electrical resistivity is observed on the reverse transformation during heating. The electrical resistivity in the low-temperature phase, of which the temperature dependence is linear, increases with increasing the number of complete thermal cycles. The temperature coefficient of the electrical resistivity in the temperature region of the high-temperature phase increases with elevating the temperature. The transformation is strongly influenced by incomplete thermal cycles to result in a peak in the resistivity even on the reverse transformation after incomplete thermal cycling. It is thought that the anomalous behavior such as enhancement of a resistivity-peak, the increase in the electrical resistivity of the low-temperature phase, and the nonlinear relation between the resistivity and the temperature in the high-temperature phase are attributable to the appearance of an intermediate phase stabilized by transformation-induced defects, the accumulation of the transformation-induced defects, and the electron scattering due to the softening of a phonon mode in the high-temperature phase, respectively. It proved useful to make more accurate measurements of the electrical resistivity in order to investigate the intrinsic behavior of the transformation in NiTi. © 2003 Elsevier B.V. All rights reserved. Keywords: NiTi; Phase transition; Thermal cycle; Electrical resistivity; Pre-martensitic phenomenon; Intermediate phase; R-phase
1. Introduction The near equiatomic NiTi alloy has the unique property of a shape memory attributable to the thermoelastic martensitic transformation near room temperature [1–3]. The transformation has often been investigated by measurements of electrical resistivity [4–7] and ultrasonic properties [8–11], and by X-ray diffraction [12,13]. An R-phase, that is, an intermediate (rhombohedral) phase appears when the high-temperature phase (CsCl structure) is martensitically transformed into the low-temperature (monoclinic) phase. The appearance of the intermediate phase or the pre-martensitic phenomenon is characterized by an increase in the electrical resistivity. On the successive transformation from the intermediate phase to the low-temperature phase, the electrical resistivity decreases drastically to bring about ∗
Tel.: +81-46-841-3810; fax: +81-46-844-5910. E-mail address: [email protected] (H. Matsumoto).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.08.067
a peak in the electrical resistivity near the start temperature of the transformation to the low-temperature phase [4,7]. However, so far the intermediate phase has often not been observed for NiTi. The investigation on the generation of the intermediate phase is important in understanding the transformation behavior of NiTi. However, the behavior concerning the transformation to the intermediate phase has not yet been satisfactorily elucidated because precise measurements have scarcely been carried out with high accuracy, although the transformation is complicated, depending on the composition [14–23] and the transformation cycles [24–29]. It is known that a measurement of the electrical resistivity is effective in detecting a trace of the intermediate phase of NiTi in comparison with the measurements of ultrasonic and thermal properties. Therefore, in the present paper, a more precise study on the electrical resistivity of the equiatomic NiTi is performed in order to reveal the intrinsic behavior of the transformation.
H. Matsumoto / Journal of Alloys and Compounds 368 (2004) 182–186
183
2. Experimental procedure The Ni50 Ti50 (in at.%) alloy was prepared from nickel (purity, 99.99%) and titanium (purity, 99.9%) by electron-beam melting on a water-cooled copper hearth in a vacuum of 10−2 Pa [30,31]. After several remelts, the alloy ingot was annealed at 1000 ◦ C for an hour in a vacuum of 10−4 Pa for homogenization. The compositional change caused by the melting procedure was negligible, as estimated from the difference between the charged and the final weights. The specimen was cut off the alloy ingot and re-annealed subsequently under the above-mentioned annealing condition. The electrical resistivity was measured precisely by a four-probe potentiometric method in the temperature range of −120 to +150 ◦ C at a thermal cycle rate of ∼ 7 × 10−3 ◦ C s−1 . The relative accuracy of the resistivity measurement was determined by the accuracy of the temperature measurement (relative accuracy of 0.1 ◦ C) and was estimated to be within 10−4 . Details for the sample preparation and the measurement are shown elsewhere [4,24]. The temperature range of the thermal cycling is much wider than the temperature region of the transformation in the Ni50 Ti50 . The complete repetition of the transformation is carried out with such complete thermal cycling. An incomplete repetition of the transformation with an incomplete thermal cycling has also been studied in order to reveal precise variations in the transformation behavior due to thermal cycles.
Fig. 1. Electrical resistivity as a function of temperature during cooling. The numbers correspond to the number of complete thermal cycles.
3. Experimental results and discussion 3.1. Effect of complete thermal cycling The electrical resistivity versus temperature curves during cooling and heating are shown in Figs. 1 and 2, respectively. The numbers correspond to the number of complete thermal cycles. The dependence of the electrical resistivity upon the temperature shows a thermal hysteresis between the transformation and the reverse one and tends to shift to the lower temperature side with increasing the number of the thermal cycles. A faint hump in the electrical resistivity during the first cooling is observed near room temperature, which is enhanced with increasing the number of the thermal cycles to result in a clear peak [25], although no peak is detected during heating. The temperature at such a peak in the electrical resistivity, which is often regarded as the start temperature of the martensitic transformation to the low-temperature phase (Ms), shifts to a lower temperature side with increasing the number of the thermal cycles, as shown in Fig. 1. The titanium-rich NiTi prepared by the same method has a higher transformation temperature and no intermediate phase. Therefore, the appearance of an intermediate phase is intimately connected with the composition, that is, the transformation temperature. The electrical resistivity in the low-temperature phase increases with the thermal cycling, although the increase
Fig. 2. Electrical resistivity as a function of temperature during heating. The numbers correspond to the number of complete thermal cycles.
184
H. Matsumoto / Journal of Alloys and Compounds 368 (2004) 182–186
in the electrical resistivity is negligibly small in the high-temperature phase, as shown in Figs. 1 and 2. Such increase in the electrical resistivity of the low-temperature phase is attributable to the electron scattering due to structural defects such as dislocations induced during the transformation, because it is independent of the temperature. The alterations in the transformation behavior during cooling occur drastically within about six thermal cycles and tend to decrease with the subsequent thermal cycles. The dependence of the electrical resistivity versus temperature curve upon the complete thermal cycle agrees with previous results on quenched specimens, qualitatively, although the decrease in the Ms and the resistivity-shift in the low-temperature phase with increasing the number of the thermal cycles are smaller than those on quenched specimens [28]. It may be thought that the quenched-in vacancies influence the generation of transformation-induced defects and the stability of each phase with the thermal cycling because of no precipitation in the equiatomic NiTi. For a better understanding of the transformation behavior, the temperature coefficient of the electrical resistivity as a function of temperature is shown in Fig. 3, which is calculated from the results shown in Figs. 1 and 2. The
numbers correspond to the number of the thermal cycles. A drastic change of the temperature coefficient during cooling is observed in the transformation region and is progressively more pronounced with increasing the number of the thermal cycles, whereas the temperature coefficient during heating is nearly constant up to about room temperature and then drastically decreases. With increasing the number of the thermal cycles, a progressive decrease in the minimum value of the temperature coefficient during heating is observed in Fig. 3, which can be regarded as the indication of a peak in the electrical resistivity, though no peak appears during heating. The temperature coefficient in the temperature range above about +50 ◦ C gradually increases from 14.8 n cm ◦ C−1 (at +50 ◦ C) to 41.6 n cm ◦ C−1 (at +150 ◦ C) which is independent of the thermal cycles. Therefore, the relation of the electrical resistivity in the high-temperature phase to the temperature is nonlinear, and the temperature region showing the pre-martensitic phenomenon is broadened to the high temperature side. The anomaly of this nonlinear behavior is taken to be attributable to the electron scattering due to the softening of a phonon mode in relation to the transformation from the high-temperature phase to the intermediate phase. On the other hand, the temperature
Fig. 3. Temperature coefficient of electrical resistivity as a function of temperature, which is calculated from the results shown in Figs. 1 and 2. Open and closed circles show heating and cooling, respectively. The numbers correspond to the number of complete thermal cycles.
Fig. 4. Electrical resistivity as a function of temperature for incomplete thermal cycling. Open and closed circles show heating and cooling, respectively. The arrow corresponds to the As. The peak in the electrical resistivity is more enhanced with the incomplete thermal cycling up to the higher limited temperature.
H. Matsumoto / Journal of Alloys and Compounds 368 (2004) 182–186
coefficient below −50 ◦ C is of 151 n cm ◦ C−1 , which is independent of the temperature and the transformation cycles. 3.2. Effect of incomplete thermal cycling After the effect of the complete thermal cycling was diminished markedly by 15 thermal cycles and the transformation behavior had become practically reversible with the thermal cycling, the cooling was stopped at a temperature of −15 ◦ C followed by incomplete thermal cycles between −15 ◦ C and a higher temperature. The upper limit of the temperature during the incomplete thermal cycling was increased up to +15, +21, and +27 ◦ C, successively. The electrical resistivity versus temperature curve during the incomplete thermal cycles is shown in Fig. 4. The electrical resistivity during heating increases linearly, of which the slope changes at about +2 ◦ C, corresponding to the start temperature of the reverse transformation (As). The peak in the electrical resistivity during cooling becomes enhanced when heating to the higher temperature above As, and a thermal hysteresis is shown depending on the temperature range of the incomplete thermal cycle. The hysteresis of the electrical resistivity versus temperature curve is irreversible for the incomplete thermal cycling within the temperature
185
region of the transformation, even after becoming reversible with many complete thermal cycles. The electrical resistivity versus temperature curve during heating after the incomplete thermal cycles is shown in Fig. 5. A peak in the resistivity is observed, although it is rather small in comparison with the peak during cooling in Fig. 1. It is assumed that the appearance of the peak on the reverse transformation is also attributable to the stabilization of the intermediate phase due to defects such as dislocations induced by the incomplete thermal cycles. Therefore, the incomplete thermal cycle is effective in forming a peak in the electrical resistivity to result in the enhancement of the irreversible behavior of the transformation.
4. Conclusion In order to reveal the intrinsic behavior of the thermoelastic martensitic transformation of NiTi, more precise measurements of the electrical resistivity on Ni50 Ti50 were performed by a four-probe potentiometric method. A hump in the electrical resistivity is detected near room temperature during the first cooling after annealing and is enhanced with increasing the number of complete thermal cycles to result in a peak. The electrical resistivity in the low-temperature phase increases with the complete thermal cycles, which is caused by the electron scattering due to transformation-induced defects. Therefore, it is thought that the defects influence the formation of the intermediate phase. On the reverse transformation during heating, no peak in the electrical resistivity appears, although the indication of a resistivity-peak is observed in the temperature coefficient of the electrical resistivity versus temperature curve. The temperature dependence of the electrical resistivity in the high-temperature phase is nonlinear, which implies that the temperature region of the transformation is broadened to the high-temperature side. Such a pre-martensitic phenomenon is taken to be attributable to the enhancement of the electron scattering due to the softening of a phonon mode in the high-temperature phase. The incomplete thermal cycling induces a peak in the resistivity not only during cooling but also during heating, and is effective for the stability of the intermediate phase in comparison with the complete thermal cycling.
References
Fig. 5. Electrical resistivity as a function of temperature during heating after the incomplete thermal cycles. A peak in the resistivity is observed on the reverse transformation.
[1] T. Saburi, in: K. Otsuka, C.M. Wayman (Eds.), Shape Memory Materials, Cambridge University Press, Cambridge, 1998, p. 49. [2] K. Shimizu, Bull. Jpn. Inst. Met. 17 (1978) 5. [3] T. Honma, Bull. Jpn. Inst. Met. 19 (1980) 366. [4] H. Matsumoto, J. Jpn. Inst. Met. 66 (2002) 1350. [5] H. Matsumoto, J. Alloys Compd. 178 (1992) L1. [6] T. Honma, H. Takei, J. Jpn. Inst. Met. 39 (1975) 175. [7] G.D. Sandrock, A.J. Perkins, R.F. Hehemann, Metall. Trans. 2 (1971) 2769.
186
H. Matsumoto / Journal of Alloys and Compounds 368 (2004) 182–186
[8] H. Matsumoto, H. Ishiguro, J. Less-Common Met. 144 (1988) L39. [9] Y.N. Wang, J.M. Guei, K.F. Sun, Y. Yuan, in: Proceedings of the International Conference on Martensitic Transformations, Japan Institute of Metals, 1987, p. 127. [10] N.G. Pace, G.A. Saunders, Solid State Commun. 9 (1971) 331. [11] N.G. Pace, G.A. Saunders, Philos. Mag. 22 (1970) 73. [12] G.M. Michal, R. Sinclair, Acta Cryst. B37 (1981) 1803. [13] K. Otsuka, T. Sawamura, K. Shimizu, Phys. Status Solidi (a) 5 (1971) 457. [14] H. Matsumoto, H. Abe, Y. Morikawa, Physica B 322 (2002) 24. [15] N. Ishii, H. Matsumoto, J. Mater. Sci. Lett. 18 (1999) 1853. [16] H. Matsumoto, Solid State Commun. 86 (1993) 755. [17] H. Matsumoto, Physica B 160 (1989) 138. [18] H. Matsumoto, H. Ishiguro, J. Less-Common Met. 153 (1989) 57. [19] S.A. Shabalovskaya, Phys. Status Solidi (b) 132 (1985) 327. [20] C.M. Hwang, M. Meichle, M.B. Salamon, C.M. Wayman, Philos. Mag. A 47 (1983) 9. [21] H. Matsumoto, K. Ojima, J. Mater. Sci. Lett. 8 (1989) 727.
[22] M. Matsumoto, T. Honma, in: Proceedings of the First JIM Institute Symposium on New Aspects of Martensitic Transformation, Japan Institute of Metals, 1976, p. 199. [23] K. Sugimoto, K. Kamei, T. Sugimoto, T. Sodeoka, in: Proceedings of the International Conference on Martensitic Transformations, Japan Institute of Metals, 1987, p. 729. [24] H. Matsumoto, J. Alloys Compd. 350 (2003) 213. [25] H. Matsumoto, Physica B 190 (1993) 115. [26] H. Matsumoto, J. Mater. Sci. Lett. 10 (1991) 408. [27] G. Airoldi, B. Rivolta, C. Turco, in: Proceedings of the International Conference on Martensitic Transformations, Japan Institute of Metals, 1987, p. 691. [28] C.M. Wayman, I. Cornelis, K. Shimizu, Scr. Metall. 6 (1972) 115. [29] F.E. Wange, B.F. DeSavage, W.J. Buehler, W.R. Hosler, J. Appl. Phys. 39 (1968) 2166. [30] H. Matsumoto, J. Mater. Sci. Lett. 10 (1991) 596. [31] H. Matsumoto, J. Mater. Sci. Lett. 10 (1991) 417.