The R-Phase Transformation in the Ti-Ni Shape Memory Alloy and its Application
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I. Ohkata*, H. Tamura** *Medical Division, PIOLAX Inc., Yokohama, 240 Japan **Yokohama Research Laboratories, The Furukawa Electric Co.., Ltd., Yokohama, 220 Japan
ABSTRACT We discuss a comprehensive design approach of Ti-Ni alloy coil springs and introduce a new application of the R-phase transformation. In order to attain high cyclic performance, one must understand the two relationships between design parameters and material characteristics and between material characteristics and cyclic performance. Metallurgical parameters and coil spring dimensions play an important role as design parameters in the former relationship. High cyclic performance of an actuator is closely related to the suppression of the monoclinic martensite. Transformation temperatures and their stress dependence is of primary importance as material characteristics in the latter relationship. A thermostatic mixing valve, which is the latest application of the R-phase transformation in Japan is then discussed as a new type of a shape memory alloy actuator. The R-phase transformation is employed to achieve not only a long cycle life but a linear operation with the set temperature to continuously control the mixing ratio of hot and cold water. This is achieved by changing the total length of the two-way actuator in a linear manner with the set temperature. The linear characteristic is satisfied between 35-50'C by optimizing thermomechanical treatment and the dimensions of Ti-Ni and biasing coil springs. INTRODUCTION The R-phase transformation in the Ti-Ni shape memory alloy exhibits a small temperature
hysteresis and excellent fatigue property[l]. The transformation also completes in a narrow temperature range, which makes it suitable for a fairly high rate actuation. However, the R-phase transformation is sensitive to thermomechanical treatment, alloy composition including the addition of a third element and repeating thermal cycles[2]. In order to utilize the R-phase transformation efficiently, one must start an actuator design from controlling material characteristics including transformation behavior and stress dependence of transformation temperatures. Figure 1 shows how coil spring dimensions and metallurgical parameters including the composition and heat treatment play an important role in determining the cyclic performance of an actuator. The relationships have been intensively but separately studied between design parameters and material characteristics and between material characteristics and cyclic performance. One purpose of this study is to comprehend how the design parameters are determined in order to attain a large number of cycles by discussing the two relationships. Since coil springs have been mostly employed in SMA applications, we focus our attention on the cyclic performance of Ti-Ni shape memory coil springs. Another purpose is to introduce a new water mixing valve which utilizes the R-phase transformation. Water mixing valves require continuous and gradual temperature control. We have succeeded in controlling the tempera
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