The R-Phase Transformation in the Ti-Ni Shape Memory Alloy and its Application

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.

R-phase transformation in NiTi alloys

“…The R-phase transformation shows a high stability during thermomechanical cycling. 52,[60][61][62][63][64] As shown in Fig. 5, 60 the Rphase transformation has superior functional fatigue resistance than the B199 martensite transformation during thermal cycling.…”

Titanium-Nickel Shape Memory Alloys in Medical Applications

“…Goodman fatigue maps [77] or stress vs. number of working cycles to failure plot [48,52] is applicable in this situation. It is convenient to take advantage of the R-phase transformation for significantly larger fatigue life compared to B2~ B 19' phase transformation [78].…”

Effect of Severe Plastic Deformation and Post-Deformation Heat Treatment on the Microstructure and Superelastic Properties of Ti-50.8 at.% Ni Alloy

“…During cooling, R is formed before the formation of B19′ because B19′ is thermodynamically preferred, but the R phase is kinetically advantageous, having low activation energy [ 17 ]. As the B2 to R phase transformation accompanies a significantly smaller transformation strain (less than 1% [ 16 , 17 , 18 ]) compared to the B2 to B19′ phase transformation (approximately 8–10% [ 17 , 19 , 20 ]), this characteristic is useful for small amplitude but higher frequency actuator and damping applications [ 21 ].…”