Effect of R-phase on shape recovery speed of Ti-Ni shape memory alloy wire for variable-stiffness mechanism using jamming transition phenomenon

The jamming transition phenomenon refers to the change in stiffness of a powder as its density changes. As a familiar example, vacuum-packed coffee grounds are hard, but when opened to allow air into the package and the density is reduced, the grounds become softer. Using Ti-Ni shape memory alloys (SMAs) and the jamming transition phenomenon, a variable-stiffness and deformable link was devised and prototyped by Takashima et al. (2022). Although the mechanism worked well, it operated slowly due to the slow shape recovery velocity of the SMA. For practical application of this mechanism, it is necessary to improve the recovery velocity of Ti-Ni SMA. Normally, Ti-Ni SMAs exhibit an austenite to martensite phase transformation; however, Miyazaki and Otsuka (1984) observed that under some conditions, R-phase transformation can occur between these phase transformations. In a previous study, Tanikata et al. (2022) observed that SMAs that show R-phase transformation during shape recovery tend to have higher recovery stress than SMAs that do not show R-phase transformation. Therefore, SMAs that show R-phase transformation during shape recovery are expected to have a high shape recovery velocity. This study investigates the effect of R-phase transformation on the recovery velocity of SMA. From the experimental results, specimens that show R-phase transformation tend to have a higher recovery velocity than specimens that do not show R-phase transformation. However, the mechanism created using materials showing R-phase transformation had a small amount of shape recovery during shape recovery. Thus, we attempted to improve the mechanical properties of the SMA by training. The device using the trained specimen demonstrated increased shape recovery compared to the device using the untrained specimen.