The Stability of NiTi shape memory alloys and actuator applications

Recent research and development in the area of shape memory alloys (SMA) continues to yield novel and unique results. Of the alloys that display the effect NiTi alloys have found the most commercial interest resulting in a number of niche applications, particularly in the medical device industry. An analysis of the market and strategic issues associated with diffusing NiTi SMA's into mainstream design use and commercial application is presented. It is concluded that for SMA's to become commercially viable, R&D must be carried out on design relevant properties and particular the cyclic stability. To address this need a systematic investigation into the durability and stability of martensitic transformations in commercial NiTi actuator alloys has been completed. A factorial design on experiments approach is employed to study: alloy type, thermal processing, heating/cooling rates, applied loads and prior cold work level. In addition, the structural integrity and internal stress effects of repeated actuation against applied external stress are considered and compared with previously published data. For the first time, this work analyses the main interactive effects and individual significance of processing and operating variables on transformation stability and actuator output. The results can therefore be used as a guide to optimizing the processing and operating conditions for long term actuator stability.