Factorial design analysis of phase transformation stability in thermally cycled NiTi alloys

This paper considers the durability and stability of thermoelastic martensitic transformations in commercial NiTi alloys subjected to repeated heating and cooling cycles. The experimental study was designed to mimic real actuator applications where the alloy is required to produce a work output. This was achieved by thermal cycling against constant loads and by studying the effect of repeated transformations on shape stability and strain output. A factorial experimental method was employed which considered: thermal processing, heating/cooling rates, applied loads and prior cold work. For the first time, this paper attempts to analyse the main interactive effects and individual significance of processing and operating factors on transformation stability and actuator output. The results presented can thus be used as a guide to optimising the processing and operating conditions required for long term actuator stability.