Caloric Effects in Shape Memory Alloys – Optimizing NiTi for Solid State Refrigeration

Recently, it was found that pseudoelastic NiTi shape memory alloys (SMAs) are very attractive as a functional material for solid state refrigeration due to their large latent heats. In theory, NiTi SMAs could even outperform conventional refrigeration processes through up to 4 times higher cooling efficiencies. Within a single unloading step from stress-induced martensite to austenite, adiabatic temperatures drops between 15 and 30K can be obtained. These caloric effects have only received little attention so far. In order to optimize NiTi for elastocaloric cooling processes, the effects of material parameters as well as loading/unloading conditions need to be understood. The goal is to identify compositions and microstructures which are associated with large latent heats, low austenite finish temperatures, and stable performance during pseudoelastic cycling. In the present work, we characterize the elastocaloric behaviour of different NiTi-based SMAs. The alloys were prepared through arc melting and thermomechanical processing. Mechanical induced caloric effects were characterized through a new designed test setup. We used a modified tensile test system where adiabatic temperature changes can be monitored through thermography imaging.  Increasing strain rate the maximum temperature change reaches saturation. We also investigate the effects of functional fatigue on caloric effects in NiTi.