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 for solid state refrigeration due to their large latent heats. NiTi SMAs 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 effects of Co, Fe, V, Cr, Cu and Ni additions, and the role grain sizes and precipitates. Therefore, different alloys were prepared through arc melting and thermomechanical processing. Caloric effects were characterized through a modified tensile test system where adiabatic temperature changes, as well a latent heats can be monitored through thermography imaging during mechanical and thermal cycling under load. A trend was observed that decreasing martensite start temperatures are generally associated with decreasing latent heats.