Compositional and Microstructural Optimization and Microstructural, Thermal and Mechanical Assessment of NiTi Shape Memory Alloys for Ferroelastic Cooling

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In the last decades, alternative refrigeration methods have increasingly received attention because they provide high efficiencies, and thus can contribute to a more efficient use of resources and lower greenhouse gas emissions. Pseudoelastic NiTi-based shape memory alloys (SMAs) represent a promising class of candidate materials for ferroelastic cooling. During mechanical loading, a stress induced martensitic transformation allows for a reversible deformation of up to 7 %. On unloading, stress-induced martensite transforms back to austenite, and recovers its initial shape. Both, forward and reverse transformations are associated with heat effects, an exothermic heat production when martensite forms on loading and an endothermic reaction when it disappears on unloading. In the present work, we aim at studying and exploiting the endothermic heat effect for cooling. The project has four objectives: First, it aims at characterizing the elementary transformation mechanisms which govern cooling during the back transformation of stress induced martensite (atomistic and microstructural reasons for elastocaloric behavior). Second, a procedure will be developed which allows to quantify the elastocaloric heat effects associated with loading and unloading. Thirdly, an effort will be made to optimize the cooling potential of NiTi based alloys through alloying and through tailoring the microstructure. And finally, functional fatigue, which may well limit the service life of a cyclically operating ferroelastic cooling device, will receive attention.