Optimizing Ni-Ti for ferroic cooling
Jan Frenzel, Ruhr-University Bochum, Bochum, GermanyAndré Wieczorek, Institute for Materials, Bochum, Germany Paulsen, Institute for Materials, Bochum, GermanyGunther Eggeler, Institute for Materials, Bochum, Germany
Pseudoelastic shape memory alloys (SMAs) on the basis of Ni-Ti are attractive candidate materials for ferroic cooling, where elementary solid-state processes like martensitic transformations yield the required heat effects. The present work aims for a chemical and microstructural optimization of Ni-Ti for ferroic cooling. The goal is to identify new alloy compositions which allow for a higher elastocaloric cooling performance. A large number of Ni-Ti-based SMAs was evaluated in terms of phase transformation temperatures, latent heats, mechanical hysteresis widths and functional stability. Different material states were prepared by arc melting, various heat treatments, and thermo-mechanical processing. Differential scanning calorimetry and uniaxial tensile testing in combination with thermography analysis were used to assess the cooling performance of selected material states. The results show that a fundamental dilemma exists. The key parameters latent heat and hysteresis width correlate. Therefore, it is not possible to select alloys which simultaneously combine low a hysteresis width and large latent heats, which is required for highly efficient ferroic cooling processes. Nevertheless, a good compromise was found in a Ni45Ti47.25Cu5V2.75 SMA. This material exhibits an extremely stable elastocaloric effect at room temperature and a very low hysteresis width. The ferroic cooling efficiency of this material is close to four times higher as compared to binary Ni-Ti. Furthermore, this material provides sufficient ductility such that sheets and wires can be prepared by thermos-mechanical processing.