Effects of alloy compositions and microstructures of Ni-Ti-based shape memory alloys on ferroic cooling performance

Pseudoelastic shape memory alloys (SMAs) are attractive candidate materials for ferroic cooling, where elementary solid-state processes like martensitic transformations yield the required heat effects. In the present work, we provide a detailed overview how specific key properties of SMAs, which are important for ferroic cooling applications, depend on alloy compositions and microstructures. We also report results from a recent study which aims for the identification of new alloy compositions with superior 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. The results show that a fundamental dilemma exists. The key parameters latent heat and hysteresis width correlate. Therefore, it is difficult to create or select alloys which simultaneously combine low hysteresis widths and large latent heats, which is required for highly efficient ferroic cooling processes. However, several compositions width well-balanced properties could be identified. The ferroic cooling efficiency of one candidate alloy, a Ni-Ti-Cu-V SMAs, is close to four times higher as compared to binary Ni-Ti.