Characterization of Thin NiTi-Based Shape Memory Alloy Sheets with Different Surface Properties for Utilization in Elastocaloric Regenerators

Abstract Elastocaloric cooling (eCC) is the most promising alternative to conventional refrigeration, leveraging the reversible martensitic transformations in nickel titanium (NiTi) shape memory alloys (SMAs) to achieve substantial adiabatic temperature changes. This study investigates the feasibility of commercially available NiTi sheets for such applications. Samples processed via electrical discharge machining (EDM), laser machining, and diamond smoothing were characterized using scanning electron microscopy (SEM), optical microscope (OM), differential scanning calorimetry (DSC), IR thermography (IR-TG), digital image correlation (DIC), and acoustic emission (AE). The sheets exhibited superelastic behavior with a critical transformation stress of ~ 400 MPa and an adiabatic temperature change of ± 20 K. However, rapid functional degradation occurred in the initial loading cycles, marked by a 50 MPa drop in transformation stress, 0.5% irreversible strain, and a temperature change reduction to ± 2 K after the first five cycles. SEM analysis after 150 cycles confirmed martensitic microstructure formation, correlating with AE signals indicative of dislocation activity and an increasing amount of retained martensite. These results highlight the microstructural challenges and issues when envisioning the use of NiTi sheets under tensile loading for elastocaloric (eC) applications. They motivate a discussion of how future studies should explore compressive loading configurations, addressing challenges such as buckling and localized martensitic phase transformation, to better exploit the potential of NiTi sheets in eC regenerators.