Impact of secondary heat treatments and mechanical cycling on elastocaloric properties of Nitinol

Elastocaloric cooling is gaining attention because of its potential for higher reliability and efficiency, making it a candidate to replace vapor-compression systems in both residential and industrial applications. Nitinol, a well-known superelastic material, has a wide range of uses in the medical field and is also regarded as a benchmark elastocaloric refrigerant. However, research on optimizing this alloy specifically for cooling applications remains limited.

In this study, we investigate the impact of heat treatments and mechanical cycling on Nitinol properties essential for cooling applications. We performed 2-minute heat treatments near 500 °C on commercial material to shift its austenite finish (A_f) temperature by +/- 10 °C. These heat treatments, along with subsequent mechanical cycling, led to reductions in plateau stresses and hysteresis while increasing the elastocaloric response by up to 30%. We also observed significant variability in the Nitinol response within a narrow temperature range when testing below its A_f. Both superelasticity and elastocaloric temperature changes were notably affected. After all mechanical tests, we reevaluated the materials’ A_f and noted substantial increases for all the samples. Our results demonstrate that short heat treatments can effectively tune and enhance the elastocaloric properties of Nitinol, thereby accelerating the introduction of efficient and environmentally benign caloric refrigeration technology to the market.

This work was supported by the Department of Defense (MIPRHQ0642384973) through the U.S. Department of Energy under Contract No. DE-AC02-07CH11358.