Large tunable elastocaloric effect in additively manufactured Ni-Ti shape memory alloys

Abstract High-efficiency elastocaloric refrigeration requires high-performance elastocaloric materials with both large surface areas to promote heat exchange rate and large elastocaloric effects to increase the amount of heat transfer. Ni-Ti shape memory alloys (SMAs) are the most promising elastocaloric materials but they are difficult to process by conventional methods due to their poor manufacturability. Here, we successfully developed Ni-Ti SMAs with large elastocaloric effects by additive manufacturing which has the capability to fabricate complex geometries with large surface areas. The phase transformation temperatures of these additively manufactured Ni-Ti SMAs, fabricated by selective laser melting (SLM), can be tuned by varying the SLM processing parameters and/or post heat treatments and thus tunable large elastocaloric effects were achieved at different temperatures, which can be used for different applications. Owing to its large transformation entropy change and high yield strength as a result of precipitation hardening, the aged SLM fabricated alloy exhibits a remarkably large elastocaloric effect with an adiabatic temperature change as high as 23.2 K, which is among the highest values reported for all Ni-Ti SMAs fabricated by both conventional methods and additive manufacturing. Furthermore, by virtue of the high yield strength and low stress hysteresis of the aged alloy, this large elastocaloric effect shows good stability during cycling. The achievement of such large elastocaloric effects in alloys fabricated by near-net-shape additive manufacturing may accelerate the implementation of high-efficiency elastocaloric refrigeration. This study is instructive for the development of advanced high-performance solid-state refrigeration materials by additive manufacturing.