Ultra-Low Fatigue Quaternary TiNi-Based Films for Elastocaloric Cooling

Elastocaloric applications require superelastic shape memory materials which show high fatigue resistance, adjustable transformation temperatures, short heat transfer times, and large elastocaloric effect sizes. Ti-rich TiNiCu films are known for their high functional and structural stability of several million cycles without any degradation, accompanied by a small hysteresis caused by the good crystallographic compatibility of austenite and martensite phase. Still, for the application of TiNiCu as an elastocaloric cooling agent, transformation temperature adjustment is necessary. Quaternary Co and Fe alloying is found to reduce the transformation temperature by 42 and 22 K at.%(-1), respectively, while maintaining high transformation enthalpies of 7.9 J g(-1) for Ti54.7Ni30.7Cu12.3 Co-2.3 films. Furthermore, this specific alloy shows a 25 % larger coefficient of performance compared to binary TiNi films. Combined with the high fatigue resistance, the small transformation strain of 1.6 %, and the operational temperature range of similar to 50 K, this material is a very attractive candidate for elastocaloric cooling applications.