Multifunctional properties in both three and one-dimensional polycrystalline Cu-doped Co–Ni-Ga shape memory alloys

The effect of Cu doping on the microstructure and functional properties of polycrystalline Co–Ni-Ga-Cu shape memory alloy is investigated with the Cu composition ranging from 0.3 to 4.5 at.%. For the miniaturization applications in actuators and sensors, the alloy with 3 at.% Cu is selected to fabricate one-dimensional microwires. The results reveal that Cu promotes the growth of the γ phase precipitates, accompanied by the decrease of the martensitic transformation temperature in the bulk Co–Ni-Ga-Cu samples. At low Cu doping, a reversible strain of 4% is obtained. By increasing Cu to 4.5%, a compressive reversible strain can reach at least 2.5% at room temperature. During the compression process, the adiabatic elastocaloric effect of a 3.3 K temperature change is also achieved. Furthermore, the polycrystalline microwire displays a great tensile superelasticity with a larger reversible strain of 5% in a temperature range varying from 223 to 373 K, which is the best among Co–Ni-Ga polycrystalline alloys. The shape memory effect of the microwire with a 3% strain output ranging from 123 K to 353 K is achieved as well. The good multifunctional properties in the Co–Ni-Ga-Cu shape memory alloy bulks and microwires lay a foundation for their applications in sensing, actuating and solid-state refrigeration fields.