Twistocaloric effect versus elastocaloric effect in shape memory alloys for low-force mechanocaloric design

Abstract The mechanocaloric effect refers to the reversible thermal effect under an external mechanical field and includes the elastocaloric effect (under the uniaxial stress field) and the twistocaloric effect (under the torsional stress field). In mechanocaloric designs, the elastocaloric effect has been the mainstream of the field with an emphasis on enhancing the performance of the elastocaloric materials and implementing them into elastocaloric systems. The twistocaloric effect has been recognized in materials implementation and exhibits the potential of miniaturized design. In this report, we compare the elastocaloric effect to the twistocaloric effect in the aspects of 1) superelastic behaviors including the critical transformation stress, transformation plateau, and transformation hysteresis, 2) reversible thermal behaviors, and 3) distribution of stress and temperature. We have provided the threshold of applied force for the twistocaloric effect under a combined set of strain and strain rates. Compared to the elastocaloric effect, the twistocaloric effect requires less force for the comparable temperature change and at a similar level of applied force generates a higher temperature change. To capture the distribution of the mechanical and thermal fields, we have conducted simulation and in-situ experiments to drive insights into the low-field activated transformation process in twistocaloric design compared to elastocaloric design. These results provide mechanical and thermal information on comparing the elastocaloric and twistocaloric effects and can facilitate the advanced mechanocaloric design for solid-state cooling technologies.