Low-force twistocaloric cooling in Ni 50 Ti 50 , Ni 43 Ti 50 Cu 7 and Ni 47 Ti 44 Nb 9 alloys assisted by axial loading

Abstract Mechanocaloric solid-state cooling based on shape memory alloys necessitates large and reversible temperature changes under actuation forces that are compatible with compact device configurations. Twistocaloric loading can reduce the force demand compared with uniaxial elastocaloric operation; however, the manner in which multiaxial stress governs martensitic transformation and the adiabatic temperature change (△ T ad ) remains unclear. Here, we have investigated Ni 50 Ti 50 , Ni 47 Ti 44 Nb 9 and Ni 43 Ti 50 Cu 7 alloys under combined torsion and constant axial tensile loading. Quasi-isothermal torsional tests quantify the torsional driving stress, and adiabatic torsional tests measure the temperature changes induced upon loading and unloading. A moderate axial tensile stress lowers the shear stress required to achieve 6% torsional strain in all three alloys. Increasing the axial tensile stress to 400 MPa decreases the required shear stress from 680 to 444 MPa for Ni 50 Ti 50 , and from 583 to 348 MPa for Ni 47 Ti 44 Nb 9 . The △ T ad is retained up to an axial tensile stress of about 200 MPa, whereas higher tensile stresses promote pre-transformation, accumulation of residual strain and reduced reversibility, leading to degradation of △ T ad . Temperature-dependent measurements on Ni 43 Ti 50 Cu 7 exhibit maximum △ T ad values of about 8.4 ℃ during loading and 7.3 ℃ during unloading. These results suggest a stress-temperature operating window for low-force twistocaloric actuation in tubular or rotary NiTi-based mechanocaloric regenerators.