Grain-size effects on the temperature-dependent elastocaloric cooling performance of polycrystalline NiTi alloy

Grain-size (GS) effects on the temperature-dependent elastocaloric cooling performance of NiTi with the average GS of 11, 22, 30, 45 and 70 nm are investigated over a temperature range from-50 degrees C to 80 degrees C. It is found that GS refinement is conducive to improving the thermal stability of superelasticity and the asso-ciated elastocaloric effect, while the trade-off between cooling temperature drop AT and effective working temperature span Tspan, more or less, is inevitable regardless of GS. The large AT of the 70 nm-GS specimen, which is characterized by sharp first-order martensitic transformation and strong temperature dependence of the transformation stress d sigma tr/dT (= 5.7 MPa/degrees C), is restricted to a narrow Tspan (= 19 degrees C). Tspan can be widened by five times via reducing GS to 11 nm but at the expense of a significant sacrifice in AT, as the combined result of high strength and small d sigma tr/dT (= 0.9 MPa/degrees C). Among the five microstructures, the 30 nm-GS one achieves a favorable compromise between AT and Tspan owing to the mild transformation nature together with robust mechanical properties. Consequently, its AT can reach 50% & ndash; 440% of that of the 70 nm-GS counterpart and the resultant cooling efficiency can be enhanced by a factor of half to six. The work demonstrates that GS engineering is a feasible approach for reconciling various elastocaloric cooling metrics of the NiTi refrigerant. (c) 2022 Elsevier B.V. All rights reserved. <comment>Superscript/Subscript Available</comment