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 ℃ to 80 ℃. It is found that GS refinement is conducive to improving the thermal stability of superelasticity and the associated elastocaloric effect, while the trade-off between cooling temperature drop ΔT and effective working temperature span Tspan, more or less, is inevitable regardless of GS. The large ΔT of the 70 nm-GS specimen, which is characterized by sharp first-order martensitic transformation and strong temperature dependence of the transformation stress dσtr/dT (= 5.7 MPa/℃), is restricted to a narrow Tspan (= 19 ℃). Tspan can be widened by five times via reducing GS to 11 nm but at the expense of a significant sacrifice in ΔT, as the combined result of high strength and small dσtr/dT (= 0.9 MPa/℃). Among the five microstructures, the 30 nm-GS one achieves a favorable compromise between ΔT and Tspan owing to the mild transformation nature together with robust mechanical properties. Consequently, its ΔT can reach 50% – 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.