Elastocaloric Properties of Polycrystalline Samples of NiMnGaCu Ferromagnetic Shape Memory Alloy under Compression: Effect of Improvement of Thermoelastic Martensitic Transformation

Abstract: Shape memory alloys (SMAs) and ferromagnetic shape memory alloys (FeSMAs) have recently attracted interest for solid state refrigeration applications. Among NiMnGa-based qua- ternary systems, NiMnGaCu exhibits an interesting giant magnetocaloric effect thanks to the overlapping of the temperatures related to the magnetic transition and the thermoelastic marten- sitic transformation (TMT); in particular, for compositions with Cu content of approximately 6 at%. In the present work, we investigated the improvement effect of TMT on the total entropy change ( ?S) in the elastocaloric performances of polycrystalline Ni50Mn18.5Cu6.5Ga25 at% alloy samples, just above room temperature. We report an extensive calorimetric and thermomechanical characteri- zation to explore correlations between microstructural properties induced by the selected thermal treatment and elastocaloric response, aiming at providing the basis to develop more efficient ma- terials based on this quaternary system. Both ?T ?and??S values obtained from mechanical curves at different temperatures and strain recovery tests under fixed load vs T were considered. Maxi- mum values of ?S = 55.9 J/KgK and ?T = 4.5 K were attained with, respectively, a stress of 65 MPa and strain of 4%. The evaluation of the coefficient of performance (COP) was carried out from a cyclic test.