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 quaternary systems, NiMnGaCu exhibits an interesting giant magnetocaloric effect thanks to the overlapping of the temperatures related to the magnetic transition and the thermoelastic martensitic 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 (DS) in the elastocaloric performances of polycrystalline Ni50Mn18.5Cu6.5Ga25 at% alloy samples, just above room temperature. We report an extensive calorimetric and thermomechanical characterization to explore correlations between microstructural properties induced by the selected thermal treatment and elastocaloric response, aiming at providing the basis to develop more efficient materials based on this quaternary system. Both DT and DS values obtained from mechanical curves at different temperatures and strain recovery tests under fixed load vs T were considered. Maximum values of DS = 55.9 J/KgK and DT = 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.

Realization of Large Low-Stress Elastocaloric Effect in TiZrNbAl Alloy

“…For both samples, the stress hysteresis ∆σ hys , defined by (σ cr + σ Mf − σ As − σ Af )/2, where σ cr , σ Mf , σ As and σ Af represent the onset and the end of forward and inverse martensitic transformation, respectively, is measured to be around 70 MPa. This value is lower than NiMnTi (100∼200 MPa) [22,24], NiTi-based (100∼300 MPa) [11][12][13][14]46] and comparable to typical low-stress hysteresis SMAs, such as Cu-based (∼60 MPa) [16,17], and NiFeGa-based alloys (∼50 MPa) [27,29,47]. The low ∆σ hys is beneficial for reducing energy dissipation and improving functional stability.…”

“…Compared with the 2Al and 3Al alloys, the 1Al alloy demonstrates an obvious advantage as an elastocaloric refrigerant, particularly attributed to its low σ cr (around 100 MPa). This value is lower than NiTi-based (250∼700 MPa) [14,[48][49][50] and NiM-nTi (300∼450 MPa) [22][23][24], and comparable to the low driving force alloy system like Cu-based (110∼200 MPa) [16,51], NiMnIn-based (50∼150 MPa) [25,26,52], NiFeGa-based (50∼180 MPa) [27][28][29]. On the one hand, the low σ cr supports the miniaturization of refrigeration equipment, paving the way for the application of micro-/nano-electronic device cooling [26,53].…”

“…Due to the large caloric effects and highly efficient heat transfer, shape memory alloys (SMAs) currently stand out as the most promising elastocaloric refrigerant [8][9][10], including Ni-Ti-based [11][12][13][14][15], Cu-based [16][17][18], Fe-based [19][20][21], and Heusler-type [22][23][24][25][26][27][28][29] alloys. Despite the enormous potential exhibited by these alloys, there is still considerable room for improvement.…”

“…For example, Ni-Ti [11][12][13][14] and Ni-Mn-Ti [22][23][24] display a giant adiabatic temperature change ∆T ad (∼30 K) but suffer from high critical stress σ cr (250∼700 MPa) and large stress hysteresis ∆σ hys (160∼350 MPa) across martensitic transformation. These issues can be addressed in Cu-based [16][17][18] and NiMn-based [25,28,30] and NiFeGabased Heusler-type alloys [27][28][29], but the inherent brittleness of these alloys introduces a new challenge [28]. Considering practical applications, an optimal elastocaloric refrigerant should exhibit high ∆T ad , low σ cr , and ∆σ hys , alongside excellent mechanical properties and high cyclic stability [31].…”