Abstract
Two Ni–Mn–Sn alloys substituted with 0.5 and 1 at.% Cr have been studied. The first alloy shows an average composition of Ni49.6Mn37.3Cr0.7Sn12.4 (e/a = 8.107), whereas the second has a multiphase microstructure with the matrix phase of an average Ni52.4Mn32.7Cr1Sn14 composition (e/a = 8.146). Both alloys undergo a reversible martensitic phase transformation. The Ni49.6Mn37.3Cr0.7Sn12.4 alloy transforms to the martensite phase at 239 K and, under the magnetic field change of μ0·ΔH = 1.5 T, gives the magnetic entropy change equal to 7.6 J/kg·K. This amounts to a refrigerant capacity in the order of 48.6 J/kg, reducible by 29.8% due to hysteresis loss. On the other hand, the alloy with a multiphase microstructure undergoes the martensitic phase transformation at 223 K with the magnetic entropy change of 1.7 J/kg·K (1 T). Although the latter spreads over a broader temperature window in the multiphase alloy, it gives much smaller refrigerant capacity of 16.2 J/kg when compared to Ni49.6Mn37.3Cr0.7Sn12.4. The average hysteresis loss for a field change of 1.5 T in the multiphase alloy is 2.7 J/kg, reducing the effective refrigerant capacity by 16.7%. These results illustrate that the key to gaining a large effective refrigerant capacity is the synergy between the magnitude of the magnetic entropy change and its broad temperature dependence.
Highlights
Inverse magnetocaloric effect (MCE) inherent to Ni–Mn–(In, Sn, Sb) metamagnetic shape memory alloys (MMSMAs) relates to a peculiar magnetic entropy change (∆SM > 0) in the proximity of the first order thermoelastic martensitic phase transformation (MPT) [1]
Seemingly detrimental for MCE [19], phase inhomogeneity is not always undesirable, e.g., in the sense of ductility enhancement in Co–Ni–(Ga, Al) due to the small volume of a precipitated γ phase [20], while more recently inhomogeneity has been reported to add to the plethora of existing functionalities in Ni–Mn–(Sn, In) MMSMA in terms of core–shell ferromagnetism [21]
Room temperature (RT) microstructure of 0.5 Cr and 1 Cr alloys was examined by scanning electron microscope (SEM)
Summary
Inverse magnetocaloric effect (MCE) inherent to Ni–Mn–(In, Sn, Sb) metamagnetic shape memory alloys (MMSMAs) relates to a peculiar magnetic entropy change (∆SM > 0) in the proximity of the first order thermoelastic martensitic phase transformation (MPT) [1]. In order to maximise MCE and bring the working temperature (MPT) close to room temperature, the composition of MMSMAs is frequently modified. This approach relies on the sensitivity of the MPT temperature (TM) and, to a lesser extent, the Curie temperature of austenite (TCA) to composition change in Ni–Mn based alloys; this sensitivity is often expressed in terms of the valence electron concentration (e/a) [9,10]. The ∆SM and RC are computed and discussed in the context of a multi-phase microstructure and mixed magnetic correlations
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