The martensitic transformation behaviors, magnetic properties, crystal structures, and entropy change (ΔS) of Ni50−xCoxMn50−ySny alloys were systematically investigated. Samples were prepared by induction melting, followed by solution heat treatment at 1273 K for 1 day and quenching. A magnetic phase diagram with a broad range of compositions was established, from which a series of alloys with varying Co concentrations was selected, such that their martensitic transformation starting temperatures (TMss) were near room temperature. The thermal and magnetic-field hystereses of these samples were systematically investigated, and a clear composition dependence was observed. The dissipation energy (Edis), which governs the transformation hysteresis, was found to be smaller for alloys with x = 5–8 than for those with x = 0–3. This behavior was found to originate from the difference in crystal structures of the martensite phases, i.e., the crystal structures of the martensite phases are 4O and 6M for x = 0–3 and x = 5–8, respectively. Moreover, the geometric compatibility between the parent and martensite phases was found to be the direct cause of the difference in dissipation energy in Ni–Co–Mn–Sn alloys.
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