Direct Observation of Microstructure Effect on Martensitic Transformation in Multifunctional Ni–Co–Mn–Ti Alloy

All-d-metal alloys have recently emerged as potential candidates for multifunctional applications in solid-state refrigeration, magnetic heat pumps, and thermomagnetic generators. Nevertheless, the limited martensitic transformation (MT) temperature span hinders their practical applications. In this study, microstructure engineering was utilized to control the MT of all-d-metal alloys. The microstructure effect on MT of Ni–Co–Mn–Ti alloy was directly observed via in situ temperature-variable Lorentz transmission electron microscopy. Magnetic domain evolutions not only highlight new application scenarios for Ni–Co–Mn–Ti alloy in magnetic storage but also reveal the mechanism of microstructure control of MT. Grain refinement and escalated local disorder significantly inhibits martensite formation in Ni_36.5Co_13.5Mn₃₅Ti₁₅ alloy, thereby decelerating the MT kinetics and lowing MT temperature. Thus, the MT temperature span of Ni_36.5Co_13.5Mn₃₅Ti₁₅ ribbons is extended and manipulated in a 60 K (255–315 K) temperature region by adjusting microstructure. The maximum entropy changes (0–3 T) of 17 J kg⁻¹ K⁻¹ in Ni_36.5Co_13.5Mn₃₅Ti₁₅ ribbons are doubling that of the benchmark magnetic refrigerant Gd. Refrigeration capacity (0–2 T) in Ni_36.5Co_13.5Mn₃₅Ti₁₅ ribbons increased by 56%, surpassing most Ni–Mn-based alloys. These findings open up new application scenarios for metamagnetic alloys in magnetic storage and highlight the pivotal role of microstructure on MT, enormously facilitating the manipulation and application of these materials.