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Solid-state refrigeration technology is expected to replace conventional gas compression refrigeration technology because it is environmentally friendly and highly efficient. Among various solid-state magnetocaloric materials, Ni-Mn-based ferromagnetic shape memory alloys (SMAs) have attracted widespread attention due to their multifunctional properties, such as their magnetocaloric effect, elastocaloric effect, barocaloric effect, magnetoresistance, magnetic field-induced strain, etc. Recently, a series of in-depth studies on the thermal effects of Ni-Mn-based magnetic SMAs have been carried out, and numerous research results have been obtained. It has been found that poor toughness and cyclic stability greatly limit the practical application of magnetic SMAs in solid-state refrigeration. In this review, the influences of element doping, microstructure design, and the size effect on the strength and toughness of Ni-Mn-based ferromagnetic SMAs and their underlying mechanisms are systematically summarized. The pros and cons of different methods in enhancing the toughness of Ni-Mn-based SMAs are compared, and the unresolved issues are analyzed. The main research directions of Ni-Mn-based ferromagnetic SMAs are proposed and discussed, which are of scientific and technological significance and could promote the application of Ni-Mn-based ferromagnetic SMAs in various fields. [ABSTRACT FROM AUTHOR]

Recently, additive manufacturing (AM) of intrinsically brittle Ni–Mn–X alloys, in which obtaining high-quality powders is imperative, has received widespread attention. Here, a batch weight of about 34 kg Ni–Mn–Sn–Co powder was prepared using industrial gas atomization equipment. To explore the sinterability of the gas-atomized powder and provide a direct comparison in caloric effects for binder-based AM techniques, we sintered Ni–Mn–Sn–Co alloy powder samples at 1173–1273 K for 2–24 h by using a powder metallurgy process with air cooling. The sintered powder samples had a relative density of about 60.3–80.1 %, and 5 M martensite and L 2 1 –ordered austenite were found to coexist at room temperature. For all sintered samples, martensitic transformation temperature width of about 11–17 K and hysteresis of about 21–23 K were obtained, and strong magneto-structural coupling was observed. The magnetocaloric and elastocaloric effects of the sample sintered at 1273 K for 2 h were examined. A magnetic entropy change of about 23.0 J kg−1·K−1 and an adiabatic temperature change (Δ T ad) of −0.97 K were achieved for magnetic fields of 5.0 and 1.25 T, respectively. Furthermore, a Δ T ad of −4.71 K was achieved upon unloading a compressive stress of 350 MPa. These competitive caloric effects in the gas-atomized powder lay the foundation for the AM of Ni–Mn–Sn–Co alloys with complex structures. This study provides a reference for the reliable manufacturing of composition-sensitive Ni–Mn–X Heusler alloys. [ABSTRACT FROM AUTHOR]

Ni45.6Fe3.6Mn38.4Sn12.4 microwires, with nanoscale γ-phase precipitates that enhance the magnetocaloric effects (MCEs) and mechanical properties, were prepared by a melt-extraction technique and subsequent high temperature annealing. The atomic ordering degree significantly increased after annealing, leading to a considerable increment in the structural entropy change (ΔStr) from 4.5 J/kg·K in the as-extracted microwire to 26.6 J/kg·K in the annealed one, and the magnetization difference (ΔM) from 35 A·m2/kg to 51 A·m2/kg under a magnetic field of 5.0 T. Consequently, a positive magnetic entropy change (ΔSM) peak of 15.2 J/kg·K with working temperature span (ΔTFWHM) of 12 K for the first-order martensite transformation followed by a negative ΔSM peak of −4.3 J/kg·K with ΔTFWHM = 50 K for the second-order magnetic transition under μ0ΔH = 5.0 T was achieved. By employing both magnetizing and demagnetizing processes for magnetic cooling, the two successive inverse and conventional MCEs in Ni–Fe–Mn–Sn microwires may show potential applications for micro-devices and systems. [ABSTRACT FROM AUTHOR]

High magnetic entropy change and wide working temperature interval are both of great importance for the application of magnetic refrigerant working materials. Here Ni–Mn–In–Co alloys with oligocrystalline structure were produced by annealing heat treatment and the giant magnetocaloric effect and large working temperature interval were confirmed. The annealing increases the magnetization difference ΔM between martensite and austenite. The annealed alloy subjects to a reverse martensite to austenite and an intermediate phase transformation upon heating. The two-stage field-induced reverse transformation, contribute to a maximum magnetic entropy change of 37.1 J/kgK at 309 K under a magnetic field of 50000 Oe, were observed for the first time. In addition, a broadened working temperature interval of ∼22 K was obtained owing to the intermediate phase transformation. The giant room-temperature MCE in quaternary Ni–Mn–In–Co alloys is promising for magnetic refrigeration. [ABSTRACT FROM AUTHOR]

The room-temperature magnetocaloric effects (MCE) in polycrystalline Ni 47.74 Mn 37.06 In 15.20 alloy were investigated. The magnetic-field-induced phase transformation from martensite to austenite phase was confirmed. Both inverse and conventional magnetocaloric effects were attained at a magnetic field up to 50,000 Oe. The inverse magnetic entropy change (Δ S m ) reaches 30.7 J/kg K around 282 K at 50,000 Oe due to the martensite to austenite first order transformation. Furthermore, the conventional Δ S m reaches −6.45 J/kg K around 294 K at 50,000 Oe related to the austenitic ferromagnetic to paramagnetic second order magnetic transition. The giant inverse and conventional magnetocaloric effects co-occur in a wide temperature range 267.5–297.5 K. This makes ternary Ni 47.74 Mn 37.06 In 15.20 alloy a promising room-temperature magnetic refrigerant. [ABSTRACT FROM AUTHOR]

Very recently, we reported that Fe-doped Ni 50-x Fe x Mn 38.0 Sn 12.0 alloys exhibited enhanced magnetization difference Δ M between the martensite and austenite phases. Of note is that the alloy with x = 4.2 exhibits a strong metamagnetic transition behavior, i.e. magnetic-field-induced reverse martensite transformation, which is favorable for the magnetocaloric effects. Here, the enhanced magnetocaloric effect related to metamagnetic transition in the selected Ni 35.8 Fe 4.2 Mn 38.0 Sn 12.0 alloy was reported. This alloy exhibits a high magnetization difference Δ M of ∼58 A·m 2 /kg between martensite and austenite phases owing to the magnetic and structural coupling. The doping of Fe leads to a strong metamagnetic behavior, with austenite peak temperature shift Δ A p / Δ H reaching −2.4 K/T. As a result, the magnetic entropy change Δ S m increases rapidly from a starting critical magnetic field ∼0.2 T and then saturates at various finishing critical magnetic fields, depending on the working temperature. A sizable peak Δ S m of 33.8 J/kg·K with a working temperature interval of 7 K is obtained under a magnetic field of 5 T, which is responsible for a high relative cooling power RCP of 237 J/kg. These results suggest that Ni 45.8 Fe 4.2 Mn 38.0 Sn 12.0 alloy may act as a potential solid-state magnetic refrigerant. [ABSTRACT FROM AUTHOR]

In this study, the microstructure, phase composition, magnetic transition, and magnetocaloric effect of the LaFe 10.6 Co 0.9 Si 1.5 alloy hot compressed at high temperatures over 1173 K with a strain rate 0.005 s−1 and reduction ratio 50%–70 % are studied. No La(Fe, Si, Co) 13 phase is formed until the hot-compression temperature is elevated to 1273 K and the reduction ratio is increased to 70 %. As a result, 86.8 ± 2 wt% 1:13 phases are rapidly obtained in the LaFe 10.6 Co 0.9 Si 1.5 ingot. The whole process can be completed within only 10 min and no post-annealing process is needed. This fast formation of the La(Fe, Si, Co) 13 phases is possibly induced by the directional atomic diffusion, accelerated atomic diffusion, and the stability of the La(Fe, Si, Co) 13 phase and structural relaxation caused by the sufficiently restored elastic energy during the hot compression. The LaFe 10.6 Co 0.9 Si 1.5 ingot hot compressed at 1273 K with a reduction ratio 70 % at a strain rate 0.005/s exhibits a second-order phase transition and negligible magnetic hysteresis. A large magnetic entropy change of 4.4 J/kg.K and a wide working temperature range of 57 K under 2 T are acquired near room temperature. This study proposes a new efficient hot compression route for processing the La–Fe–Co–Si alloy and enhancing the magnetocaloric effect. • 86.8 wt% 1:13 phases are formed within 10min in LaFe 10.6 Co 0.9 Si 1.5 ingot during hot compression. • The | Δ S M | max is rapidly enhanced to 4.4 J/kg.K at 2 T during the hot compression process. • No 1:13 phase is formed when the compression temperature <1273 K or reduction ratio <70 %. • Dynamic recrystallization occurs in α-Fe phase when the compression temperature reaches 1373 K. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ni 41.3 Co 6.3 Mn 42.4 Sn 10.0 polycrystalline microwires with diameter 40–80 μm and length of 3–8 cm were fabricated on a large scale. • The martensite transformation temperatures and T c A shifted to higher temperatures after annealing at temperatures between723K and 1173 K. • A weak first-order phase transformation occurred prior to the strong metamagnetic transition in the wire annealed at 1073 K, while partial transition arrest appeared at 1173 K, leading to the reduction of ΔM. • The microwires annealed at 1023 K produce ΔS M 20.1 J/kg·K, ΔT FWHM 20 K and RC eff 197 J/kg due to strong metamagnetic transition under 5.0 T. Effect of annealing on martensite transformation (MT) and magnetocaloric effect (MCE) in a Ni 41.3 Co 6.3 Mn 42.4 Sn 10.0 microwire was investigated. After annealing at temperatures 723–1173 K, the MT temperatures and Curie points shifted to higher range; meanwhile, the thermal hysteresis (ΔT hys) decreased. However, partial transition arrest appeared when annealed at temperatures > 1073 K, leading to decreased magnetization difference (ΔM) between martensite and austenite phases. The maxima ΔM and minima ΔT hys were produced at an annealing temperature 1023 K. Consequently, a large magnetic entropy change (Δ S M max) 20.1 J/kg·K, wide working temperature span (ΔT FWHM) 20 K and thus high effective refrigeration capacity (RC eff) 197 J/kg were obtained under a magnetic field 5.0 T after annealing at 1023 K for 1 h. These facts make Ni-Co-Mn-Sn microwires promising for miniature refrigeration devices due to their high specific surface area, low thermal hysteresis and markedly reduced annealing temperature and time. [ABSTRACT FROM AUTHOR]

• Ni 40.7 Mn 42.9 Sn 10.4 Co 6.0 foam with porosity 56% was created by replication casting and annealing. • The foam had reduced thermal hysteresis (9.0 K). • The foam showed entropy change 4.7 J/kg·K with working temperature span 40 K under 5.0 T. • The foam exhibited a small hysteresis loss (AHL) 17.4 J/kg under 5.0 T. • The foam produced refrigeration capacity 155 J/kg and relative cooling power 188 J/kg. Enhanced magnetic refrigeration performance via introducing pores in a ferromagnetic shape memory Ni-Mn-Sn-Co alloy was demonstrated. The Ni 40.7 Mn 42.9 Sn 10.4 Co 6.0 foam, with a porosity of 56% created by replication casting technique, exhibited reduced thermal hysteresis because of the high specific surface area. A maximum magnetic entropy change (Δ S M) 4.7 J/kg·K with a very wide working temperature span (Δ T FWHM) 40 K under 5.0 T were found in the annealed foam. Furthermore, the foam showed a small hysteresis loss (AHL) 17.4 J/kg under 5.0 T. As a result, refrigeration capacity (RC) 155 J/kg and relative cooling power (RCP) 188 J/kg were produced. [ABSTRACT FROM AUTHOR]

Magnetocaloric and magnetoresistance properties originated from a metamagnetic structural transition were investigated in a Fe-doped Ni 46.8 Mn 38.1 Sn 11.6 Fe 3.5 alloy. After annealing, the alloy exhibited the intermartensite phase that contributed to the multiple abrupt magnetization and resistivity changes during heating process of martensite transformation. As a result, three successive magnetic entropy change ΔS M peaks of 10.7, 14.5 and 9.7 J/kg·K at temperatures 275, 279 and 285 K respectively were observed, which was responsible for a wide working temperature span ΔT FWHM of 273–287 K and thus a large effective refrigeration capacity RC eff of 98.1 J/kg under 5.0 T. At the same time, a large negative magnetoresistance MR (e.g. over 26.7% at 275 K) was also revealed during heating process under 5.0 T. Such multi-functional magnetocaloric and magnetoresistance effects render Ni–Mn–Sn–Fe alloys promising working materials for magnetic refrigerants, information storage and thermal magnetoelectricity conversions. [Display omitted] • The intermartensite phase divided the martensite transformation into 3 parts. • Three successive magnetic entropy change ΔS M were obtained, which were responsible for a wide working temperature span. • A large negative MR value was reached with several small negative MR peaks during heating. [ABSTRACT FROM AUTHOR]