D.Y. Cong

Giant magnetic refrigeration capacity near room temperature in Ni40Co10Mn40Sn10 multifunctional alloy

We report a giant effective magnetic refrigeration capacity in a Ni40Co10Mn40Sn10 multifunctional alloy. With a large magnetization difference between austenite and martensite, this alloy shows a strong magnetic field dependence of transformation temperatures. Complete magnetic-field-induced structural transformation and a considerable magnetic entropy change are observed in a broad operating temperature window of 33 K near room temperature. Consequently, an effective magnetic refrigeration capacity of 251 J/kg for 5 T is achieved, which is the largest value for Ni-Mn-based Heusler alloys and comparable to that of the high-performance Gd-Si-Ge and La-Fe-Si magnetocaloric materials. Incorporating the advantages of low cost and non-toxicity, this alloy shows very promising prospects for room-temperature magnetic refrigeration.

Direct evidence for stress-induced transformation between coexisting multiple martensites in a Ni–Mn–Ga multifunctional alloy

The structural response of coexisting multiple martensites to stress field in a Ni-Mn-Ga multifunctional alloy was investigated by the in situ high-energy x-ray diffraction technique. Stress-induced transformation between coexisting multiple martensites was observed at 110 K, at which five-layered modulated (5M), seven-layered modulated (7M) and non-modulated (NM) martensites coexist. We found that a tiny stress of as low as 0.5 MPa could trigger the transformation from 5M and 7M martensites to NM martensite and this transformation is partly reversible. Besides the transformation between coexisting multiple martensites, rearrangement of martensite variants also occurs during loading, at least at high stress levels. The present study is instructive for designing advanced multifunctional alloys with easy actuation.

Large and reversible inverse magnetocaloric effect in Ni48.1Co2.9Mn35.0In14.0 metamagnetic shape memory microwire

High-performance magnetocaloric materials should have a large reversible magnetocaloric effect and good heat exchange capability. Here, we developed a Ni48.1Co2.9Mn35.0In14.0 metamagnetic shape memory microwire with a large and reversible inverse magnetocaloric effect. As compared to the bulk counterpart, the microwire shows a better combination of magnetostructural transformation parameters (magnetization difference across transformation ΔM, transformation entropy change ΔStr, thermal hysteresis ΔThys, and transformation interval ΔTint) and thus greatly reduced critical field required for complete and reversible magnetic-field-induced transformation. A strong and reversible metamagnetic transition occurred in the microwire, which facilitates the achievement of large reversible magnetoresponsive effects. Consequently, a large and reversible magnetic-field-induced entropy change ΔSm of 12.8 J kg−1 K−1 under 5 T was achieved in the microwire, which is the highest value reported heretofore in Ni-Mn-based mag...

Large internal stress-assisted twin-boundary motion in Ni2MnGa ferromagnetic shape memory alloy

The twin boundary motion driven by thermo-magnetic coupling was in-situ studied in a NiMnGa single crystal using high-energy x-ray diffraction technique. An unstable martensite with an internal stress of ∼8 MPa was obtained through a thermo-magnetic training. The triple martensite variants assisted by internal stress are distinct from the self-accommodated martensite twin variants with a stress-free state, and a single martensite-variant can be actuated only by a magnetic field of ∼0.34 T, equivalent to an actuator stress of about 1.3 MPa. The generation of so large internal stress among variants is attributed to the altered martensite nucleation sites triggered by external fields during thermo-magnetic training.

Giant tensile superelasticity originating from two-step phase transformation in a Ni-Mn-Sn-Fe magnetic microwire

Large recoverable strain of more than several percent arising from superelasticity in shape memory alloys is important for actuators, sensors, and solid-state refrigeration. Here, we report a Ni50.0Mn31.4Sn9.6Fe9.0 magnetic microwire showing a giant tensile recoverable strain of about 20.0% along the ⟨001⟩ direction of austenite at 263 K. The recoverable strain represents the largest value reported heretofore in Ni-Mn-based shape memory alloys and is also larger than that of the Ni-Ti wire available for practical applications at present. This giant tensile superelasticity is associated with the stress-induced two-step transformation, and the transformation sequence could be L21 (austenite) → 6M (six-layered modulated martensite) → NM (non-modulated martensite), as suggested by the temperature-dependent in-situ synchrotron high-energy X-ray diffraction experiments and the transformation strain calculation based on the crystallographic theory of martensitic transformation. In addition, this Ni50.0Mn31.4Sn9.6Fe9.0 microwire shows a transformation entropy change ΔStr of 22.9 J kg−1 K−1 and has the advantages of easy fabrication and low cost, promising for miniature sensor, actuator, and solid-state refrigeration applications.Large recoverable strain of more than several percent arising from superelasticity in shape memory alloys is important for actuators, sensors, and solid-state refrigeration. Here, we report a Ni50.0Mn31.4Sn9.6Fe9.0 magnetic microwire showing a giant tensile recoverable strain of about 20.0% along the ⟨001⟩ direction of austenite at 263 K. The recoverable strain represents the largest value reported heretofore in Ni-Mn-based shape memory alloys and is also larger than that of the Ni-Ti wire available for practical applications at present. This giant tensile superelasticity is associated with the stress-induced two-step transformation, and the transformation sequence could be L21 (austenite) → 6M (six-layered modulated martensite) → NM (non-modulated martensite), as suggested by the temperature-dependent in-situ synchrotron high-energy X-ray diffraction experiments and the transformation strain calculation based on the crystallographic theory of martensitic transformation. In addition, this Ni50.0Mn31.4Sn9....

Giant negative thermal expansion in Fe-Mn-Ga magnetic shape memory alloys

Fe-Mn-Ga magnetic shape memory alloys can undergo martensitic transformation (MT) from a paramagnetic cubic phase to a ferromagnetic tetragonal phase. The MT is accompanied by a large volume change; yet, these alloys have never been explored for technological applications as negative thermal expansion (NTE) materials. Here, by careful chemical modification, tunable NTE characteristics including wide operating temperature windows (ΔT) and large negative linear coefficients of thermal expansion (αl) have been achieved in Fe44−xMn28Ga28+x (x = 1, 2, and 2.5) alloys. Typically, a giant NTE ΔT of 81 K and αl = −50.2 × 10−6 K−1 were realized in the Fe43Mn28Ga29 alloy upon cooling from 290 K. The relationships between the NTE features, the MT, and the substitution of Ga for Fe were discussed. Furthermore, the Fe-Mn-Ga alloys possess excellent mechanical properties, high electrical conductivity and high thermal conductivity. With these advantages, the Fe-Mn-Ga magnetic shape memory alloys show promising prospects for use as advanced NTE materials.Fe-Mn-Ga magnetic shape memory alloys can undergo martensitic transformation (MT) from a paramagnetic cubic phase to a ferromagnetic tetragonal phase. The MT is accompanied by a large volume change; yet, these alloys have never been explored for technological applications as negative thermal expansion (NTE) materials. Here, by careful chemical modification, tunable NTE characteristics including wide operating temperature windows (ΔT) and large negative linear coefficients of thermal expansion (αl) have been achieved in Fe44−xMn28Ga28+x (x = 1, 2, and 2.5) alloys. Typically, a giant NTE ΔT of 81 K and αl = −50.2 × 10−6 K−1 were realized in the Fe43Mn28Ga29 alloy upon cooling from 290 K. The relationships between the NTE features, the MT, and the substitution of Ga for Fe were discussed. Furthermore, the Fe-Mn-Ga alloys possess excellent mechanical properties, high electrical conductivity and high thermal conductivity. With these advantages, the Fe-Mn-Ga magnetic shape memory alloys show promising prospects...