In the field of solid-state refrigeration, the elastic thermal refrigeration technology has comprehensive advantages such as diversified driving methods, large adiabatic temperature changes, and easy expansion of the operating temperature range. The variable magnetic NiMn-based Heusler shape memory alloy has the properties of low critical stress and low hysteresis, and includes the characteristics of magnetoelastic coupling. The elastic thermal performance can be enhanced by applying stress and magnetic field simultaneously or stepwise in an appropriate order. But the intrinsic brittleness of such intermetallic compounds is the main bottleneck restricting the development of solid-state refrigeration devices.
In previous studies, elemental doping, microstructure optimization and texture control were generally used to solve the brittleness problem of alloys. In recent years, researchers in China have discovered a new type of fully transitional Heusler magnetic alloy NiMnTi. In addition to the advantages of the traditional Heusler shape memory alloy, it also has good mechanical properties. Its toughness and compressive strength are traditional Ni-Mn. Several times the base alloy. In the early research, the Key Laboratory of Magnetic Materials and Devices of the Chinese Academy of Sciences reported that Ni (Co) MnTi alloy has a large elastic thermal reversible adiabatic temperature change (Applied Physics Letters 2019, 114, 101903), but the critical stress and hysteresis are large The problem remains to be solved.
Recently, the laboratory Wei Zhiyang, Shen Yi and others prepared the Ni35.5Co14.5Mn35Ti15 full transition group Heusler alloy with <001> preferred orientation by directional solidification, and studied the mechanism of solidification texture and elastic-thermal properties. Through electron backscatter diffraction, it is found that the alloy has a crystallographic genetic relationship from austenite <001> orientation to 5-layer modulation martensite <105> orientation. Multiple modulation of martensite was observed using transmission electron microscopy, which originated from the weak d-d covalent hybridization of the alloy. In addition, in-situ high-resolution digital image correlation technology (DIC) was also introduced to monitor the two-dimensional strain field during the whole process of stress-induced martensitic transformation. Good consistency and coordination. This explains the source of low critical stress and small stress hysteresis of the alloy from the crystal structure and microstructure. The combination of infrared thermal imaging technology and DIC technology found that the local strain inhomogeneity caused by the unsynchronized internal phase change of the sample during the adiabatic compression cycle will result in a steeper stress-strain curve, temporary residual strain and increased hysteresis.
The alloy achieves an adiabatic temperature change of 11.5K at a low critical stress of 38MPa and a small hysteresis of 54MPa. Its important elastothermal effect measurement index-the adiabatic temperature change value under unit critical stress reached 0.31K / MPa, which surpassed the typical first-order phase change elastothermal material reported previously. This work provides important guiding significance for in-depth understanding of the martensitic transformation behavior of the fully transitional Heusler magnetically variable shape memory alloy and the promotion of the design of high-performance elastic-thermal materials.
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Elastocaloric effect of all- d -metal Heusler NiMnTi (Co) magnetic shape memory alloys by digital image correlation and infrared thermography
We have studied the stress-induced martensitic transformation behaviors and the associated elastocaloric effect (eCE) for non-textured polycrystalline all-d-metal Heusler alloys of Ni 50 Mn 32 Ti 18 and Ni 35 Co 15 Mn 35 Ti 15 by a combination of Digital Image Correlation (DIC) and Infrared (IR) thermography techniques. A large but irreversible adiabatic temperature change (ΔT ad) of 10.7 K at a strain level of 3.9% is observed for Ni 50 Mn 32 Ti 18, whereas Ni 35 Co 15 Mn 35 Ti 15 exhibits a reversible eCE with ΔT ad = 9.0 K at a strain level of 4.6%. At lower strain levels (<2.4%), both specimens exhibit full superelasticity without residual strain. While in a higher strain range (> 3.2 %), Ni 50 Mn 32 Ti 18 is plastically deformed with small strain variation in space from the DIC map. In contrast, Ni 35 Co 15 Mn 35 Ti 15 can be deformed superelastically accompanied by large strain variation in space, which can be ascribed predominately to the crystalline orientation dependence of Both the transformation strain and the Young's modulus from different orientated grains under mechanical loading. The improved reversibility of eCE for Ni 35 Co 15 Mn 35 Ti 15 is supposed to be associated with the enhancement of d-d hybridization by the introduction of the element Co.