Magnetic transformations during mechanical loading of magnetocaloric materials based on Heusler alloys

基于磁热效应的磁制冷因其在节能方面的潜在影响而备受关注。到目前为止，由于材料的限制，这项技术只被用于小众应用。然而，在过去的十年里，已经开发出了新的合金，这些合金表现出很大的磁热效应，并且不依赖于稀土元素。Ni45Co5Mn36.7In13.3是一种很有前途的新候选材料，其马氏体相变温度与磁场有很大的温度依赖关系，约为4K/T。此外，由于该材料具有伪弹性，还可以通过叠加机械应力来调节相变。因此，可以通过施加应力和磁场来控制这种材料中的相变和相关的放热和吸热。然而，众所周知，反复循环加载会严重影响伪弹性材料的微观结构，进而导致功能退化。因此，本研究的目的是双重的。首先，基辅磁学研究所的小组将专注于改进NiCoMnIn系统，以在实际应用中足够低的磁场中获得足够大的磁热效应。在基辅开发的合金将在目前的项目中循环加载，以了解导致功能退化的微观结构机制。这部分研究将包括循环机械加载和使用电子显微镜进行详细的微结构表征。作为功能退化的衡量指标，将监测循环应力应变响应和磁导率的变化。拟议的研究结果将允许同时使用磁场和叠加应力来扩大磁制冷的温度范围。同时，了解支配微结构演变的机制将有助于设计新材料，以显示下一代磁制冷系统的功能退化减少。

Magnetic refrigeration based on the magnetocaloric effect is of a great interest due to its potential impact on energy saving. So far, this technology has only been employed for niche applications due to material limitations. Over the last decade, however, new alloys have been developed that demonstrate a large magnetocaloric effect and do not rely on rare earth elements. A promising new candidate material is Ni45Co5Mn36.7In13.3, where the martensitic transformation temperature shows a large temperature dependence on magnetic field of about 4 K/T. Moreover, the phase transformation can also be tuned by a superimposed mechanical stress as this material shows pseudoelasticity. Thus, the phase transformation and the associated heat release and absorption can be controlled in such a material by both applied stress and magnetic field. It is well known, however, that repeated cyclic loading can drastically affect the microstructure of a pseudoelastic material, which in turn leads to functional degradation.Thus, the objectives of the proposed research are two-fold. Firstly, the group at the Institute of Magnetism in Kyiv will focus on improving the NiCoMnIn system, to obtain a sufficiently large magnetocaloric effect in magnetic fields that are low enough with respect to actual applications. The alloys developed in Kyiv will be cyclically loaded within the current project in order to understand the microstructural mechanisms that lead to functional degradation. This part of the research will involve both cyclic mechanical loading and detailed microstructural characterization using electron microscopy. As measures of functional degradation both the changes in cyclic stress-strain response and magnetic permeability will be monitored.The result of the proposed research will allow for using both magnetic fields and superimposed stresses to widen the temperature range of magnetic refrigeration. At the same time understanding of the mechanisms that govern microstructural evolution will help to design new materials that demonstrate reduced functional degradation for the next generation of magnetic refrigeration systems.