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

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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.