Magnetocaloric effect in Ni-Mn-Ga-Si alloy subjected to plastic deformation by the multiple isothermal forging

In Heusler alloys, in the region of martensitic and magnetic phase transformations, a magnetocaloric effect is observed, which allows considering these alloys perspective for applying as solid refrigerant in cooling units. However, these alloys possess degraded performance properties. This study presents results of the investigation of the magnetocaloric effect in Ni-Mn-Ga alloy in two structural states: the initial as-cast state and subjected to deformation processing by multiple isothermal forging at 953 K. It is shown that in the as-cast state microstructure of the alloy is represented by equiaxial grains with a size of several hundreds of microns. As a result of forging, a binary microstructure is formed, where initial coarse grains are surrounded with a layer of a fine-grained structure. The investigation of the temperature dependence of the thermal expansion shows that the martensitic transformation in the as-cast state alloy occurs in the temperature range from 194 K to 214 K. In the treated state the phase transformation in the alloy shifts to the low temperature range by 10 K. In both states the value of magnetocaloric effect in the region of the martensitic transformation equals 0.15 K in the magnetic field of 1.8 T, and in the region of the Curie point — to 0.9 K. The only difference of the treated state is a shift of the $\Delta T\left(T\right)$ dependence to the low temperature range by 10 K.