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This paper presents the results of studies of the effects of deformation obtained by the method of multiple isothermal forging on the microstructure and magnetic properties of the NiMnGa alloy system. It is shown that the microstructure of the initial alloy during deformation undergoes changes, grain size decreases and a two-component structure is formed. The magnetic domain structure of the original and deformed alloys was visualized by magnetic force microscopy. It is shown that the distortion of the domain shape is associated with the presence of a martensitic relief and grain boundaries, 180-degree magnetic domains are continuous within the crystallites, while they cross the flat parallel boundaries of the martensite plates. The study of temperature and field magnetization dependences of the initial and isothermal forging samples demonstrates an insignificant magnetization decrease and a shift of the phase transition temperature toward low temperatures. It’s concluded that deformation by the isothermal forging method can be considered as a way to correct the phase transition temperature in NiMnGa alloys with preservation of magnetization.

This paper presents the results of complex studies of the effect of deformation obtained by the multiple isothermal forging method on the magnetocaloric properties of the Ni-Mn-Ga Heusler alloy system. Direct measurements of the adiabatic temperature change show that cycling the sample in a magnetic field μ0ΔH=1,85 T can decrease the maximum of ΔTad(T)0→H by the value corresponding to the latent heat of transition. Neglecting this transformation effect leads to an overestimation of the magnetocaloric effect of materials with the first-order transition. An analysis of the deformation treatment effect on the phase transformation temperatures and the magnitude of the magnetocaloric effect indicates that the application of multiple isothermal forging leads to a small decrease of the magnetization and the magnitude of the magnetocaloric effect. Deformation also causes a shift in the phase transition temperature towards low temperatures and decrease of the temperature hysteresis width.

The paper describes the results of the scanning electronic and atomic force microscopy research of the surface structure of the Ni57Mn21Al22, Ni57Mn21Al19Si3, Ni40Co10Mn34Al16 rapidly quenched ribbons. The influence of the chemical composition on the size, boundary geometry and structure of the grains is considered. It is shown that all the test samples have a nano-sized martensitic relief, and its parameters are determined. It has been established that the cross section of the ribbon is represented by crystalline grains of different shapes which are due to differences in the cooling rates along the edges and in the center of the samples. Elemental mapping of the cross-sectional surface of the ribbons was carried out using an X-ray energy dispersive spectrometer, and a uniform distribution of chemical elements in the samples was established. It is shown that the cobalt doping of Ni57Mn21Al22 ribbon causes changes in the micromorphology of the surface and has a significant effect on the magnetic properties of rapidly quenched ribbons: field dependences of themagnetization and domain structure.

Copyright of Metallophysics & Advanced Technologies / Metallofizika i Novejsie Tehnologii is the property of G.V. Kurdyumov Institute for Metal Physics, N.A.S.U and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Metallophysics & Advanced Technologies / Metallofizika i Novejsie Tehnologii is the property of G.V. Kurdyumov Institute for Metal Physics, N.A.S.U and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Metallophysics & Advanced Technologies / Metallofizika i Novejsie Tehnologii is the property of G.V. Kurdyumov Institute for Metal Physics, N.A.S.U and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)