Influence of deformation processing on magnetocaloric effect of Heusler alloys

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

Influence of deformation on the microsructure and magneic properties of Heusler alloys

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

Comparative studies of the strength properties of germanum and silicon single crystals

In this paper, we present the results of microhardness tests performed by Vickers indentation of germanium and silicon single crystals. It’s shown that in the investigated samples there is a dependence of microhardness on the crystallographic directions and the nature of the alloying impurity. Microhardness anisotropy coefficients are calculated: for germanium KII=1,2 and for silicon KII=1,3. The analysis of high-temperature annealing influence on microhardness value of germanium and silicon crystals is carried out. It has been established, that the microhardness of Ge(111) crystals grows on 12% after annealing at 550°С, the further thermal processing of germanium crystals at T=650°С considerably changes the structure and surface relief which contribute to a decrease in microhardness values. It is shown that the microhardness of silicon crystals increases by 10% after annealing at 750°C, further annealing to T=850°C leads to a decrease in microhardness. The surfaces of single crystals after high-temperature annealing have been studied; it has been established that thermal treatment at T≈0.6 Tm (Tm – the melting temperature of the single crystal) leads to the appearance of defects and a tenfold increase in the maximum height of the surface profile (from 10-12 nm to 100-200 nm)