Thermal effects during the formation of the structure in highly coercive Nd-Fe-B-based alloys in the HDDR process

Abstract

The phase transformations during the HDDR process used to obtain an anisotropic structure of permanent magnets based on the Fe-Nd-B system have been studied by differential thermal analysis (DTA) and X-ray diffraction methods. It is found that sharp single-stage changes in the hydrogen pressure during the HDDR process lead to strong changes in the temperature of the reacting mixture of phases, which is why the formation of the structure occurs at non- optimal temperatures; this worsens the anisotropy of the distribution of the axes of light magnetization of ferromagnetic nanoparticles and the characteristics of the magnetic material. Heat treatments were carried out with a stepwise change in the pressure of hydrogen during isothermal direct and reverse reactions. The DTA method shows a 2-4-fold decrease in undesirable abrupt changes in the temperature of direct and reverse reactions. The completeness of the forward and reverse reactions and the absence of the formation of undesirable phases are shown by X-ray diffraction methods.