Dependence of the Phase Composition of Nd16Fe73.9Zr2.1B8 Alloy on the Conditions of Milling in Hydrogen

Abstract

We reveal the competitive influence of two factors in the process of "dry" milling of Nd 16 Fe 73.9 Zr 2.1 B 8 ferromagnetic alloy in hydrogen. As the rotation frequency of the mill chamber and the duration of the process increase, the range of particle sizes of the powder increases but particles become isotropic as a result of agglomeration. We indicate the possibility of application of a liquid medium (oleic acid) with an aim to prevent mechanical alloying of highly dispersed powder particles as a result of long highenergy milling. Highly dispersed anisotropic powders are obtained by milling of the alloy saturated with hydrogen in oleic acid at a rotation frequency of the mill chamber of 100-200 rpm for 12 h.The development of contemporary advanced technologies requires the uninterrupted improvement of permanent magnets. Among all known permanent magnets, the permanent magnets based on rare-earth metals (REM) exhibit the highest characteristics including, in particular, the coercive force, magnetization, and specific magnetic energy. According to the theoretical calculations [1, 2], their properties can be improved by the formation of nanocrystalline states in these materials [3]. There are numerous technological approaches developed in this direction [4][5][6][7]. As their common disadvantage, we can mention the difficulties connected with the appearance of anisotropy.For the formation of anisotropic structures with the help of hydrogen-vacuum thermal treatment by the method of hydrogenation, disproportionation, desorption, and recombination (HDDR), it is customary to use magnetic alloys of the Nd-Fe-B system [8]. There are several assumptions concerning the mechanism of formation of anisotropy after HDDR connected with the state of the material after the first stage of treatment, namely, after hydrogenation and disproportionation (HD). In particular, it is believed that anisotropy can be caused by the presence of residues of the nondisproportionated ferromagnetic phase among the products of HD, by a metastable phase of unknown nature, and by a certain ordering between the crystallographic directions of the phases in the intact material and in the disproportionated mixture.In the investigation of alloys of the Sm-Co system, it was established that anisotropy is attained if the HDDR procedure is realized for the values of parameters guaranteeing that an insignificant amount of ferromagnetic residues of the ferromagnetic phase remains in the alloy after disproportionation. These residues serve as crystallization centers in the course of desorption and recombination and play a decisive role in the formation of