Multiphase Characterization of Phase Equilibria in the Tb-Rich Corner of the Co-Cu-Tb System

In recent years, the grain-boundary diffusion (GBD) and grain-boundary restructuring processes used in manufacturing Nd-Fe-B magnets show promise as procedures that allow one to increase their hysteretic characteristics. The processes are realized by adding various amounts of heavy-rare-earth metals (in the form of hydrides, oxides, intermetallic compounds, etc.) to powder mixtures. The additions decompose or melt during subsequent heat treatment, and their components diffuse into grains and remain within the grain-boundary phase and thus, increase the anisotropy field of the main-magnetic (Nd2Fe14B-based) phase and improve the grain-boundary structure, respectively. In the present study, we consider alloys near the Tb3(Co0.6Cu0.4) composition as such an addition, which is of importance in designing the microstructure of Nd-Fe-B permanent magnets allowing us to economically alloy them with terbium (via GBD) simultaneously making copper and cobalt parts of the magnet composition. The phase equilibria in the Tb-rich corner of the Co-Cu-Tb system near the Tb3(Co0.6Cu0.4) composition, which was found to be multiphase, are assessed based on electron microscopy studies, data from electron microprobe, x-ray diffraction, and differential thermal analyses, and from magnetic measurements. A portion of the isothermal section for compositions Tb-40 at.% Co-50 at.% Cu at 600°C was constructed. The copper solubility in Tb3(Co,Cu) and Tb12(Co,Cu)7, and Co solubility in the Tb(Cu,Co) compound were determined, which are up to 6.5, 14.5, and 10 at.%, respectively. The possibility of hydrogenation of the multiphase composition with the formation of TbHx, and fine Co and Cu powders, which are components for diffusion in manufacturing permanent magnets, is demonstrated.