Microstructural Analysis During the Step-Cooling Annealing of Iron-Rich Sm(Co0.65Fe0.26Cu0.07Zr0.02)7.8 Anisotropic Sintered Magnets

Microstructural analysis during the step-cooling procedure of iron-rich Sm(Co0.65Fe0.26Cu0.07Zr0.02)7.8 anisotropic sintered magnets was investigated systematically. The sintered magnet exhibits the maximum energy product of about 32 MGOe together with a remanence of about 11.5 kGs. It is found that the cellular structure has formed at the stage of isothermal annealing (heat treating at 1103 K for 20 h). Moreover, the average cell size of the 2:17 cell phase in magnets decreases from ~150 to ~105 nm, and the density of the lamella phase increases from ~0.03 to ~0.05 1/nm during the step-cooling procedure. It is also found that the Cu concentration is homogeneous in the specimen quenched at a temperature of 1103 K and then becomes nonuniform, especially enriched in the cell boundaries during the step cooling to 873 K. Interestingly, the typical twining structure along the [010] zone of the main phase exists in the specimen quenched at 873 K. Furthermore, it is inferred that the magnetization mechanism of the specimens transforms from the nucleation-dominated type to the coexisting type of pinning and nucleation effect during the step-cooling procedure.