Evolution of the microstructure of rapidly solidified Nd-Fe-B permanent magnets

Nd-Fe-B permanent magnets prepared by rapid solidification have been extensively characterized by transmission electron microscopy at all stages of their development. Grains in the as-spun ribbons range in size up to several micrometers in their largest dimension. Excess Nd is trapped within some of the larger grains during solidification; this excess Nd separates from the lattice during subsequent processing, forming Nd-rich, coherent, precipitates. During consolidation of the ribbons, the surface oxide films are broken, creating intimate metallurgical bonds and allowing grain growth to occur across the original ribbon interfaces. The bulk of the grains in the final magnet are plate shaped, having a c axis in common and parallel to the axis of die upsetting; these grains are surrounded by the Nd-rich grain-boundary phase.