The development of the microstructure of die-upset Nd-Fe-B magnets

Observation of the microstructure of Nd-Fe-B permanent magnets, melt-spun and then die upset to varying extents, establishes a mechanism for the development of crystallographic texture and favorable magnetic properties. During die upsetting, grain growth occurs much more rapidly normal to the crystallographic c-axis than parallel to it. This anisotropic grain growth results in platelet shaped grains of Nd2Fe14B, surrounded by continuous films of a Nd-rich grain boundary phase. At the temperature of die upsetting, this phase is molten; it serves to enhance grain boundary sliding and allows the platelet shaped grains to slip past each other until they end up with their wide flat faces normal to the axis of compression. In so doing, they establish common c-axis orientations and correspondingly high remanence and energy product. Deformation twins and precipitates inside Nd2Fe14B grains are produced during high temperature deformation and are believed to have minor effects on processing and properties.