DISSOCIATION OF ND2FE14B DURING HIGH-TEMPERATURE OXIDATION

The dissociation of the Nd2Fe14B matrix phase of Nd-Fe-B magnets durin g oxidation leads to the formation of an adherent grey surface layer, which was investigated using optical microscopy; electron probe microa nalysis (EPMA) and transmission electron microscopy (TEM). This layer was observed to be composed mainly of a textured mosaic of alpha-iron grains with fine precipitates within. After further oxidation, the iro n within the dissociated layer is itself oxidised forming another surf ace layer, which results in extensive cracking of the magnet. Along wh at was the basal plane of Nd2Fe14B, the texture consists of iron grain s with a common [111] crystal axis; each crystal being rotated by 30-d egrees from its neighbours. The initial steps in the formation of this texture were observed by oxidising a TEM sample in-situ. This texture results from an epitaxial growth of alpha-iron from the Nd2Fe14B subs trate during the oxidation process, with growth in four possible cryst allographically equivalent orientations upon the basal plane.