HDDR HOT-PRESSED MAGNETS - MAGNETIC-PROPERTIES AND MICROSTRUCTURE

The recently developed HDDR process has now been extended to production of hot-pressed magnets. The samples prepared to date exhibit intrinsic coercivities of approximately 1200 kA/m, remanence of around 690 mT and BH(max) values of approximately 90 kJ/m3. Careful studies of the microstructures reveal that the magnets consist of Nd2Fe14B grains predominantly in the range 0.1-mu-m less-than-or-equal-to x less-than-or-equal-to 1.0-mu-m with a very small number of grains in the range 10-mu-m less-than-or-equal-to x less-than-or-equal-to 35-mu-m. These larger grains exhibit a very regular morphology and appear to be the result of a very rapid growth of some of the smaller sub-micron grains. The starting composition of the cast material Nd16Fe76B8 ensures that the magnets have approximately 12% Nd-rich intergranular material; this low melting point phase has been found to assist in the densification process. However, SEM studies have shown that the Nd-rich material is very unevenly distributed throughout the hot-pressed magnet and little evidence can be found for its presence between the grains of Nd2Fe14B.