GRAIN-GROWTH KINETICS IN SINTERED ND-FE-B MAGNETS

The topology of the microstructure and the grain growth kinetics of Nd -Pe-B magnets have been examined after sintering-time intervals betwee n 0.5 and 19 h by metallographic analysis. In dense magnets about 80 % of the grains have 5 to 7 corners. The fraction of grains with >8 cor ners increases with sintering-time and the amount of liquid phase duri ng sintering. The grain growth exponents amount to n approximate to 25 to 30 for small grains (much less than 10 mu m) and to n approximate to 1 to 4 for large grains (much greater than 10 mu m) revealing the i ntensive growth of large grains. Dense Nd-Fe-B magnets with an average grain size between 6.2 and 7.4 mu m reach a maximum energy density of 53 MGOe.