THE PRODUCTION OF HIGH COERCIVITY CAST MAGNETS USING THE HDD PROCESS

Nd-Fe-B as-cast alloys in the form of small ingots 4-5 mm thick and with composition Nd2.1Fe14B were transformed into high coercivity magnets using the hydrogenation disproportionation desorption (HDD) process without decrepitation of the blocks taking place. Magnetic measurements revealed that it was possible to produce blocks with coercivity of approximately 780 kA m-1, remanence of approximately 680 mT and a BH(max) of approximately 68 kJ m-3 (8.6 MGOe). The high coercivities in these samples were attributed to the disproportionation and subsequent recrystallization process. SEM metallographs of the samples processed at optimum temperature revealed an absence of the original large (approximately 200-mu-m) grains of Nd2Fe14B containing areas of free iron; instead an extremely fine grained, submicron, microstructure of single phase Nd2Fe14B grains was observed. In samples processed at temperatures higher than that found to be the optimum, larger Nd2Fe14B grains with extremely rectangular morphologies could be observed growing from surrounding submicron matrix grains. Domain patterns observed on these larger grains, using the Kerr effect, indicated that Nd2Fe14B matrix grains appear to grow predominantly with rectangular morphologies from the disproportionated material with the a and c crystal axis running parallel to the edges of the rectangle.