A study of high-energy milling for the production of sintered PrFeB magnets

Sintered Pr16Fe76B8 magnets have been produced using the hydrogen decrepitation (HD) process and high-energy planetary ball milling. Investigations have been carried out to evaluate the influence of the milling speed and time. The best magnetic properties obtained were Br = (1020 ± 20) mT, µ0iHc = (1420 ± 30) mT and (BH)max/= (200 ± 4) kJm-3, for a magnet prepared with the alloy milled at 200 rpm for 4.5 ks. Magnets prepared from this powder exhibited a superior intrinsic coercivity compared to that of magnets produced using low-energy ball milling. However, the remanence and energy product of the latter were somewhat lower. An important feature was the dramatic reduction in the processing time (about 90%). Microstructural observation have shown that increasing the milling time and keeping constant the rotational mill speed caused an exponential grain size reduction in the sintered magnet. Increasing the milling speed also reduced the grain size and influenced both remanence and intrinsic coercivity.