The effects of average grain size on the magnetic properties and corrosion resistance of NdFeB sintered magnets

Magnets with a nominal composition of Nd14.5Fe79.5B6 have been prepared by the conventional powder metallurgy technique. Precursor alloy powders with average particle sizes of 2.5, 3.0, 3.5, and 4.2 mu m were included in this study. Average particle size and oxygen content were effectively manipulated to control the average grain size in the sintered magnets. Typically, for NdFeB sintered magnets, the corrosion resistance of these magnets was improved with increasing oxygen content. The corrosion resistance of magnets varied significantly with the average particle size of precursor alloy powders. For a fixed oxygen content, magnets made from powders of larger sizes exhibited a higher weight loss (a poor corrosion resistance) when compared to those made from smaller alloy powders. However, the H-ci of magnets made from 2.5, 3.0, and 3.5 mu m precursor alloy powders was found to decrease drastically with increasing oxygen content while magnets made from the 4.2 mu m powder was found to remain relatively constant with increasing oxygen content. To optimize magnetic performance, one needs to compromise the corrosion resistance and the H-ci obtained by balancing the average particle size of the precursor alloy powder for magnet fabrication, as well as the oxygen content and the average grain size in the finished magnet. (C) 1996 American Institute of Physics.