The mechanism of coercivity enhancement by the grain boundary diffusion process of Nd-Fe-B sintered magnets

We discuss the mechanism of the coercivity enhancement by the grain boundary diffusion process (GBDP) using Dy vapor based on detailed microstructural characterizations. Scanning electron microscopy and electron probe microanalysis showed that a (Nd,Dy)(2) Fe14B shell formed in the outer region of Nd2Fe14B grains while its thickness decreased from the surface to the center of a cube-shaped sample. Atom probe tomography showed that the Dy content at grain boundaries (GBs) was close to that in the (Nd,Dy)(2)Fe14B shell. High-temperature annealing (at 900 degrees C) of a GB diffusion processed magnet led to the disappearance of the GB layers, which resulted in a substantial reduction in coercivity. This suggests that both the (Nd,Dy)(2)Fe14B shell and the Nd-rich GB phase layer are required microstructural features for the coercivity enhancement by the GBDP. (c) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.