Faceted shell structure in grain boundary diffusion-processed sintered Nd-Fe-B magnets

Dysprosium enriched shell structure formed by the grain boundary diffusion process (GBDP) of a sintered Nd-Fe-B magnet was characterized by using scanning electron microscopy, electron back-scattered diffraction and transmission electron microscopy. Faceted core-shell interfaces with an abrupt change in Dy concentration suggest the Dy-rich shells are formed by the solidification of the liquid phase during cooling from the GBDP temperature. The Nd-rich phases are almost free from Dy, and their quantity near the surface of a bulk sample is much higher than that in the center, indicating that a higher fraction of a liquid phase exists near the surface during processing at 900 degrees C. These microstructural features are explained on the basis of the phase equilibrium between Nd and Nd2Fe14B at the processing temperature and subsequent cooling. Based on the results, we discuss the coercivity enhancement by the GBDP using Dy vapor. (C) 2014 Elsevier B.V. All rights reserved.