Microstructure of Nd-Fe-Co-Ga-B hot-deformed magnet improved by the Dy diffusion treatment was investigated with high-resolution scanning electron microscopy (HRSEM), analytical transmission electron microscopy (ATEM) and high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). It was found that Dy diffuses along the thin grain boundary (GB) phase between Nd2(Fe,Co)14B grains, and then the Dy substitutes for a part of Nd near the surface of the Nd2(Fe,Co)14B grains. Consequently, two-layered-wrapping structure of thin GB phase and thin (Nd,Dy)2(Fe,Co)14B layer is formed on each Nd 2(Fe,Co)14B grain. Moreover, the Dy substitution occurs preferentially at the Nd-sites of a-planes compared with that of c-planes in the Nd2(Fe,Co)14B grains. These results indicate that the Dy diffusion treatment improves significantly the coercivity by forming the thin (Nd,Dy)2(Fe,Co)14B layer on the surface of Nd 2(Fe,Co)14B grains, although the treatment leads to grain growth and deterioration of the c-axis alignment of Nd2(Fe,Co) 14B grains.
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