The effect of Dy-Cu co-deposition and grain boundary diffusion on the microstructure and magnetic properties of sintered NdFeB magnets

In this paper, Dy-Cu coating was electrodeposited on the surface of commercial sintered NdFeB magnet as a diffusion source and the grain boundary diffusion was carried out on the sintered NdFeB magnet. The experimental results show that the coercivity of the diffusion magnet is increased from 993 kA/m to 1241 kA/m, an increase of 24.95 %, and the remanence is only reduced to 1.366 T, a decrease of 0.44 % compared with the original magnet. The electrochemical behavior analysis shows that with the addition of Cu2+, the reduction potential of Dy3+ appears a large positive shift, and the Dy-Cu co-deposition process is a three-dimensional instantaneous nucleation process. The SEM images show that the Dy-Cu deposition layer has a dense and flat surface with small grain size. The energy spectrum analysis shows that the Dy-Cu elements obtained by electrodeposition are uniformly distributed, and there is no massive agglomeration phenomenon. SEM-EDS line scan observation shows that the Dy elements diffused along the grain boundary phase mainly accumulate in the grain boundary phase and the epitaxial layer of the main grain. EPMA shows that Cu and Dy elements are evenly distributed in the grain boundary phase after grain boundary diffusion. SEM-BSE shows that the near-surface grain boundary phase structure is significantly optimized after grain boundary diffusion. XRD shows that Dy partially replaces the NdFeB main phase, leading to the migration of the main peak to the large angle direction. At the same time, due to the existence of low melting point metal, Dy preferentially enters the magnet internal, resulting in the decrease of the content of (Dy, Nd)2Fe14B phase. The optimization of the microstructure and grain boundary phase composition and the formation of (Dy, Nd)2Fe14B structure are the main reasons for the significant enhancement of the coercivity According to Kronmüller plots, the coercivity mechanism of diffusion magnet is still the magnetic domain nucleation inversion mechanism. After the Tafel test, it is found that the diffusion Dy magnet has a relatively positive corrosion voltage Ecorr and a relatively low corrosion current density Icorr, and its corrosion resistance is greatly improved.