Effects of Dy_71.5Fe_28.5 intergranular addition on the microstructure and the corrosion resistance of Nd-Fe-B sintered magnets

To satisfy high-temperature applications, heavy rare-earth (RE) Dy is commonly introduced into the Nd-Fe-B sintered magnets to improve the coercivity. In addition to forming (Nd, Dy)₂Fe₁₄B, Dy also exists in the intergranular RE-rich phase. Hence, understanding the effect of Dy on the electrochemical characteristics of the RE-rich phase and corrosion resistance of the magnet is of importance. In this work, eutectic alloy Dy_71.5Fe_28.5 powders were added into the (Pr_0.2Nd_0.8)_12.3Fe_balB_6.1 magnet through binary-alloy approach to investigate the corrosion resistance of the magnet in electrochemical and hot/humid environments. The results demonstrate that Dy is enriched in the intergranular phase, improving its electrode potential and stability due to the higher electrode potential of Dy than Nd or Pr. As a consequence, the electrode potential difference between the 2:14:1 phase and the RE-rich phase is reduced, improving the corrosion resistance. Furthermore, formation of (Pr, Nd, Dy)₂Fe₁₄B shell with stronger local anisotropy surrounding the 2:14:1 phase grains improves the coercivity with a slight remanence loss. Therefore, intergranular adding Dy-Fe alloy powders can obtain both high magnetic properties and good corrosion resistance simultaneously.