Effects of Dy71.5Fe28.5 intergranular addition on the microstructure and the corrosion resistance of Nd–Fe–B sintered magnets

• Eutectic Dy71.5Fe28.5 powders were intergranular added to NdFeB sintered magnets.
• The doped magnet showed improved corrosion resistance compared to Dy-free magnet.
• Dy enrichment in RE-rich intergranular phase improved its electrode potential.
• (Nd, Dy)2Fe14B shell was expected to form in the surface of Nd2Fe14B grains.
• Both corrosion resistance and coercivity were improved in Dy–Fe doped magnet.

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 Dy71.5Fe28.5 powders were added into the (Pr0.2Nd0.8)12.3FebalB6.1 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.