Coercivity and thermal stability improvement in sintered Nd-Fe-B permanent magnets by intergranular addition of Dy-Mn alloy

To increase coercivity and thermal stability of sintered Nd-Fe-B magnets for high temperature applications, Dy88Mn12 (wt%) alloy powders were intergranular added into (Pr0.25Nd0.75)30.6Cu0.15FebalB1 (wt%) starting magnet. The magnetic properties, microstructure and thermal stability of the sintered magnets with different amounts of Dy88Mn12 were investigated. By adding a small amount of Dy88Mn12, the coercivity was significantly increased from 12.56 kOe to 17.49 kOe. Microstructure analysis showed that a optimized microstructure, i.e. continuous, uniform grain boundary phase was achieved with Dy88Mn12 alloy addition, and Dy was enriched in the outer region of the Nd2Fe14B matrix grains during the sintering process, which favored to substitute for Nd in matrix grains to form the (Nd,Dy)2Fe14B core-shell phase. The greatly increased magnetocrystalline anisotropy of the core-shell phase and the improved decoupling by the continuous grain boundary phase accounted for the coercivity enhancement. Furthermore, by adding 0-4 wt% Dy88Mn12, the reversible temperature coefficients of remanence (α) and coercivity (β) of the magnet were improved from −0.115%/ºC to −0.107%/ºC and −0.744%/ºC to −0.696%/ºC in the range of 20-100 °C, respectively. In addition, the irreversible flux loss of magnetic flow (hirr) decreased sharply as Dy88Mn12 addition. The temperature-dependent magnetic properties results indicated that with intergranular addition of Dy88Mn12 alloy, the thermal stability of the magnets was effectively improved.