Re-exchange of Fe and Cu at the interface in sintered Nd–Fe–B magnets: A method to eliminate Fe precipitation at grain boundaries

Highlights: • Fe precipitated at the grain boundaries in Nd–Fe–B magnet was explained. • Low Gibbs free energy (13.6 meV, 600 °C) in Cu-added magnets. • Little amount of Ag will reduce Fe precipitation greatly. - Abstract: According to the decoupling hypothesis for magnetic grains, the coercivity in sintered Nd–Fe–B magnets is increased after Cu doping, which is due to the formation of non-magnetic grain boundaries. However, this method partially fails, and ferromagnetic Fe-segregation occurs at the grain boundary. We discovered both experimentally and through calculation that the Fe content at the grain boundaries can be tuned across a wide range by introducing another element of Ag. Segregated Fe at high temperature at the grain boundary re-dissolves into Nd₂Fe₁₄B grains during annealing at low temperature. Both configurable and magnetic entropies contribute a large driving force for the formation of nonmagnetic grain boundaries. Almost zero Fe content could be achieved at the grain boundaries of sintered Nd–Fe–B magnet.