Microstructure and annealing effects of NdFeB sintered magnets with Pr-Cu boundary addition

•Pr-Cu boundary addition was beneficial to the Hcj enhancement of NdFeB magnets.•4.3 and 6.2 kOe Hcj enhancements were obtained in 5 and 10 wt% Pr-Cu samples.•The Hcj enhancements for Pr-Cu samples were mainly related to the 480 °C annealing.•The micromagnetic parameter α increased from 0.491 to 0.656 after optimal annealing treatment. The annealing effects of NdFeB magnets were usually related to the eutectic liquid segregation from junction to grain boundary regions, atoms solid diffusion at grain boundary regions or structure transition of junction phases. In this work, the microstructure and annealing effects of sintered NdFeB magnets with eutectic Pr-Cu boundary additions (0, 5 and 10 wt%) have been systematically investigated. There were two kinds of boundary phases (Cu-lower phase: fcc REO2; Cu-higher phase: h-RE2O3) in Pr-Cu processed magnets. Pr-Cu showed no positive influence on the coercivity of as-sintered magnets and the first-step annealing at 900 °C followed by quenching was also useless. The best annealing temperature (480 °C) for Pr-Cu processed magnets was slightly higher than the lowest eutectic temperature (476 °C). Microstructural investigations demonstrated that the 480 °C annealing showed no influence on the structure transition of boundary phases but changed their distribution patterns. The dominant annealing effect was the formation of strong exchange-decoupling grain boundary phases (GBPs) which were originated from the eutectic liquid segregation. The repeated heating to 900 °C followed by quenching/furnace cooling for optimal annealed (900 + 480 °C) magnet demonstrated reversible eutectic liquid segregation. Annealing under the lowest eutectic temperature (460 °C) was harmful which indicated that the atoms solid diffusion mechanism was not accessible in Pr-Cu processed magnets.