Improvement of Thermal Stability in Hot-Deformed Nd-Fe-B Magnets by Grain Refinement

By doping 8 wt.% Nd₆₀Fe₁₅Ga₂₀Cu₅ alloy powders, the room-temperature coercivity of the hot-deformed magnet was enhanced from 1.21 to 1.86 T and the coercivity at 180°C was enhanced from 0.33 to 0.70 T. Temperature coefficient of coercivity improved from -0.469 to -0.402 %/°C and is superior to most of the hot-deformed magnets. Microstructure revealed that the Nd₂Fe₁₄B grains were isolated by Nd-Ga-Cu-rich intergranular phase. Statistics indicated that the average grain size was reduced by about 25% from 286±60 to 217±57 nm. Ultra-fine grains are the main reason for the improved thermal stability of the magnets without using heavy rare-earth elements.