Comparison on the coercivity enhancement of sintered NdFeB magnets by grain boundary diffusion with low-melting (Tb, R)75Cu25 alloys (R= None, Y, La, and Ce)

A significant coercivity enhancement of the commercial NdFeB magnets with the magnetic properties of (BH)(max) = 48.4 MGOe and H-i(c) = 17.5 kOe through grain boundary diffusion (GBD) with low-melting Tb55R20Cu25 alloys is demonstrated. Adopting Tb55R20Cu25 alloys as GBD sources is effective in increasing coercivity to 29.0 kOe for R = None, 23.8 kOe for R = Y, 25.6 kOe for R = La, 28.0 kOe for R = Ce, respectively. Yet, (BH)(max) is slightly reduced to 46.2-48.2 MGOe. The preferential appearance of Cu at grain boundary and triple junction of the grains, and the core-shell structure occurred due to Tb at grain surface remarkably enhance the coercivity. Interestingly, higher coercivity enhancement per wt% Tb usage (Delta H-i(c)/wt%Tb) of 7.2 kOe/wt% for the magnet with Tb55Ce25Cu25 than 5.9 kOe/wt% for that with Tb75Cu25 has been found due to the magnetic isolation effect caused by the preferential appearance of Ce at grain boundary, though a slight lower coercivity enhancement was found for the samples with R = Y and La. Lower melting point (637 degrees C) for Tb55Ce20Cu25 than Tb75Cu25 (743 degrees C) leads to larger diffusion depth of Tb into the magnet and therefore contributes to higher efficiency of coercivity enhancement for the magnet with R=Ce.