Effect of Heat Treatment on Microstructure and Thermal Stability of Nd–Fe–B Sintered Magnets

The effect of heat treatment on the microstructure and thermal stability of sintered Nd-Fe-B magnets was investigated. The results show that the microstructure of the magnets was greatly affected by heat treatment. The intrinsic coercivity (H
_cj
) was increased and the temperature coefficient of coercivity (β) was decreased by heat treatment. As the annealing temperature changes from 500°C to 770°C, H
_cj
first increases and reaches a maximum, then starts to decrease, but β remains unchanged from 500°C to 680°C and increases rapidly when annealed over 680°C. A magnet with H
_cj
= 30.55 kOe, β (from 20°C to 200°C) = -0.4079%°C was obtained after annealed at 620°C for 2 h. The relationship between the annealing temperature and magnetic properties was studied.