Effect of post-sintering annealing on microstructure and coercivity of Al_(85)Cu_(15)-added Nd-Fe-B sintered magnets

摘要

Effects of post-sintering annealing on the microstructure and coercivity have been investigated for the Al_(85)Cu_(15)-added (Pr, Nd)_(14.8)Fe_(78.7)B_(6.5) sintered magnets. It is found that the optimum annealing temperature at which the coercivity _iH_c reaches a maximum decreases from 550 ℃ for the magnets added with 0.3% and 0.6% Al_(85)Cu_(15) to 480℃ for the magnets added with 0.9% and 1.2% Al_(85)Cu_(15). The decrease in optimum annealing temperature is related to the precipitation of Al-Cu or (Pr, Nd)-Cu liquid phase among (Pr, Nd)-rich phases during annealing. Existence of Al-Cu or (Pr, Nd)-Cu liquid phase is beneficial to dissolve the irregularities of (Pr, Nd)_2Fe_(14)B grain interface and increase the quantities of (Pr, Nd)-rich phases at the grain boundary, thus optimizing the grain boundary microstructure. The modifications of the microstructure are helpful to decouple the exchange interaction between (Pr, Nd)_2Fe_(14)B hard magnetic grains, thereby increasing the coercivity.