Effect of annealing on the microstructure of sintered Nd-Fe-B magnets

A transmission electron microscopy study of sintered Nd-Fe-B magnets after annealing above 650 °C shows the formation of a thin intergranular Nd-rich layer extending to pockets of polycrystalline fcc Nd crystals. The grain boundary phase acts as a pinning site for the magnetic domain walls and is necessary for good magnetic hardening. On the other hand, the Nd1+εFe4B4 phase is an unavoidable product of the sintering process but need not be present for good coercivity.