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.
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Mishra et al. (1986) studied this question.
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