Microstructural evidence for the magnetic surface hardening of Dy2O3-doped Nd15Fe77B8 magnets

Microstructures of (NdDy)15Fe77B8 prepared by alloying with Dy, and using Dy2O3 as a sintering additive, have been determined using electron microprobe and transmission electron microscopy. A two-step, post-sintering heat treatment has been applied to both systems in order to improve the coercivity. The results show that the second-step heat-treatment temperature is lower for the oxide-doped magnets for an optimum intrinsic coercivity. In a typical Dy2O3-doped sample, a high concentration of Dy was found close to the grain boundary of the hard 2-14-1 phase. In contrast, Dy was found uniformly substituted in a magnet to which Dy was added in the alloying step. It is proposed that a high concentration of Dy at the surface of the (NdDy)2Fe14B improves the coercivity by increasing the required field for domain reversal nucleation at or near the grain boundaries