Dissolution of two types of grain boundary phases in a hot-deformed Nd-Fe-B magnet by high-temperature annealing

Two main types of grain boundary phases have been found in magnets under low magnification after Nd70Cu30 alloy grain boundary diffusion: an Nd-Cu-rich phase and an Fe-Nd-rich phase. The Nd-Cu-rich phase originated from the diffusion of Nd70Cu30 alloy. The Fe-Nd-rich phase originated from the hot-deformed magnet itself and was confirmed to be the Nd6Fe13Ga phase. After Nd70Cu30 eutectic diffusion, the magnets were subjected to annealing at 800 °C for ~10 min with a height reduction of 15 or 30% to optimize the grain boundary phase distribution. A coercivity of 1.21–2.20 T and remanence of 1.16–1.46 T were achieved depending on the processing parameters. The squareness factor increased from 0.87 to 0.96 after short-term annealing at 800 °C. SEM analysis showed that the Nd-Cu-rich phase further diffused into the intergranular region of the nanocrystalline Nd2Fe14B grains and that the Nd6Fe13Ga phase almost dissolved in the matrix. Detailed TEM observations indicate that the intergranular region is mainly Nd- and Cu-rich, with a few regions rich in Nd, Cu and Ga. The NdCu phase and Nd6Fe13Ga phase coexist in some blocky intergranular regions, indicating diffusion of the Nd6Fe13Ga phase. The relation between the increased squareness and evolution of the NdCu and Nd6Fe13Ga phases is discussed.