The basic composition of the material affects the performance of the material. Sintered NdFeB permanent magnets are mainly produced by powder metallurgy. It has at least the following four different phases:
① Matrix phase (main phase): Nd2Fe14B phase. It is formed by peritectic reaction at about 1200℃ and is the only magnetic phase in the alloy. The excellent magnetic properties of NdFeB magnets are mainly attributed to the high saturation magnetization (μMs=1.6T) and anisotropy field (7.3T) of Nd 2Fe14B phase.
② Nd-rich phase: Its melting point is 650~700℃. It is the last solidified in the alloy. It exists in thin layers and blocks and is distributed at the intersection of grain boundaries or on the grain boundaries of Nd 2Fe14B. Although it is a non-magnetic phase, due to its low melting point, it is dispersed around the main phase during sintering, which not only plays a role in densifying the sintered body, but also inhibits the growth of grains and promotes the improvement of coercivity. Therefore, it is indispensable.
③ B-rich phase Nd1+εFe4B4: It is formed when the boron content in the alloy exceeds the normal composition of Nd2Fe14B. It does not contribute to the magnetic properties. Generally, the amount is very small and has little effect on the magnetic properties.
④ α-Fe: Its melting point is 1520℃. It is the phase with the highest melting point in the alloy. It is the first to precipitate from the liquid alloy. α-Fe is a soft magnetic phase. Its existence leads to the reduction of the main phase and the increase of the neodymium-rich phase, destroying the optimal ratio of the main phase and the neodymium-rich phase, damaging the magnetic orientation of the main phase grains, and coarsening the grains in the local area during the sintering process, which not only deteriorates the magnetic properties, but also deteriorates the structure of the electroplating layer, affecting the protective effect. Therefore, measures are taken from the manufacturing process to minimize or eliminate the generation of α-Fe phase, such as sheet casting process and rapid quenching process.
