What are the phase compositions of sintered NdFeB? What is the function?

Matrix phase (main phase) Nd2Fe14B

It is formed by peritectic reaction at about 1200 ° C and is the only magnetic phase in the alloy. The excellent magnetic properties of NdFeB magnet are mainly due to the high saturation magnetization of Nd2Fe14B phase（ μ 0ms = 1.6T) and anisotropic field (7.3t), which are mainly used to provide high MS and high HA.

Nd rich phase (75% - 85%) NdFe (wt%)

Its melting point is 650 ~ 700 ° C. It is the last solidified phase in the alloy and is between the solidified grains. It is a thin layer phase covering the matrix phase. Although it is a non-magnetic phase, due to its low melting point characteristics, it is dispersed around the main phase during sintering, which not only plays a role in densifying the sintered body, but also inhibits the grain growth and promotes the improvement of coercivity. Therefore, it is essential and distributed among grains.

B-rich phase ND1 1Fe4B4

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.

α- Fe

Its melting point is 1520 ° C. It is the phase with the highest melting point in the alloy, which is first broken out from the liquid alloy, α- Fe is a soft magnetic phase. Its existence leads to the reduction of main phase and the increase of neodymium rich phase, destroys the optimal ratio of main phase and neodymium rich phase, damages the magnetic orientation of main phase grains, and coarsens the grains in local areas during sintering, which not only worsens the magnetic properties, but also worsens the structure of electroplating layer and affects the protective effect.

Therefore, measures shall be taken from the manufacturing process to minimize or eliminate α- The generation of Fe phase, such as sheet casting process and rapid quenching process.