1. Matrix phase (main phase) Nd2Fe14B
It is formed through a peritectic reaction at around 1200°C and is the only magnetic phase in the alloy. The excellent magnetic properties of NdFeB magnets are mainly attributed to the high saturation magnetization (uOMS=1.6T) and anisotropic field (7.3T) of the Nd2Fe14B phase. Its main function is to provide high Ms and high Ha.
2. Nd-rich phase (75%~85%) NdFe (wt%)
Its melting point is 650~700°C. It is the last to solidify in the alloy. It is located between the solidified grains and 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 and distributed around the main phase during sintering. It not only densifies the sintered body, but also inhibits the growth of grains and promotes the increase of coercive force. Therefore, It is essential and distributed among the crystals.
3. Rich B phase
Generally, the quantity is very small and has little effect on the magnetic properties. Formed when the boron content in the alloy exceeds the normal composition of Nd2Fe14B, it does not contribute to the magnetic properties.
4.a-Fe
Its melting point is 1520°C, which is the phase with the highest melting point in the alloy. It is the first to be folded out of the liquid alloy. a-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 main phase. The optimal ratio of the phase and the neodymium-rich phase damages the magnetic orientation of the main phase grains, and also coarsens the grains in local areas during the sintering process, which not only deteriorates the magnetic properties, but also deteriorates the structure of the electroplated layer, affecting Protective effect.
Therefore, measures should be taken from the manufacturing process to minimize or eliminate the generation of α-Fe phase, such as sheet casting process and quick quenching process.
