| Double-phase nanocomposite permanent magnet materials cause wide attention due to excellent magnetic properties. How to produce permanent magnets with high properties and cheapness is one of the hot points at present.Considering the problem existing in the development of nanocrystalline composite magnets, nanocomposite Fe3B/Nd2Fe14B permanent magnets are prepared by rapidly quenching, post heat treatment and Spark plasma sintering (SPS) in this paper. Effect of preparation parameters and component on microstructure and magnetic properties of nanocomposite permanent magnets have been investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and atomic force microscopy (AFM), scanning electron microscopy (SEM).For Nd4.5Fe77B18.5, at lower quenching speed, the amount of nuclear increases. It is easy to grow coarsely for crystallized compound, and result in worsen the magnetic properties. While at the higher quenching speed, and the smaller nucleation center the as-spun alloy has during the crystallization process. As a result, the magnetic properties are low because of coarse grain of Fe3B. Only when the quenching speed is 8m/s, which has a fine and homogenous microstructure and excellent combination of magnetic properties are obtained.When the crystallization temperature is low, the magnetic phases precipitate incompletely, and amorphous phases existing in Nd4.5Fe77B18.5 ternary alloys. And thereby the exchange coupling interaction is weak and the magnetic properties are low. When the crystallization temperature is high, the exchange coupling interaction is weak due to the coarse grain of Fe3B, and the magnetic properties such as Br and (BH)m degraded. Magnetic properties, such as Br, Hcj and (BH)m, increase with the crystallization time increasing at first, and then decrease, that is to say it exists a maximum. As a result, the optimal magnetic properties of Nd4.5Fe77B18.5 are obtained after dynamic crystallization heat treatment at 710℃for 10min.The influence of Nd,B,Zr, Cu and Co to the magnetic properties are also discussed in this paper. With the increasing of atom percent of Nd for NdxFe81.5-xB18.5 alloy, the magnetic properities first increase and then decrease. When atom percent of Nd is 4.5, the magnetic properties reach the maximum: Br =0.80T, Hcb=206kA/m, Hcj=259 kA/m and (BH)m=48kJ/cm3. At the same time,the high content of Nd retards the formation of Fe3B and Nd2Fe14B, promote the stability of the nonmagnetic phase Nd2Fe23B3.Increasing the B content or Zr content can promote the formation of amorphous phases. But if the content of B exceeds a certain level, nonmagnetic phase Nd1.1Fe4B4 occurs during crystallization of the alloys, and increase with the increase of atom percent of B. As a result, the exchange coupled interaction between the grains becomes weak and the magnetic properties decrease with the B content increasing. The optimal magnetic properties of Nd4.5Fe95.5-yBy are obtained when y=18.5. The magnetic properties decrease by adding Zr element.The high content of Zr can also retard the formation of Fe3B and Nd2Fe14B and promote the stability of the nonmagnetic phase Nd2Fe23B3.Adding Cu can reduce the formation of amorphous phases, which is helpful for the alloy crystallising at low temperature. It can be clearly seen that the Cu atoms form clusters. They are located at the interface between the Fe3B crystals and the amorphous matrix, and each cluster is in direct contact with the Fe3B particles. Those clusters serve ad heterogeneous nucleation sites for the Fe3B particles,thereby increasing the number density of the particles. The exchange coupled interaction enhances and the magnetic properties increases too.The element Co can dissolve in Nd2Fe14B and Fe3B phase by replacing Fe atom, which improve the magnetic properties. The Nd4.5Fe76.9Cu0.1B18.5 quartemary alloy obtained the optimal properties by adding Cu after treated by conventional heat treatment in the mass:Br=0.82T, Hcb=219kA/m,Hcj=277kA/m,(BH)max=53kJ/cm3.It has been determined that, compared with the conventional heat treatment,spark plasma sintering can promote crystallization of melt-spun NdFeB powder from amorphous phase, decrease the crystallization temperature and shorten the crystallization time. Moreover SPS can enhance the exchange coupled interaction between the grains and increase the magnetic properties. As a result, the optimal magnetic properties of Nd4.5Fe76.9Cu0.1B18.5 (Br=0.95T, Hcb=233kA/m,Hcj=275kA/m,(BH)max=69kJ/cm3,the density is 6.89g/cm3) are obtained after sintered at 50MPa, 670℃for 10min.
|
PDF Full Text Request