Microstructure And Magnetic Property Research Of(Nd,Ce)-Fe-B Sintered Magnets With High Ce Substitution

Nd-Fe-B sintered magnets,well-known by their excellent magnetic performance,most widely use and the most rare earth consumption,is widely used in information,transportation,energy,national defense and other fields.Using cheap and high abundance ratios of rare earth Ce to product Nd-Fe-B,not only can promote the efficient and balanced use of rare earth strategic resources,but also can greatly reduce the cost of production of rare earth permanent magnet,so it becomes the most important direction of rare earth permanent magnet material basic research.But when we substitute Nd with Ce homogeneously,the intrinsic magnetic properties of 2:14:1 phase determine,thus magnet magnetic performance is deteriorated seriously.Based on the new prototype of multi main phase(MMP)approach by mixing the Ce-free and Ce-rich RE2Fei4B terminal powders,we utilize the BMP method to fabricate the(Nd,Ce)-Fe-B sintered magnet.Its product has higher magnetic performance than that of single alloy method.Therefore,it has great significance to research magnetic mechanism of main phase chemical heterogeneity of the MMP(Nd,Ce)-Fe-B magnets,then we can improve the magnetic performance and Ce alternative quantity.In this paper,by optimizing the sintering and annealing process,the key influence of heterogeneity,the composition distribution,microstructure and magnetic properties of the MMP(Nd,Ce)-Fe-B magnets is studied.The main results are as follows:Ce-free terminal with(Pr,Nd)30.5FebalM1.39B1.0 and Ce-rich terminal with(Pr,Nd)15.25Ce15.25FebalM1.39B1.0 are designed to prepare the MMP(Nd,Ce)-Fe-B magnets.A perculiar microstructural feature is observed that RE concentration in the main phase differs a lot either over local regions within an individual grain or across grains,being completely different from the single main phase(SMP)magnet in which Ce and Nd are homogenously distributed in the main phase.The resultant short-range exchange couplings of local regions within an individual 2:14:1 main phase grain,and the long-range magnetostatic interactions among different 2:14:1-type grains in these magnets facilitate superior magnetic performance including Br and Hcj.At 27 wt.%Ce,(BH)max of the MMP magnet reaches-41.6 MGOe,basically comparable to that of the Ce-free starting magnet(～47.9 MGOe).Meanwhile,the Br and Hcj are well maintained at～11.3 kGs and～13.1 kOe,even superior to those of the SMP one with only 9 wt.%Ce(Br=12.65 kG,Hcj= 9.8 kOe,and(BH)max= 38.3 MGOe).To optimize the magnetic performance of MMP(Nd,Ce)-Fe-B magnets,chemical heterogeneity,microstructure and magnetic property changes as a function of the sintering and annealing condition have been investigated.As a consequence,preferable magnetic properties of Br=12.5 kG,Hcj = 8.9 kOe,(BH)max = 37.5 MGOe can be obtained for the chemically inhomogeneous BMP magnet with 27 wt.%Ce.Firstly,after adding Ce,the sintered(Nd,Ce)-Fe-B has good microstructure,reaches a certain magnetic properties.So we use only one step heat treatment process,finally obtains the optimal heat treatment temperature at 580℃×4h step of tempering later,the coercivity of Hcj=9.5kOe,through the analysis we find that increasing the tempering temperature or prolonging holding time will aggravate the magnets inside the main phase grain between element interdiffusion,destroying the uneven distribution of the magnet significantly,the coercivity decreases rapidly.When the amount of Ce is more than 27wt.%,due to the low melting point of Ce2Fe14B and the sintering aid of Ce,the magnetic performance is best at about 1040℃.The coercive can be improved by one step tempering process,heat treatment affects the magnetic performance in two aspects:grain boundary phase magnetic separation and the main phase and the chemical composition of grain boundary phase gradient distribution,the magnetic seriously deteriorated influence of sintering(Nd,Ce)-Fe-B magnets by the direct adding Ce cerium can be avoided.Elements in different grain interior and grain boundary keep homogeneously by optimizing the heat treatment temperature,which can strengthen the exchange coupling effect in grain and between different grains and significantly improves magnetic coercivity.