Study On Structural Evolution And Properties Of RE-Fe-B Based Sintered Mgnets With A High Amount Of High-abundance Rare-earth La/Ce/Y

The large usage of RE-Fe-B-based rare-earth permanent magnets leads to an excessive consumption of rare-earth elements Nd/Pr/Dy/Tb as well as a large backlog of rare-earth elements La/Ce/Y with high-abundaice and low cost,which not only uinbalances the utilization of rare-earth resources but also raises the price of RE-Fe-B based rare-earth permanent magnets.Therefore,it has become a hot topic in the rare-earth permanent magnet indtustry or sciencetific community,that is,the magnets are prepared by substituting the expensive and tight rare-earth elements Nd/Pr/Dy/Tb with La/Ce/Y so as to realize the comprehensive and balanced utilization of rare-earth resources and reduce the production cost of rare-earth permanent magnet.materials.The substitution of high-abundance rare-earth elements La/Ce/Y for Nd/Pr/Dy/Tb reduces the intrinsic properties of RE2Fei4B tetragonal phase,and the addition of La/Ce/Y has adverse effects on the phase constitutes and microstructures of RE-Fe-B magnets,which are significant factors of restricting the large-scale application of high-abundance rare-earth elements.Regarding these issues,this thesis firstly investigated the migration characteristics of La/Ce/Y in the preparation processes of magnet,and clarified the influence of high-abu1ndance rare-earth elements on the phase constitutes and microstructures of sintered magnets.On this basis,Y-containing magnets with core-shell structure were designed to study the influence mechanism of different Y substitutions on the performance of sintered RE-Fe-B magnet,followed by the fabrication of high performance Nd-?Y,Ce?-Fe-B magnets using Y/Ce synergistic effect.Finally,the distribution of Ce in the Ce-containing magnet was regulated by the double-alloy method and the grain boundary structu re of the Y-containing magnet was optimized to achieve the optimal distribution of the high-abundance rare-earth elements in the magnet,by which the high performance permanent magnets with high-abundance rare-earth were obtained.The main research results of this thesis are as follows:The migration characteristics of La/Ce/Y and their influences on the phase constitutes and microstructures of sintered magnet during the preparation processes were investigated.In the processes of strip casting,sintering and annealing,La gradually diffused into the grain boundary phase from the matrix phase grains,while Ce/Y progressively migrated from the grain boundary phase into the matrix phase grains of the magnet.In addition.Y entered the matrix phase crystal more easily than Ce.At the same time,it was also found that a core-shell structure with Y-rich core and Nd-rich shell was formed in 2:14:1 matrix phase grains of the Nd-Y-Fe-B magnet.The crystallographies of matrix phase and grain boundary phase were respectively P42/mnm and hcp-Re₂O₃ during the whole fabrication processes,indicating that there is no effect of La/Ce/Y on the crystal structure of the magnet.According to the elemental migrations and phase formation characteristics of high-abundance rare-earth magnet,Y-magnets with core-shell structure were designed,and the magnetic properties of the magnets were also improved by using the synergistic effect of Y/Ce.Designing core-shell structure with Y-rich core and Nd-rich shell in the matrix phase grains of the Y-containing magnets could not only optimize the microstructures of the magnet but also increase the surface anisotropy field of the matrix phase grains,thereby meet the purpose of increasing the properties of the magnet.The magnetic properties of the Y-containnig magnets decreased with the increase of Y content.However,the properties of the magnet were Br=13.25 kG,Hcj=11.73 kOe,?BH?max=43.13 MGOe when Y content was 20%,which could still reach the level of commercial N45 magnet.In Nd-?Ce,Y?-Fe-B magnets,the coercivity could improve from 7.13 kOe to 9.73 kOe when the ratio of Y/Ce was 0.15/0.85?wt.%?.The dual-alloy method was used to regulate the distribution of Ce and the grain boundary structure of the Y-containing magnet to obtain high performance magnets.The properties of?Nd0.75Ce0.25?30.5FebalAl0.1Cu0.1B1 magnet with dual-alloy were Br=13.33 kG,Hcj=12.14 kOe,?BH?max = 40.56 MGOe,while the properties of the same nominal composition magnet with single-alloy?Br= 13.07 k0,Hcj=10.26 kOe,?BH?max=39.44 MGOe?were much lower than that of the dual-alloy magnet.It was also found that after adding 1 wt.%,2 wt.%and 3 wt%NdPrHx,the coercivities of Y-containing magnets were 8.59 kOe,9.55 kOe and 9.88 kOe,respectively,while the remanence of the magnets decreased slightly.