Study On Microstructure Design And Properties Optimization Of Permanent Magnets Containing High Abundant Rare Earth

Nd-Fe-B magnets have been widely used owing to its excellent magnetic properties,which leads to the sharply consumption of the critical raw material-Pr,Nd,Dy,Tb,whereas La,Ce with high abundance and low cost are stacked largely,meanwhile the separation and purification processes of rare earth cause serious environmental pollution.So researches on the massive application of high abundance rare earth and misch-metal rare earth have been paid much attention recently,which is beneficial to the balanced utilization of rare earth.On the other hand,the formation of La/Ce₂Fe₁₄B compound is difficult which leads to the destruction of microstructure and the worsen of the magnetic properties.So the challenge to develop rare earth-containing magnets with low cost and high abundance is to suppress the deterioration of the intrinsic magnetic properties of the matrix phase and destruction of the microstructure owing to the incorporation of La or Ce.The theme of this work has a discussion from the aspects of the grain boundary modification and the elemental manipulation at the surface of the matrix phase grains.In Ce-containing magnet,Nd/Ce-based grain boundary additives with low melting point are designed and the distributional features of Nd/Ce at the grain boundary are investigated,therefore achieving the high magnetic properties in magnet with high Ce content and clarify the manipulating principles of the elements in Ce-containing magnets.In LaCe misch metal containing magnet,on the basis of fabrication process optimization,the features of Pr,Nd enriching at the surface of the matrix phase is obtained through dual-alloy method and grain boundary diffusion method,therefore reduing the magnetic dillusion effect caused by the La/Ce incorporation,laying a foundation of the production of high LaCe-containing magnets.The main conclusions are listed below:1.In Ce-containing magnets,the distribution of Ce/Nd at the grain boundary is manipulated via the additives with low melting point and the wettability of the grain boundary improves.After the additives of Ce-Fe/Nd-Fe,REFe₂ phase and RE₆Fe₁₃Cu phase forms respectively at the triple junction,broadening the thickness of the grain boundary and improve the decouple effect.After doping 1wt.%Ce-Fe additives the coercivity increases by 1.4 kOe and the magnetic properties of Ce-containing magnet with the composition of Ce/RE=0.28 reaches to 35M grade.Additionally,in comparison with the magnetic properties and microstructure of the magnets processed with Nd-Fe/Ce-Fe additives whose nominal compostion is same,the additives of Nd-Fe leads to the formation of 6:13:1 phase with low melting point,Nd enriches at the outer region of the matrix grains which leads to the magnetic harden of the grains and is beneficial to the production of Ce-containing magnets with high coercivity.The additives of Ce-Fe leads to the formation of 1:2 phase,Ce enriches at the outer region of the matrix grains which leads to the worsen of magnetic harden,while Ce prefers to occupy at the grain boundary owing to its high formation energy of Ce₂Fe₁₄B and that reduces the content of Ce in matrix phase,so as to obtain high saturation magnetization.Finally,at the Ce-containing magnet with the composition of Ce/RE=0.34,the magnetic proeperties corresponding to N40 grade with high energy product.2.In LaCe-containing magnets,the law of the strip cast flake melting,sintering and annealing processes with LaCe additive amount have been systematically investigated.The optimal casting temperature decreases 5-10°C with per LaCe misch-metal incorporation in weight owing to its lower peritectic temperature of matrrix phase due to the incorporation of LaCe misch-metal,Besides,the suppression of Fe-rich phase gets harder and the Fe content at the grain boundary increases with the increasing incorporation of LaCe misch-meal.The densification of the magnet with high LaCe content gets harder owing to the formation of Rare earth oxides with high melting temperature caused by high activity of La,Ce and the annealling temperature enhances for the magnet with high LaCe content owing to the formation of 5:17 phase with high melting temperature.3.On the basis of systematical investigations on the critical processes of LaCe-containing magnets,the elemental manipulation of the magetic harden at the surface of the matrix phase grains haven been obtained through the dual-alloy method and grain boundary diffusion method,therefore improving the magnetic properties and laying a foundation for large scale production of LaCe-containing magnets.Firstly,LaCe-containing magnets with different elemental distribution features are prepared with dual alloy method,among them magnet blending thin LaCe-lean powders and coarse LaCe-rich powders have superior magnetic properties compared with LaCe-containing magnet prepared with conventional dual-alloy method and the coercivity increase by 1.3 kOe.Additionally,the content of Pr,Nd at the triple junction of the surficial region of the diffused magnet is higher than that of the original magnet,and dilutes the content of Fe at the triple junction.While for the matrix phase grains Pr,Nd enrich at the outer shell of the grains and form the core-shell structure.The Lorentz-TEM images indicate that the domain walls interdict at the triple junction and thin grain boundary for the diffused magnets,manifesting strong magnetic decouple effect.Finally for the magnet with LaCe/RE=0.3,the coercivity obviously increases by 2 kOe and the energy product retains 36.2 MGOe through the grain boundary diffusion method with?PrNd?H_x powders.