Effect Of Ce-rich Liquid Phase Alloy Addition On Magnetic Properties Of Sintered Ce-Fe-B Magnets

The fabrication of traditional Nd-Fe-B permanent magnets relies heavily on Nd and Pr,as a result,the high-abundant and low-cost Ce was overstocked for a long period of time.To develop high abundance Ce-based magnets can not only greatly reduce the cost of raw materials of rare earth permanent magnets,but also double the usable rare earth resources for RE magnet preparations,which will be of great significance for the balanced utilization of rare earth resources in China.In recent years,the research and development of high abundance Ce-based magnets has become a common hotspot of academia and industry.However,when Ce substitution amount,defined as Ce/?50?RE??50?RE is total rare earth content?,is more than50%for Ce-based magnets,especially for Ce/?50?RE>80%,the microstructure observations show that large volume fraction of REFe₂ phase?Laves phase?are aggregated at grain boundary junctions and the grain-boundary microstructures are worsened,as a result,the magnetic properties of magnets have been deteriorated significantly.Up to now,the research on improving the microstructure of sintered Ce-Fe-B magnets by modifying the compositions and structures of Ce-rich grain-boundary phase,has not been reported.In the paper,the microstructure and magnetic performance of Ce-Fe-B sintered magnets with the nominal composition of[Ce_0.8?Gd,Ho?_0.2]_15.1Fe_75.7B_7.2M₂?at.%?were firstly studied;then the micro-morphology,phase constitutions and thermal behavior of Ce-rich liquid phase alloys with different Al/Cu doping content were investigated systematically;finally,a dual alloy method was adopted to prepare sintered Ce-Fe-B magnets.The microstructure and magnetic properties of the Ce-Fe-B magnets have been obviously improved.The main conclusions in this paper are as follows:?1?A non-uniform lamellar crystalline structure has been formed in the experimental Ce-Fe-B strip casting flakes,the refined grain structure exists on the wheel surface of the flakes,and the coarsened grain structure is on the free surface.It may be attributed to the narrow solidification zone of Ce₂Fe₁₄B.The flakes are mainly composed of RE₂Fe₁₄B and REFe₂ phases,no?-Fe phase was found by SEM observation.There are two types of island-like phases surrounded by REFe₂:RE₂Fe₁₇phase and transitional phases with atomic ratio of RE:Fe being 1:3¹:4,which may be caused by the incomplete peritectic reaction.?2?Al/Cu additions have changed the microstructure,phase constitutions and melting points of Ce-rich liquid phase alloys.The original Ce-rich alloy is mainly composed of CeFe₂,Ce₂Fe₁₇ and?Ce,the Ce₂Fe₁₇ phases appeared as dendrites at local areas.The volume fractions of CeFe₂ and Ce₂Fe₁₇7 decrease with increasing Al/Cu content in the alloys,and the dendrite morphology of Ce₂Fe₁₇7 phase disappears.When Al substitution amount is 10 at.%,multiple Ce-Al eutectic structures have been formed,the melting point of the alloy has been reduced from 942?C to 856?C;when Cu content is 6 at.%,single eutectic structure of CeCu and?Ce has appeared,and the melting point of the alloy has been reduced from 942?C to 895?C.?3?Sintered Ce-Fe-B magnets have been prepared by Ce-rich liquid phase alloy addition,the magnet microstructure has been optimized,the flowability of grain-boundary phase during heat treatment has been improved and the volume fraction of grain-boundary phase has also been increased.The coercivity H_cj,energy product?BH?_max and squareness of final magnets have been improved by 8%,22%,24%?Al-doped alloy?and 16%,35%,17%?Cu-doped alloy?,respectively.In addition,it was found that Ce and Cu elements present completely different distribution characteristics at different heat treat conditions.Ce enriches in grain boundary during sintering and annealing processes,while trace Cu is partially dissolved in the 2:14:1phase grains for as-sintered magnet and will enrich in the grain boundaries after annealing.