| Due to their high energy density,Nd-Fe-B sintered magnets have been widely applied in various fields such as sensors,direct-drive wind power generators,motors of electric and hybrid vehicles,and wind turbines.However,the low coercivity and poor thermal stability are still main obstacles that limit their further applications in high temperature environments.To overcome those problems,heavy rare earth elements such as Dy have been added to increase the coercivity by partially replacing Nd to form a(Nd,Dy)2Fel4B magnetic hardened layer.The question of how to improve the utilization of Dy introduction without sacrificing the coercivity has received a lot of attention recently.It is common that sulfur and phosphorus have the characteristics of low melting point,segregation on steel grain boundaries.Therefore,the optimization of grain boundary is promising to obtain by introducing S and P in Nd-Fe-B sintered magnets.In this study,the site preference of S and P in Nd-Fe-B sintered magnets was simulated by first principle.Then,the S and P doping effect on microstructure and magnetic properties in Nd-Fe-B sintered magnets was systematically studied.In addition,the distribution and role of S and P in grain boundary,the interactions of Dy and S atoms,and the mechanism of coercivity enhancement were emphasized.Firstly,the boundary structure between Nd2Fe14B phase and Nd2O3 phase was built by Materials Studio(MS).Then,the corresponding cohesive energies of S or P atom,occupied in the Nd2Fe14B or the Nd2O3 phase,were simulated by Vienna Ab-initio Simulation Package(VASP)which was based on the first principle.Calculations showed that the cohesive energies of S or P atom occupied in the Nd2O3 phase were lower than that in the Nd2Fe14B phase.It can be inferred that the S or P atom was preferred in the Nd2O3 phase of Nd-Fe-B sintered magnets,which would mainly precipitate in Nd-rich phases in the grain boundary phases.Nd-Fe-B powders with a nominal composition of(NdPr)31.0Febal(Cu,Al,Co,Cr,Ga)4.0B1.0(wt.%)were mixed with various amounts of sulfur(0,0.1,0.2,0.3,0.5wt.%).The compact density increased with increasing S content.When the S content increased to 0.5wt.%,the compact density increased from 3.87g·cm-3 to 4.17g·cm-3.It was significant to eliminate the process of cold isostatic pressing,and reduce the shrinkage and distortion during sintering.In addition,the coercivity increased firstly and then decreased with the increase of S content.With 0.2wt.%S addition,the coercivity reached its maximum value.The corresponding coercivity increased from 1236.3kA·m-1 to 1326.2kA·m-1 with a peak value 7.3%higher than that of the undoped magnets.There was no notable decrease in the remanence and the maximum magnetic energy product.Continuous grain boundary phases were formed in the S-doped magnets with smaller grain size.The average grain size was 7.83?m,which was approximately 1.29?m smaller than that of the undoped magnets(9.12?m).The melting point of Nd-rich phases decreased from 1038K to 1021K.The optimization of grain boundary and grain size refinement were the reasons for the coercivity enhancement in Nd-Fe-B sintered magnets.However,the distribution of S element was non-uniform,building obvious clusters.The S-rich phases at the triple junctions can be hexagonal Nd2O2S phase,or tetragonal NdS2 phase.Aiming to address the uniform distribution problem,the S element was introduced by jet milling.Due to the less amount of S addition and the obvious density distinction between S and magnets,pyrite was chosen was chosen as the raw material for obtaining the S element.In the magnets with 0.2wt.%S doped by jet milling,a coercivity enhancement of 13.4%was obtained.Compared to the undoped magnets,the corresponding coercivity increased from 1236.3kA·m-1 to 1401.8kA·m-1.The microstructure analysis showed that the distribution of S element was more uniform in the magnets with pyrite doped by jet milling.The distribution of grain boundary phases was optimized.The grain refinement were more obvious,with the average grain size decreasing from 7.86?m to 6.67?m.The effect of Dy2O3/S co-doping on the microstructure and magnetic properties was studied in the Nd-Fe-B sintered magnets.Firstly,Nd-Fe-B powders with a nominal composition of(NdPr)31.0 Febal(Cu,Al,Co,Cr,Ga)4.0B1.0(wt.%)were mixed with various amounts of Dy2O3(0,0.5,1.0,2.0,3.0wt.%).The results showed that the coercivity incread with the increase of Dy2O3 content.With 3.Owt%Dy2O3 addition,the coercivity increased from 1236.3kA·m-1 to 1665.9kA·m-1.In the Dy-containing magnets,continuous grain boundary phases were formed.Dy atom diffused into the outer region of Nd2Fe14B phase to form a(Nd,Dy)2Fe14B magnetic hardened layer,resulting in the enhancement of coercivity.However,the EPMA and EDS results showed that Dy was mainly enriched in the Nd-rich phases at the triple junctions,resulting in the low utilization of Dy.Therefore,the Dy2O3(3.0wt.%)and S(0,0.1,0.2,0.3,0.5wt.%)were co-doped in the magnets.The coercivity increased firstly and then decreased with the increase of S content in the co-doping magnets.The coercivity reached a maximum in the 3.0wt.%Dy2O3/0.2wt.%S co-doped magnets,increasing from 1665.9kA·m-1 to 1816.2kA·m-1,9.1%higher than the 3.0wt.%Dy2O3 doped magnets.In addition,there was a slight decrease in remanence and maximum magnetic energy product.The EPMA and TEM results showed that Dy atoms,enriching in the triple junctions,avoided the hcp-Nd2O2S phases,resulting in more available Dy atoms diffusing into the outer region of Nd2Fe14B phase grains in the co-doped magnets.This in turn made the most of Dy element to enhance the anisotropy field,resulting in the coercivity enhancement.Nd-Fe-B powders with a nominal composition of(NdPr)31.0 Febal(Cu,Al,Co,Cr,Ga)4.0B1.0(wt.%)were mixed with various amounts of phosphorus(0,0.05,0.08,0.1,0.2wt.%).The distribution of P element in the P-containing magnets was non-uniform.P element was enriched in grain boundary and triple junctions,partly overlapping the distribution of Nd and O elements.The TEM analysis showed that the P-rich phases in the grain boundary were NdP04 phases with a hexagonal structure,and that at the triple junctions were matched with NdPO4 phases with a monoclinic structure.With 0.05wt.%P addition,the melting point of Nd-rich phases and the average grain size decreased from 1038K to 1022K and 9.12?um to 8.43?m,respectively.Though the melting point of Nd-rich phases were lower with P addition,the NdPO4 phases appeared as a big chunk inclusion in the grain boundary,without there being a notable increase in the coercivity. |
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