Magnetic Properties,Microstructure,and Corrosion Resistance of Single Alloy RE2Fe14B Magnets with Different Ce Content
The development of high-performance Ce-containing sintered magnets based on a single alloy process remains a bottleneck problem.In addition,the effect of Ce content variation on the corrosion resistance of single alloy Ce-containing sintered magnets is not clear.In this paper,Ce30,Ce35 and Ce40 sintered magnets containing Ce were prepared by a single alloy process,and their Ce/TRE(TRE=total rare earths)were 30％,35％and 40％,respectively.The magnetic properties of Ce30 and Ce35 magnets reached N40 and N38 grade of common commercial sintered magnets(coercivity ≥955 kA·m-1).Using the ratio of(BH)max to the cost per kilogram the magnet as a comparison standard,the costs of the three Ce-containing sintered magnets were reduced by 33.7％～41.1％compared to those of the common commercial N grade sintered NdFeB magnets with similar(BH)max.The phases analysis by X-ray diffraction(XRD)and Rietveld fitting showed that the content of REFe2 phase increased from 4.35％for Ce30 magnets to 5.31％for Ce40 mag-nets with the increase of Ce substitution.The microstructures of three Ce-containing sintered magnets were observed,and it was found that the thin layers of grain boundary phases gradually became blurred and discontinuous with the increase of Ce content in the mag-nets.In addition,Ce elemental content in the agglomerated triangular grain boundary phases was higher than that of Nd,and there was also a higher Fe concentration.The agglomerated triangular grain boundary region with high Ce and Fe content and low Pr and Nd con-tent was analyzed for the elemental mass ratio,and RE/Fe atomic ratio showed that the region was REFe,phase.The content and distri-bution state of the REFe2 phase produces some changed in the microstructure of the magnets,which therefore caused variations in the magnetic properties as well.The effect of the Ce content of sintered magnets on the corrosion resistance of magnets was further investi-gated:under the corrosion conditions of high pressure accelerated aging,the corrosion weight loss gradually decreased as Ce content in magnets increased.The corrosion weight loss of Ce30 magnets was much larger than those of Ce35 and Ce40 magnets,especially when the corrosion time reached 96 h,the weight loss of Ce30 magnets began to show a clear increasing trend.When the corrosion time reached 168 h,the corrosion weight loss of Ce30 magnets was 106.8 mg·cm-3,which exceeded 3.1 times to Ce40 magnets.The neutral salt spray corrosion experiments conducted under 5％NaCl(mass fraction)solution also showed that Ce30 magnets were more susceptible to corrosion.The polarization curves of the magnets with different Ce contents were measured using 3.5％NaCl solution as the corrosion medium and Tafel slope extrapolation method was used to determine the corrosion current density(icorr).The corrosion current density of Ce30,Ce35,and Ce40 magnets were 3.17725×10-5,2.62734×10-5 and 1.83278×10-5 A·cm2.With the gradual in-crease of Ce content in the magnet,the corrosion current density was gradually becoming smaller,and the corrosion rate was propor-tional to the corrosion current density from the perspective of corrosion kinetics.This indicated that the corrosion rate of Ce-containing sintered magnets gradually decreased with the increase of Ce content in the magnets,and the chemical stability was higher with the in-crease of Ce content in the magnets.Electrochemical impedance spectroscope(EIS)impedance tests of magnets with different Ce con-tent reflected the same trend.The diameters of the capacitive resistance arc of the magnets became progressively larger with the increas-es of Ce content,indicating that the electrode reaction needed to overcome a larger potential barrier and the electrode reaction rate be-came slower.This indicated that the corrosion resistance of the magnets was gradually improved with the increase of Ce content.It could be concluded that the content of the REFe2 phase with higher Fe concentration in the Ce-containing sintered magnets increases with the increase of Ce content in the magnet.The higher Fe concentration resulted in higher chemical stability than the conventional RE-rich phase and REFe2 phase was distributed in agglomerates in the grain boundary phase,which made the continuity of the grain boundary phase poor.On the one hand,the deterioration of grain boundary phase continuity decreases the coercivity(H)and deterio-rates the magnetic properties.Under the combined influence of the blocked corrosion channels due to the deterioration of the grain boundary phase continuity and the better chemical stability of REFe2 phase,the Ce-containing sintered magnets showed better corro-sion resistance with the increase of Ce content in high-pressure accelerated corrosion environment,neutral salt spray environment,and electrochemical corrosion environment.The results could provide a reference for explaining the relationship between Ce content of sintered magnets and magnetic properties of magnets and magnet corrosion resistance.
single alloy Ce-containing sintered magnetmagnetic propertiesmicrostructurecorrosion resistance
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