Study On The Microstructure Regulation,Properties And Mechanism Of NdFeB Magnets

NdFeB magnet possesses good function conversion and magnetic effects and has been widely used in core components of high-tech fields.However,the defect of low coercivity and poor corrosion resistance have not been fundamentally solved,which severely limit the application of NdFeB magnet.Therefore,the methods have been explored for regulating magnet’s microstructure and properties by composite and grain boundary modification.This paper strives to make new breakthrough in the basic theory and process control of NdFeB magnets such as compounding,interface control,magnetic and corrosion resistance mechanisms,to provide theoretical guidance for NdFeB magnet’s performance improvement and application,which has important theoretical value and practical significance.This paper focuses on the interfacial reaction and controlling of NdFeB/SmCo₅ magnet and the grain boundary modification of NdFeB magnet.According to the Miedema theory,the interfacial reactions that may occured during the preparation of NdFeB/SmCo₅ magnet were preliminary explored,and the phase selection rule during interface reaction was elucidated.The influence of non-magnetic interfacial modification phase with low-melting on the interfacial reaction was studied.The control rule and mechanism of interfacial regulation were revealed.Meanwhile,starting from adjusting phase composition and microstructure of NdFeB magnet through Ni intergrainular addition,the modification rule of microstructure and properties were studied in details.The evolution of grain morphology and component in main phase and grain boundary phase’s component and distribution are revealed.And then its influence mechanism on magnetic properties is clarified.The corrosion behavior and mechanism of sintered NdFeB magnets in different electrochemical environments were illustrated.The main findings of this paper are as follows:According to thermodynamic calculation and analysis,the interface reaction and controlling mechanism of NdFeB/SmCo₅ magnets are expounded.The Sm-Co alloy system tends to form SmCo₂ when rare earth metal content is in 26%-53%.The thermodynamic driving forces Fe replaced by Co is higher than Sm replacing Nd in Nd₂Fe₁₄B phase.Thus NdFeB/SmCo₅ magnets preferentially to form Nd₂（Fe,Co）₁₄B and SmCo₂ phase when interfacial reaction occurs.The disadvantage of it is to improve magnetic properties.Pr-Cu alloy can effectively control interfacial reaction of NdFeB/SmCo₅ magnets.The substitution of Pr for Nd in Nd₂Fe₁₄B main phase has little effect on its formation enthalpy.A small amount of Pr repalcing Nd can improve the formation ability of Nd₂Fe₁₄B main phase.However,Cu replacing Fe in Nd₂Fe₁₄B main phase reduces its formation ability.Pr replacing Sm or Cu replacing Co can change SmCo₅formation enthalpy and reduce its forming ability.Pr-Cu alloy grain boundary modification is beneficial to improving magnetic properties through effectively inhibited the interfacial reaction between NdFeB and SmCo₅,maintaining the composition and structure of SmCo₅ hard magnetic phase which contributed to the formation of high anisotropy（Nd,Pr）₂Fe₁₄B hard magnetic layers.Grain boundary addiing ultra-fine Ni powders effectively improve the microstructure of sintered NdFeB magnet and significantly increase magnetic properties and corrosion resistance.It has been found that adding a small amount of Ni to the grain boundary can increase the intrinsic coercivity.This is mainly because most of the Ni accumulated in the grain boundary,which refine the main phase grain making their distribution more uniform and effectively increase the nucleation field of anti-magnetic domain.Physicochemical properties of grain boundary phase are changed.Wettability between the main phase and the grain boundary phase is increased.Nd phase in the grain boundary is more evenly distributed around the main phase.Magnetic exchange coupling between the main phase grains is reduced.However,as the Ni increase,the remanence Br and the maximum energy product（BH）_max gradually decrease,which is caused by the decrease of the magnet density and saturation magnetization.When Ni content is less than 0.3wt%,the comprehensive magnetic properties are optimized.In addition,the grain boundary addition of Ni increases the open circuit potential of magnet in different etching solutions,which reduces the erode tendency of magnet,making the corrosion potential more positive,decreasing the corrosion current density and enhancing the corrosion resistance.This is mainly because the addition of Ni to the grain boundary alters the composition,increases the electrode potential of Nd-rich phase and reduces potential difference between main phase and Nd rich phase.The main phase grain is refined and intergranular corrosion channel is narrowed.Thus intergranular corrosion is suppressed.