Effect Of Grain Boundary Modification On Mechanical Properties And Fracture Behaviors Of Nd-Fe-B Magnets

Sintered Nd-Fe-B magnet is a kind of important magnetic functional material that is commonly applied in various motors,consumer electronics and medical equipments.In recent years,with the increasing application of sintered Nd-Fe-B magnets in energy-saving motors and smart phones,the high performance and miniaturizing magnets require higher mechanical properties.The cracking problem of magnets during processing into miniature products,magnetization and service has been concerned.In view of the poor mechanical properties of sintered Nd-Fe-B magnets,the fracture behavior of sintered Nd-Fe-B magnets has been studied in this paper.On the basis of analyzing the fracture mechanism of magnets and the key influencing factors,the improvement of mechanical properties of magnets by modifying grain boundaries have been explored.The fracture behavior of sintered Nd-Fe-B magnets was investigated.A variety of sintered Nd-Fe-B magnets with different characteristics were selected as the research objects,that was,the magnets prepared by different processes such as ingot casting-mechanical crushing-ball grinding powder,strip casting-hydrogen breaking-jet mill powder,and double alloy method,the magnets with different properties such as high remanence magnets and high coercivity magnets,and the magnets riched in Ce element.The fracture morphology,fracture mode,fracture location,and phase structure,micro structure,composition at near cracks,were comprehensively analyzed.The results showed that intergranular fracture was dominant in these magnets.The cracks were located in the triple junction phase(TJP)and at the interfaces between main phase and thin Nd-rich phase.The surface of some main phase grains was not covered by the Nd-rich phase,and some main phase grains were attached by Nd-rich phase.The mismatch degree between main phase and thin Nd-rich phase in magnet was between 60%and 90%,which may be the key factor for the crack along interface between main phase and thin Nd-rich phase.The modification of grain boundary structure of sintered Nd-Fe-B magnet by grain boundary diffusion and its influence in bending strength and fracture behavior of magnet were studied.The metal elemental(Tb)film was coated on magnet by magnetron sputtering,and low melting point Pr-Cu,Pr-Tb-Cu,Pr-Tb-Cu-Al alloy ribbons were respectively put on magnet.The grain boundary diffusion was carried out on N52 magnets and 52M magnets,respectively.The effect of diffusion treatment on grain boundary phases,grain boundary structure,lattice mismatch between main phase and boundary phase,and their effects on magnetic properties,mechanical properties and fracture behavior of magnet were studied.The results showed that the bending strength of the diffused magnet by Pr-Cu increased obviously.Compositional and microstructural ananlysis showed that Cu element was mainly distributed at grain boundaries,and the lattice mismatch between main phase and thin Nd-rich phase decreased obviously.However,after the diffusion of Tb metal,Pr-Tb-Cu alloy and Pr-Tb-Cu-Al alloy,the bending strength of the magnet decreased obviously.The Tb element entered into main phase grain and formed Tb-rich shells,contributing to the coercivity enhancement of magnet.It was observed that after the diffusion of Tb elemental,Tb containing alloy and Al containing alloy,a large number of transgranular fractures appeared near the surface of the magnet,and with the increase of the diffusion depth,the transgranular fractures gradually decreased,which was believed to be caused by the internal stress of the lattice caused by the entry of Tb and Al elements into the main phase.This may be the reason for the decrease of the bending strength of the diffused magnet by Tb metal,Pr-Tb-Cu alloy and Pr-Tb-Cu-Al alloy.The effect of nano-particles on mechanical properties and fracture behavior of sintered Nd-Fe-B magnets was studied.Jet milled Nd-Fe-B powders were mixed with slight nano-TiC particles,then aligned and pressed,furthermore sintered and tempered.It was found that the nano-TiC particles dispersed in the triple junctions,boundary phases at triple junctions were strengthened and the bending strength of the magnet was increased obviously.The effect of C element on bending strength and fracture behavior of sintered Nd-Fe-B magnets was also studied.C content was regulated by proportions of powder additives.It was observed that the bending strength of the magnet increased with increasing C content.The microstructural and compositional analysis showed that Nd2C3 phase was formed and dispersed at triple junctions,contributing to the increase of bending strength.