The Grain Boundary Engineering Of Sintered Nd-Fe-B Permanent Magnet With High Performance

At present,sintered magnet is the leading material in the Nd-Fe-B global markets. With ever-increasing magnetic performances,the thermal stability,corrosion and brittleness of Nd-Fe-B have become matters of concern.These performances partly attributed to the intrinsic properties of Nd-Fe-B,can be strongly affected by the composition and microstructure of grain boundaries.In this dissertation,the NdFeB sintered magnets were prepared by powder metal. The effects of Nd and B on the composition and microstructure of grain boundaries, magnetic properties have been investigated systemically.X-ray diffraction(XRD), scanning electron microscope(SEM),energy dispersive spectrometer(EDS) and optical microscope(OM) have been employed.Effects of annealing temperatures and the ZrC nano-powders additions on the microstructures,corrosion resistance,magnetic and mechanical properties were also studied.The results showed that:The neodymium content of the magnet varied from 11.76%(atom fraction) to 13.6%(atom fraction) and the boron content in the magnet gradually varied from 5.53%(atom fraction) to 6.16%(atom fraction).The effects of the contents of neodymium and boron on the mechanical properties and microstructure of sintered Nd-Fe-B magnets were studied.Found the optimum compound addition ratio of Nd and B,and got the right composition of Nd_13.31Dy_0.48Feb_bal(AlGaNbZr)_0.48B_5.76 for high properties sintered magnet,which exhibited good stability and homogeneity for mass production.Heat treatment studies had permitted to establish some very strong correlation between the magnet microstructure and the variation in properties.The main finding is the systematic presence of the RE-rich phase film in the grain boundaries.The morphology of the RE-rich film/RE₂Fe₁₄B grain interface is the most important factor controlling the resistance to demagnetization.The annealing treatment and the ability to dissolve the interface irregularities at the solid state largely influence this morphology.The results shown that the optimum annealing temperature was 900℃for Nd_13.31Dy_0.48Fe_bal(AlGaNbZr)_0.48B_5.76 magnet. ZrC nano-powders were added into the sintered NdFeB magnets in order to improve their magnetic properties and corrosion resistance.It was found that both remanence and maximum energy product were increased at minor additives,and coercivity was almost invariable.Microstructure observation revealed that ZrC additives were mostly incorporated into the intergranular phases in the magnets. Furthermore,mass loss of magnets with ZrC additives was few,which indicated that intergranular addition of ZrC could improve corrosion resistance of NdFeB magnets without a cost of magnetic properties.