A Study Of HDDR Process Utilized In Grain Boundary Control And Surface Processing In Sintered Nd-Fe-B Permanent Magnet

As demands of permanent magnets of high performance from fields of wind turbines,electric automobiles,consumer electronical devices and intelligent manufacturing increasing,sintered Nd-Fe-B magnets have received more attentions.After more than thirty years of technical progress,the maximum magnetic energy of sintered Nd-Fe-B magnet comes near to its theoretical limit,almost perfect.The challenge this material faces now,is to satisfy harsher working environments.On one side,for device like electric motor,a higher coercivity of ambient temperature is required to service in high temperature environment to resist demagnetization.At present,the most efficient coercivity elevation technique is grain boundary diffusion process（GBP）.A shallow layer with high magnetocrystalline anisotropy filed forms by infiltration of heavy rare-earth element（HRE）into the surface layer of grains,achieving utilizing HRE with high efficiency,however,the depth of diffusion is limited.On the other side,severe intergranular attacking failure in environment with high temperature and humidity exists,due to microstructure and chemical properties of the material itself.As for elevating corrosion resistance,metal coating is a primary solution now,however,the costs are high and pollutions are huge.Hydrogenation-Disproportionation-Desorption-Recombination（HDDR）is a technique which can refine grains of some alloys containing rare-earth element（RE）and Fe or Co to nanocrystallines through a reversible hydrogen reaction at elevated temperatures.This dissertation utilized the reaction traits of HDDR elevating coercivity efficiently and improving corrosion resistance.On the one hand,utilized HDDR to refine grains of RE-Fe/Co compounds as secondary alloys to enhance magnetic properties of sintered Nd-Fe-B magnet through precise controls of grain boundary,studied processes of microstructure evolution and element migration during sintering processes of main alloy and secondary alloy and how grain boundary phases and element migration affecting the final magnetic properties.On the other hand,prepared a 50μm thick layer of nanocrystallines on the surfaces of sintered magnets,by a designed HDDR process.Studied main mechanisms of enhanced corrosion resistance and surface mechanical strength by HDDR surface nanocrystallization,elucidating microstructural evolution and its impacts on magnetic properties throughout the process.Main results of this dissertation had concluded were:In the sintering processes of magnets with addition of nanocrystalline secondary alloy,liquid phase had huge impact on diffusion behavior of HRE element into main phase grains.In the area with less liquid,diffusion of HRE element stayed mainly in the shallow surfaces of grains,HRE concentration difference was large at outer and inner parts of grains;in the area with more liquid,the difference of the HRE concentration was smaller.For main alloy with a RE content of 29wt.%,efficiency of coercivity increment had reached 4.17 k Oe/1wt.%Dy.In studies on addition of Dy-Fe and Nd-Co alloys,The well-distributed formation of Nd₆Fe₁₃Ga and Nd₂O₃ with Ia-3 structure were main reasons for the elevation of the final magnetic properties.Had developed about 50μm thick nanocrystalline layer in regular sintered magnet by a well-designed HDDR process.Mechanisms for improved corrosion resistance are,firstly,Nd-rich phases in the suface layer were dispersed,poor in continuity,which blocked intergranular attacking;secondly,a chemically homogenous nano-structure with lower chemical potential formed in the nanocrystallized surface in corrosion environment which made corrosion less active and inhibit formation of corrosion cells;thirdly,refined grains had increased density of grain boundaries,decreasing corrosion current density by decreasing conductivity of surface.In the study of process of the surface HDDR reaction,during the disproportionation process,rod-like structure formed near the grain boundaries first and expands inwards.Oxide grains formed from rod-like structure embedded in the main phase grains and distorted crystal structure formed in the 50～400μm area were main reasons for the deterioration of coercivity of these areas.Impacts of the surface area on the demagnetization process on the bulk magnet can almost be neglected with an increasing size of magnet due to decreases of the volume proportion of the surface layer.