Study On Microstructure Inhomogeneity Evolution And Regulation Of Hot Deformed Nd-Fe-B Magnets

It is generally believed that with the application of heat and plastic deformation,the polydedron-shaped isotropic grains in the original melt-spun ribbon rapidly grow into Nd₂Fe₁₄B platelets with their c-axis aligned roughly parallel to the applied press pressure and normal to the direction of plastic material flow.Theoretically,the magnet in the region with uniform flow stress should have a high remanence and structural uniformity.However,the remanence of the currently prepared hot deformed magnet can only reach 80%of the theoretical value,and the squareness is also poor,which is mainly due to the presence of some inhomogeneity microstructures in the magnet,thereby affecting the magnetic properties of the hot deformed magnet.At present,researchers have tried to improve this phenomenon and some progress has been achieved.But,the reasons for the formation of inhomogeneity structural regions in hot deformed magnets are not clear yet,the lack of theoretical and experimental guidance hinders the structure optimization.In this paper,four main kinds of inhomogeneous structures in hot deformed magnets are systematically studied,and the reasons for formation are also analyzed.It is confirmed that the quasi-periodic coarse grains region in the magnet is derived from the wheel contacted surface of the melt spun ribbons,and the mechanism of coarse grain formation is also analyzed.On this basis,melt spun powder homogenization pretreatment and high melting point second phase addition was designed to improve the microstructure uniformity of the hot deformed magnet,and the mechanism of suppression and orientation of the coarse grain at the interface are studied.Finally the magnetic properties of the hot deformed magnets are successfully improved.The main results achieved are as follows.Firstly,the formation of the coarse grain region in the hot-deformed Nd-Fe-B magnet was studied.Studies have shown that the coarse grain region is formed by the grain growth of the wheel contacted surface of melt spun ribbons.The surface of the melt spun powder has a 30 nm thick oxide layer,which prevents the coarse grains growing across the ribbon interface in hot deformed magnet.Since the melt spun powder in the hot deformed magnet has a stacking structure of wheel contact?W?surface/W surface,W surface/free?F?surface,and F surface/F surface,the combination structure of coarse grain/coarse grain,coarse grain/fine grain and fine grain/fine grain will be formed at the interface of the ribbons.It was found that the grain size of the main phase of the melt spun powder was distributed in gradient from the wheel contacted surface to the free surface.The grain size of the wheel contacted surface is about 20³0nm,and the shape of grains are anisotropy,the grain boundary phase around these extremely small grains is not evenly distributed.Some Nd2Fe14B grains are directly contacted with each other.At hot deformation temperature,due to the high energy of the grain boundary,the contacted small grains grow rapidly through the migration of grain boundary and form equiaxed coarse grains.The grain size of the inner and F side of ribbons is larger than W side,the size is about 50¹00nm,the shape is equiaxed,and the grain boundary phase is evenly distributed.At the temperature of hot deformation,the interface reaction controlled grain growth prevails and forms a anisotropic platelets shape.The grain size in F surface of some thicker ribbons is about 200³00 nm.Due to its large grain size and stable grain boundary,it is not easy to grow at hot deformation temperature,so as to maintain the original sub-micron equiaxed grain structure.Secondly,according to the previous analysis of the formation mechanism of the coarse grain region,melt spun powder pretreatment process was carried out to optimize the orientation of the grains near ribbon interface of the hot deformed magnet and enhance the remanence of the magnet.It is found that the coarse grain region of the magnet can be effectively inhibited by optimizing the grain size and Nd rich phase distribution of the wheel contacted surface of the ribbons.The grain size of wheel contacted surface was optimized by anneling the ribbons at 600?to make the grain size of W surface equal to that of F surface¹00nm.The grain growth reduced the grain boundary energy and promoted the redistribution of Nd-rich grain boundary phase around the grains at W surface.Finally,a hot deformed magnet with remanence of 13.81 kGS and squareness of 0.91 was obtained.In addition,by means of PrCu diffusion treatment of melt spun powder,large amounts of grain boundary phases is introduced in,the average diffusion distance between the Nd₂Fe₁₄B grains is increased,So the growth rate of Nd₂Fe₁₄B grains are slow,and the grain size is almost the same as the original one after diffusion for 2h.During hot deformation process,due enough Nd rich phase,the W surface grains grows to paltelets shape and obtain good orientation through grain boundary sliding.Finally,the effect of high melting point nano-additives on the ribbon interface of hot deformed magnets was systematically investigated.The addition of high melting point phase directly interacts with the interface of ribbon during hot deformation to inhibit grain growth,avoiding the phenomenon of magnet coercivity reduction caused by 600?anneling.The magnetic properties of the hot deformed magnet with nano-additives are all improved,the remanence of the hot deformed magnet with nano BN doping is 14.06 kGs,and the squareness is 0.93 close to the sintered magnets,so it can be deduced that the microstructure uniformity in hot deformed magnet is improved significantly.The inhibition of ribbon interface grain growth by BN and WC nano particles are mainly due to the introduction of vacancy defects,so that the excessive Nd-rich phase in the ribbon will inject into the defect position during the hot deformation process,and the thickness of grain boundary phase of the W surface increases during the injection process,which inhibit the formation of coarse grain.The grain orientation is due to grain boundary sliding and diffusion under uniform stress which is introduced by the nano paticles.Finally a hot deformed magnet with high microstructure uniformity and high degree of texture is obtained.