Texture Improvement And Controllable Structural Design Of Hot-deformed Nd-Fe-B Mganets

The hot-deformed Nd-Fe-B magnet with an ultrafine-grained structure has an excellent thermal stability and corrosion resistance,which is usually fabricated into the radial ring-like permanent magnets for preparing high-efficiency motors.Thus it has become a powerful competitor for sintered Nd-Fe-B magnets.Restricted by the inhomogeneous microstructures of starting melt-spun ribbons,the as-periodic coarse grains with random orientation emerge at the boundaries of melt-spun ribbons,threatening the integrate magnetic properties of hot-deformed magnets.Besides the unfavorable coarse grains,the highly textured structure of hot-deformed magnets is realized by pressure-induced preferential growth,whereas the crystallographic c-axis of individual Nd2Fe14B grain has misorientation from those of adjacent grains.Due to the rapid hot deformation and cooling processs,the crystallographic misorientation of neighboring Nd2Fe14B grains acclerates the lattice mismatches at the grain boundaries,where the gap of strain energy is pronounced.In view of the deformation mechanism,considerable friction between die and hot-pressed magnets largely suppresses the formation of the near-surface texture.Therefore the[001]texture of the centre region is perior to that of the near-surface region.To eliminate the microstructural and macroscopic defects are of importance to improve the mganetic properties of hot-deformed Nd-Fe-B magnets.This thesis breaks through the already-existing restriction for bulk Nd-Fe-B permanent magnets by grain size.Fistly,the effect of the heat treatment on the microstructural evolution is studied.A remarkable enhancement of remanece can be obtained in hot-deformed magnets after 1st heat treatment(900?).The critical reason for the high-temperature remanence enhancement is discussed by analyzing the grain growth,texture evolution and domain structure.In addition,the microstructure of the ultrathin grain boundaries is studied to reveal the crystallographic misorientation and lattice distortions as well,where the rare twin and moire fringes demonstrate the strain energy.The 2nd heat treatment(700?)promotes the formation of continuous Nd-rich intergranular phases,benefical to the degraded coercivity.Secondly,a special Dy-Cu diffusion technique is introdued into the hot deformation to further improve the coercivity.The good fluidity of Dy-Cu eutectic reduces the near-surface friction and improves near-surface texture.On basis of the optimized microstructure,a high-performance hot-deformed magnet with large maximum energy product of 53 MGOe and coercivity of 0.97 T is achieved after dual-step heat treatment.Inspired by the high-energy-product magnet with a macroscopic gradient upon Dy element and grain size,a macroscopic multilayer composite structure,consisting of high-coercivity and high-remanence melt-spun powders,is designed to overcome the short-range diffusion depth.Heavy rare-earth Dy element diffusion at layers'interfaces accelerates the chemical homogeneity,improving the demagnetization curve.The long-range magnetostatic interaction bewteen adjacent layers largely suppresses the reverse magnetization in the low-coercivity layers.