Study On The Phase Equlibria Of La-Fe-B System And Crystal Structure And Magnetic Properties Of Compound

Rare earth Nd-Fe-B permanent magnet material has been widely used in medical equipment,wind power generation,new energy vehicles and other fields due to its superior magnetic properties.However,due to its shortcomings such as low coercivity,poor thermal stability and easy corrosion,which limits its further development and application.Too much reliance on heavy rare earth elements such as Ho,Dy,Tb to improve various properties,eventually,the production cost will be greatly increased.At present,the researchers hope to reduce the production cost and meet the demand of most production fields by adding cheap and abundant rare earth elements such as La and Ce into ndfeb magnets,so as to realize the balanced development of resources.However,the magnetic properties of Nd-La-Ce-Fe-B magnets with La and Ce added are not satisfactory.This is due to the lack of research on the phase equilibrium relationship of La-Fe-B and Ce-Fe-B ternary systems and the thermal stability of the compounds.It is impossible to obtain a phase structure with strong magnetic properties and good intrinsic permanent magnetism through the best heat treatment organization.This is quite disadvantageous for the development of high-performance multi-main phase Nd-La-Ce-Fe-B high-abundance rare earth permanent magnets.At present,it is very necessary for researchers to study the phase equilibrium relationship of La-Fe-B and Ce-Fe-B ternary systems and the crystal structure and thermal stability of related compounds.This is conducive to the preparation of high-performance multi-phase rare earth permanent magnet materials and the realization of the balanced development of resources.The development of multi-main phase Nd₂Fe₁₄B/La₂Fe₁₄B/Ce₂Fe₁₄B high-abundance rare earth permanent magnets is also a hotspot issue in current research.In this paper,the isothermal phase diagram,vertical section phase diagram of La-Fe-B ternary system,crystal structure and thermal stability of the compound were studied in depth.Finally,the high-performance dual-main phase Nd₂Fe₁₄B/La₂Fe₁₄B alloy was designed to obtain the preparation method of dual-main phase rare earth permanent magnet material.The research content can be divided into the following four parts:?1?The phase equilibrium relationship of La-Fe-B system at 600?and 800?was studied by using the method of diffusion couple technique and key equilibrium alloys.Electron probe?EPMA?and electron backscatter diffractograph?EBSD?were used for metallographic analysis of the diffusion interface to obtain the phase equilibrium relationship of La-Fe-B system at 600?and 800?.In order to further determine the accuracy of the phase diagram determined by the diffusion couples,XRD diffractometer and EPMA were used to analyze the phase composition of the key equilibrium alloy at600?and 800?.The experimental results showed that the phase equilibrium relationship of the equilibrium alloy was consistent with the phase diagram determined by the diffusion couples.?2?The electron probe?EPMA?was used to study the solidification characteristics and phase equilibrium relations of 18 as-cast alloys on three vertical sections of La-Fe-B system,La_xFe₈₂B_y?x+y=18?,La_xFe₇₀B_y?x+y=30?and La_xFe₅₃B_y?x+y=47?.A differential thermal analyzer?DSC?was used to measure the heat flow-temperature curves of 18 alloys after annealed at 600?and equilibrium,and the exact temperature of each alloy phase transition was obtained.Finally,the phase transition diagrams of three vertical cross sections with temperature changes are drawn.?3?Single-phase alloy samples of magnetic compound La₂Fe₁₄B were prepared.The crystal structure,thermodynamic stability and generation/decomposition behavior of single-phase alloy samples of La₂Fe₁₄B were obtained by differential heat analyzer?DSC?,electron probe microanalysis?EPMA?combined with X-ray diffraction?XRD?and Rietveld finishing technology.The experimental results show that the compound La₂Fe₁₄B is a high-temperature stabilizer,and the decomposition reaction occurs below 792?:La₂Fe₁₄B?La+Fe+La Fe₄B₄,and the compound reaction occurs from high temperature to 927?:Fe+La+Fe₂B?La₂Fe₁₄B,exists between 792?-927?.?4?Based on the phase diagram and the thermodynamic stability of the compound La₂Fe₁₄B,the double-main phase?La_xNd_y?₂Fe₁₄B rare earth permanent magnet alloy was designed.The composition of the dual main phase alloy after heat treatment at 900?was analyzed by scanning electron microscope?SEM?and XRD diffractometer to determine the composition of the dual main phase permanent magnet alloy.The vibrating sample magnetometer?VSM?was used to test the magnetic properties and absorbing properties of the rapidly quenched thin strip of the dual main phase?La_xNd_y?₂Fe₁₄B alloy.In this paper,the phase diagram of the phase equilibrium relationship of La-Fe-B ternary system,the solidification characteristics and phase transition relationship of the alloy,the crystal structure and thermal stability of the compound,and the design of the dual main phase magnetic properties alloy are deeply studied.This work provides an important theoretical basis for the development of high-performance multi-main phase Nd-La-Ce-Fe-B rare earth permanent magnet materials.