Research On Grain Growth Mechanism And Grain Boundary Modification Of Hotdeformed Nd-Fe-B Material

Nd-Fe-B permanent magnetic material have a wide and important applications in the fields of telecom,electronic and automobile and so on based on their excellent magnetic properties.To meet the demands of Hybrid Electric Vehicle and Wind Generator developed in recent years on magnetic materials with high-coercivity and high-remanence,the Nd-Fe-B magnets with good performance is studied extensively at home and abroad.The magnetic properties of Nd-Fe-B material is controlled by the microstructure,such as the grain size,grain boundary chemistry and structure.Hot-deformed nanocrystal Nd-Fe-B magnets exhibit great potential in the coercivity enhancement for its fine grain size.But at present,some unsolved problems,such as the mechanisms of grain alignment and anisotropic grain growth,remanence decrease obviously after grain boundary diffusion process?GBDP?and so on,are still exist.In this research,the mechanism associated with grain alignment and anisotropic grain growth of hot-deformed magnets are explained and the effect of intergranular phase on grain growth and microstructure evolution are studied as well.The effect of anisotropic microstructure on GBDP are analyzed.A method defined as secondary deformation process after GBDP is introduced to improve the comprehensive performance of magnets.The effect of grain size and grain boundary phase on coercivity is discussed.Based on the above research,a new technology defined as high-temperature short-term annealing treatment process is proposed,which can effectively improve the coercivity as well as the remanence of hot-deformed Nd-Fe-B magnets.The relevant research results will provide some solid scientific credentials and technology supports for the development of high performance Nd-Fe-B magnets.The main research contents and results are as follows:Firstly,based on the enough microstructure characterizations,technics optimization and magnetic properties measurements,the controversial grain alignment and grain growth mechanisms of hot-deformed Nd-Fe-B magnets were clarified.The results indicated that the anisotropic grain growth behavior of melt spinning nanocrystal Nd-Fe-B is unrelated with the external pressure.The mechanisms of grain alignment and anisotropic growth of hot-deformed Nd-Fe-B magnets are results of deformation induced grain rotation and grain coalescence.The presence of intergranular Nd-rich phase not only inhibit grain coalescence and growth during hot-deformation but can facilitate the microstructure evolution.The driving force of microstructure evolution is the area decrease of the crystal plane with high surface energy,which is a results of surface diffusion.Secondly,we developed an efficient technology to produce high-coercivity and high-remanence hot-deformed Nd-Fe-B magnets.SEM and XRD analyses showed that this technology can effectively control the volume fraction of the Nd-rich phase and the alignment of Nd₂Fe₁₄B grains in the secondary deformation process.The magnetic property test results indicated that the SD20 sample shows the high-coercivity of 1.97T and high-remanence of 1.30 T.When secondary deformation increases to 50%,the SD50 samples shows the highest?BH?max of 411 kJ/m³.TEM and STEM analyses indicated that intergranular phase of SD20sample are enriched in Nd,Cu and Ga elements and depleted from Fe and Co elements.Further studies indicate that the volume fraction of Nd-rich intergranular phase plays a more important role in affecting the coercivity of magnets than grain size while the grain size in the range from 236 to 444 nm.Meanwhile,the thesis has further studied the effect of anisotropic grain microstructure and anisotropic distribution of intergranular phase on GBDP.The results indicated the hot-deformed Nd-Fe-B magnets have obvious diffusion anisotropic behavior between c-plane and ab-plane of magnets.The diffusion rate and diffusion depth of ab-plane diffused sample are higher than c-plane diffused sample.Thirdly,we proposed a high-temperature short-term annealing treatment process to prepare high performance Nd-Fe-B magnets.The effects of annealing temperature on microstructure,magnetic properties and grain alignment of hot-deformed Nd-Fe-B magnets are studied carefully.The results indicated that the coercivity,remanence and square degree are improved after heat treated at 800??HT800?and 850??HT850?.XRD,SEM,TEM and HRTEM observations indicates that the high-temperature short-term annealing treatment process can effectively optimize the distribution of intergranular phase and Nd₂Fe₁₄B grains alignment of hot-deformed magnets.In specific,XRD partterns and Gaussian fitted curves of orientation deviation indicates that the grain alignment of hot-deformed magnets increases with the increase of annealing temperatre.The coercivity of hot-deformed sample increased from 1.05 T to 1.28 T,remanence improved from 1.41 T to 1.45 T and the maximum energy product?BH?_max increased 382 kJ/m³ to 402.7kJ/m³ after annealing at 800?.The remanence of hot-deformed sample increases with the increment of annealing temperature and the highest remanence of 1.49 T is obtained from sample annealing at temperature of 900?.In addition,with the increase of the annealing treat temperature,the maximum energy product?BH?_max of hot-deformed sample increases first and then decreases.The highest?BH?_max of 405.7kJ/m³ is obtained from HT850 sample,increased by 6%in respect to the hot-deformed sample.Based on these investigations,we suggest that the high-temperature short-time annealing treatment process possess the great potential for the development of the hot-deformed Nd-Fe-B magnets with high-coercivity and high-remanence.