晶界内多层结构设计及其对钕铁硼磁性和抗蚀性的作用机制研究

Nd-Fe-B magnets, known by their excellent magnetic performance, has become the most widely used member among the permanent magnets family. However, low coercivity and poor corrosion resistance are still the major concerns due to the multi-phase morphology, where the magnetic anisotropy field and electrode potential of Nd-rich intergranular phase are much lower than Nd2Fe14B matrix phase, and by now, the design of the fine structures of grain boundary can not be realized, these has limited the further applications of Nd-Fe-B magnets. In current work, a multilayer grain boundary with A/B/A structure is designed, where A refers to the thin (Nd,Dy)2Fe14B or (Nd,Tb)2Fe14B shell with high magnetic anisotropy field and B is the central intergranular phase with high electrode potential. Magnets with multilayer grain boundary are prepared through blending near-stoichiometric Nd2Fe14B powders with low melting-point Dy/Tb containing alloy powders followed by proper heat treatment. Then the powders are further mixed with low melting-point high electrode potential alloy powders to make the magnets. 3D atom probe (3DAP) and high resolution transmission electron microscope (TEM) will be used to analyze the composition and the phases of the multilayer grain boundary. Based on these results, the influence mechanism of the multilayer grain boundary structure on the magnetic properties and corrosion resistance will be discussed. In this project, fine control of the grain boundary structure can be realized, which is beneficial to improve the general properties with much less heavy rare earth.

钕铁硼磁体因其优异的磁性能，已经成为目前应用最广泛的稀土功能材料。但是，由于尚未实现对晶界结构的精细设计和调控，晶界的磁晶各向异性场和电极电位远低于主相，导致磁体矫顽力低、抗腐蚀性能差，限制了钕铁硼磁体的进一步应用，是目前该领域亟待解决的重要问题。基于这一组织结构根源，本项目拟通过双合金工艺将低稀土钕铁硼主相粉与低熔点重稀土晶界重构合金粉均匀混合并热处理，在主相晶粒边界形成一层高磁晶各向异性的重稀土薄壳层，之后再与低熔点高电位晶界重构合金粉混合并烧结、热处理，最终获得具有重稀土薄壳层/高电位晶界中心层/重稀土薄壳层多层晶界结构的钕铁硼磁体。同时利用高分辨电镜、三维原子探针等手段对晶界多层结构的物相和成分进行细致的表征和分析，并在此基础上深入研究晶界多层结构对磁体磁性能和抗腐蚀性能的影响机制。该研究可实现对晶界结构的精细调控，在提高钕铁硼磁体矫顽力和抗腐蚀性能的同时大大降低重稀土用量。

钕铁硼材料因其优异的磁性能被称为“磁王”，广泛应用在风力发电、汽车制造、医疗器械、电子器件等领域。但其较差的抗腐蚀性能和低矫顽力限制了其进一步应用。晶界富钕相的低电位和高活性是其低抗蚀性的根源，晶界相的成分、结构和分布对磁体的矫顽力影响较大。为了实现磁体磁性能和抗腐蚀性能的同步提升，本项目通过晶界重构技术和双合金工艺开展钕铁硼磁体晶界多层结构的精细调控研究，将低稀土含量的Nd-Fe-B主相粉末和高电位、含重稀土的重构晶界相粉末混合，并严格控制烧结和热处理工艺，制备出了具有稀土薄壳层/高电位晶界中心层/重稀土薄壳层的多层晶界结构的钕铁硼磁体。Nd65Ni35晶界重构磁体晶界中的富Nd相和富Ni相在烧结过程中依次熔化，形成了高电位富Ni层/富Nd层/高电位富Ni层的晶界结构，减少了主相与晶界相的电位差，提升了磁体的抗腐蚀性能。经过590℃热处理的重构磁体，其腐蚀速率仅为商用磁体的十分之一。同时，低熔点的Nd65Ni35晶界相增加了两相的润湿性，形成了连续分布的晶界相，提升了磁体的矫顽力。Dy69Ni31和Dy6Fe13Ga-H含重稀土的晶界重构磁体形成了富Dy高电位层/富Nd相层/富Dy高电位层的晶界结构，在提升磁体抗腐蚀性能的同时在主相晶粒边界形成了高磁晶各向异性的Dy2Fe14B相，在保持磁体抗腐蚀性能的同时进一步提升了磁体的矫顽力。EPMA和TEM分析结果证实了晶界相的多层结构并揭示了磁体抗腐蚀性能和磁性能改善的物理化学机制。项目通过合理设置烧结和热处理温度可以实现对钕铁硼晶界微结构的调控。进而可以实现磁体抗腐蚀性能和磁性能的调控。与已有技术相比，晶界显微结构调控不受磁体尺寸和形状的限制，而且几乎不增加生产工序和能耗，可降低磁体总的稀土用量及高矫顽力磁体中重稀土的用量，对研发具有不同性能特点的钕铁硼新材料和降低磁体生产成本具有普遍指导意义。