基于晶界调控的热变形纳米复合永磁取向及性能增强机制研究

There are still some technical and theoretical challenges to obatain the crystallographic alignment of hard magnetic phase and improve the magnetic properties and mechanical properties of bulk nanocomposite permanent magnets. In this research topic, the melt spun Nd2Fe14B/α-Fe alloy was selected as study subject with regard to its modification of the intergranular phase. Based on the addition of high-melting-point alloy （Nb and Zr） and the liquid diffusion of low-melting-point alloy (Zn,Cu,Ga and Al) in the processing, during which the crystallographic alignment of die-upset magnet was therefore enhanced, its microstructure and performance was improved. By the analysis of magnetic properties, mechanical properties and microstructure changes for the alloy under different preparation process, the effect mechanisms of alloy additives on its intergranular phase composition of the nanocomposite magnet were clarified, and the mechanism about the texture, magnetic and mechanical properties enhancement by the grain boundary control was illuminated. Based on further research on the magnetic domain structure and magnetization and reverse magnetization process, the effect mechanisms of the grain boundary control on the magnetization reversal and coercivity of the die-upset nanocomposite magnets was also explored. This study will provide a new approach to prepare bulk anisotropic nano-composite magnets and understand the texture formation mechanism, and provide theoretical and experimental basis to improve the machinability of permanent magnets, which also has a important role in guiding the design, production and application of new high-performance rare-earth permanent magnets.

如何实现全致密纳米复合永磁的硬磁相晶粒一致取向、并提高其磁性能和力学性能仍面临理论上和技术上的挑战。为此，申请者提出利用纳米晶Nd2Fe14B/α-Fe合金的晶界相调控，通过高熔点合金元素（铌和锆）添加以及低熔点金属（锌、铜、镓和铝）在热变形过程中的液相扩散，增强热变形磁体硬磁相取向能力，并改善微观结构和提高磁体性能。通过研究不同制备工艺下磁体的磁性能、力学性能及微观结构的变化，掌握以不同方式添加的合金元素对纳米复合永磁晶界相组成的影响规律，阐明晶界调控增强磁体取向及提高磁体磁性能和力学性能的机制；并进一步研究磁体磁畴结构变化以及磁化与反磁化过程，探讨晶界调控对热变形纳米复合磁体反磁化及矫顽力的影响机制。本项目为全致密各向异性纳米复合稀土永磁的制备及其硬磁相取向机理研究提供一种新思路，为改善其加工性能提供理论和实验基础，对新型高性能稀土永磁的设计、生产和应用具有重要的指导意义。

如何提高永磁磁性能以减小器件体积、减少永磁中稀土含量以减少对日益紧张的稀土资源的依赖是稀土永磁发展的主要趋势。纳米复合Nd2Fe14B/α-Fe永磁合金具有稀土含量低，理论磁性能高的特点。然而，由于其稀土含量较低、相结构不同，利用传统技术很难获得纳米晶复合磁体的理想取向。为改善合金磁性能，项目围绕纳米复合永磁的致密化以及高取向展开一系列研究。主要利用热压热变形技术，结合高熔点合金元素以及低熔点金属的添加，实现合金晶界相改性，从而增强热变形磁体硬磁相取向能力，并改善微观结构和提高磁体性能。.本项目首先研究了Nb的添加对热压、热变形理论配比Nd2Fe14B合金磁性能及微观结构的影响。结果表明， Nb添加改变了晶界元素分布，并可能改善晶界物理性能，从而为Nd2Fe14B颗粒的优先长大创造条件，并导致合金取向和磁性能的提高。以Nd-Fe-Nb-B合金为母合金，并降低稀土含量至11.5at.%。采用混合添加适量Zn粉的方法，热变形后，合金微观结构和磁性能得以明显改善，最大磁能积可达20MGOe。为进一步提高合金矫顽力，利用添加适量Dy以取代Nd，热变形合金矫顽力可达12300Oe。.为弄清元素添加对热变形纳米晶低稀土NdFeB合金的取向增强的影响，研究了Nb和Zn的复合添加、Zr及Co添加对热变形纳米晶NdFeB合金磁性能的影响规律。结果表明，合金磁性能随Nb含量的增加而增加，但只有含Nb合金的磁性能可通过混合Zn粉的方式得以进一步提高。Nb的添加可增加晶界相含量，促进Zn原子扩散进入晶界。这也导致了合金微观结构和c轴织构的改善。在此基础上，制得了各向异性致密Nd10Nb2Fe82B6合金，并研究了热压、热变形合金磁畴的变化。Zr添加对热变形纳米晶Nd-Fe-B-Zn合金的影响与Nb添加类似，均可改善合金取向和磁性能。适量Co取代Fe可提高热变形合金剩磁，但Co含量大于2at.%时，合金磁性能下降。.此外，为提高无稀土MnBi合金磁性能，项目组初步研究了大块各向异性MnBi/Co NWs及MnBi/NdFeB混合磁体的制备，并取得了较好的成果。制备的混合磁体均呈单相退磁特征，Co NWs的添加可提高合金Ms，但并不降低矫顽力；NdFeB的添加则可明显提高合金综合磁性能，而MnBi的添加也可使混合磁体具有较好矫顽力温度系数。