热变形Nd-Fe-B永磁材料晶粒长大及形貌演变机制的多尺度及原位透射电镜表征

Nd-Fe-B permanent magnetic material have a wide applications in the modern industry. Hot-deformed nanocrystal Nd-Fe-B magnets exhibit great potential in the coercivity enhancement for its fine grain size. However, we are still failed to clarify appropriate and clear mechanisms on grain growth and morphological evolution of hot-deformed Nd-Fe-B magnets over the years. This is because that the nanocrystalline grains grow rapidly and microstructure evolution is a complex procedure during hot-deformation process. Also, it is difficult to determine interface information of Nd2Fe14B compound associated to the driving force of grain growth by experimental method. This program aims to investigate the mechanisms of Nd2Fe14B grain growth and morphological evolution during hot deformation process by using multi-scale and in situ TEM and first-principle calculation. The dynamic observation process of grain growth and morphological evolution will be performed by using in situ TEM experiments. We will pay special attention to the origination and evolution rules of crystal defects, such as stacking fault, dislocation and twin boundary, and distribution of alloying elements by using advanced aberration-corrected electron microscopy. The interface energy related to the driving force of grain growth will be calculated based on the first principle calculation. The aim of the present study is to fully reveal the mechanisms of grain growth and morphology evolution of Nd2Fe14B grains. The research results will provide direct guidance information for our understanding and optimizing the microstructure of hot-deformed Nd-Fe-B magnets.

稀土永磁材料在现代工业中有着广泛的应用基础。具有纳米级晶粒尺度的热变形Nd-Fe-B材料因其晶粒尺寸细小在改善矫顽力方面展现出极大的发展潜力。然而，Nd2Fe14B晶粒在热变形过程中长大迅速，且难以直接测量与晶粒长大、晶体形貌演变驱动力相关的界面信息，导致多年来未能给出准确、清晰的晶粒长大及形貌演变机制。本项目拟借助多尺度及原位透射电镜表征，结合第一性原理计算，对纳米晶Nd2Fe14B晶粒的长大及形貌演变机制做系统深入的研究。主要借助透射电镜原位加热技术，动态观察Nd2Fe14B晶粒的长大及形貌演变行为；采用先进的球差矫正电镜技术在原子尺度研究热变形磁体中的孪晶、层错和位错等晶体缺陷的形成和演化规律，同时结合第一性原理计算研究系统中与晶粒长大驱动力相关的界面性质。以期全面揭示Nd2Fe14B晶粒的长大及形貌演变机制。研究结果将为我们更好的认识和调控Nd-Fe-B材料的微观结构提供指导。

澄清具有复杂结构的Nd2Fe14B化合物的孪晶界面的原子排列和界面能是领域内面临的重要难题，本研究首先在热变形纳米晶的粗大晶粒内观察到了大量的(101)孪晶，研究借助具有原子尺度的HAADF-STEM球差校正电镜观察了孪晶界面的原子排列规律，在此基础上建立了(101)孪晶界面的原子模型，并借助第一性原理计算了孪晶界面的能量，研究结果显示，Nd-Fe-B材料中的(101)孪晶界面具有非常低的能量，为0.008 J/m2，研究认为低的孪晶界能诱发了晶粒的快速长大，是出现晶粒粗化的主要原因。项目还采用高温短时退火处理工艺对热变形Nd-Fe-B磁体晶粒长大机制、矫顽力、剩余磁化强度以及方形度的影响，研究表明磁体的矫顽力、剩磁和方形度可以同时获得提高，如经过800℃处理后，磁体的矫顽力由1.05T提升到1.28T；在800-900℃温度范围内，磁体的剩磁由1.41T逐渐提升至1.49T；而方形度则由0.91先增加至0.97，然后下降至0.74。原子尺度的HAADF-STEM分析显示相邻晶粒之间位向差为3°时，出现了明显的晶粒合并行为，这表明在高温短时退火处理过程中位向差较小的相邻晶粒的合并是导致晶粒长大的主要原因。项目还提出采用Tb-Cu合金掺杂结合Nd-Cu合金扩散工艺提高热变形磁体的矫顽力和矫顽力温度稳定性，原子尺度的HAADF-STEM和相应的EDS研究显示，在Tb70Cu30掺杂量为2%和5%的热变形态磁体的晶界处往往存在薄层的富Fe非晶晶界相；同时，在Tb70Cu30掺杂量为5%的热变形磁体中还观察到了厚度约为20 nm厚的核-壳结构；当进一步对掺杂量为2%Tb-Cu掺杂磁体进行Nd-Cu晶界扩散后，在磁体中观察到了明显增多的Nd3Co相和NdCu相以及富Tb的壳层结构，这种微观组织也导致磁体的矫顽力由1.77T提升至2.68T,矫顽力温度系数则由-0.452%/℃提升至-0.358%/℃；研究还进一步分析了核壳微观结构的演化机制及相结构演变机制。

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