Grain Boundary Restructuring Of Nd₂Fe₁₄B Sintered Magnets By REH_x

Nd-Fe-B sintered magnets, well-known by their excellent magnetic performance, have become the most widely used permanent magnets and the largest products of rare earth (RE) resource consumption. Recently, main research interests of Nd-Fe-B magnets have focued on the two aspects:coercivity enhancement and high efficiency utlization of abundant RE elements. In Nd-Fe-B sintered magnets, the tetragonal 2:14:1 phase determines the theoretical magnetic performance, but the grain boundary phase determines the practical magnetic performance, especially the coercivity (Hcj). In the present work, rare earth hydrides REHX with high reactivity after dehydrogenation have been developed to restructure the grain boundary of RE-Fe-B magnets to form hardening shell with locally higher magnetocrystalline anisotropy field, thus effectively improving Hcj.The main results are as follows:The anisotropic phenomenon caused by grain boundary restructuring of Nd-Fe-B sintered magnet has been observed. Based on comprehensive study of the anisotropic microstructural characteristics of sintered Nd-Fe-B magnet, diffusing DyHx along the direction parallel and perpendicular to the alignment direction (i.e. c-axis) has been conducted, respectively. The results show that the relative volume fraction of Nd-rich grain boundary phase with low melting point in the plane of c-axis is significantly higher than that in the plane of a-axis, thus providing more Dy diffusion channels, increasing diffusion depth and containing higher Dy content in the plane parallel to c-axis during processing. Therefore, more magnetic hardening shells with higher magnetocrystalline anisotropy field have been formed in the plane of c-axis. Magnetic measurement shows that Hcj increases more rapidly to 20.61 kOe (5.76 kOe higher than that of the as-sintered state) when diffusing along the c-axis, which is faster than that diffusing perpendicular to c-axis (18.85 kOe,4.00 kOe higher than the as-sintered state). This work reveals that diffusing the heavy rare earth along c-axis is more effective to enhance the coercivity of the aligned Nd-Fe-B sintered magnets.A small amount of NdHx has been introduced into the Ce-rich (Nd, Ce)-Fe-B sintered magnet to restructure the grain boundary, which significantly improves the coercivity. Based on the binary main phase (BMP) method whose product has higher magnetic performance than that of single alloy method, we utilized the BMP method to fabricate the (Nd, Ce)2Fe14B sintered magnet. However, when the ratio of Ce is higher than 20 wt.%, the coercivity still cannot meets the standard commercial magnet. NdHx was incorporated by BMP method to restructure the grain boundary of (Nd, Ce)2Fe14B (Ce/TRE=27 wt.%) magnet. Results show that Hcj increases from 8.2 to 11.2 kOe by 6.8% of the incremental ratio with remanence matained in the adding amount of 2.0 wt.% NdHx powders. Furthermore, when the amout is 4.0 wt.%, Hcj increases continuously to 13.12 kOe, but its remanence reduces to 1.19 kGs from 1.29 kGs. When optimized addition amout is 2 wt.% NdHx the magnetic performance can reach a pratical level of N40 commercial magnet. The material cost decreases by 25%. EPMA and TEM characterizations show that the thickness of grain boundary phase in the magnet with NdHx addition becomes more obvious than that of initial magnet, which is benefical for demagnetization exchange coupling. The volume fraction of the matrix phase, Ce-rich core but Nd-rich shell, is increases. The depth and content of Nd in the outer regions also rise, thus inhibiting the reverse domain nucleation in the demagnetization process and effectively improving Hcj. The volume fraction of matrix phase decreases, the alignment degree decreases and new phase REFe2 forms due to excessive NdHx addition, are unfavorable for maintaining the remanence.