Study on Ce₂Fe₁₄B-based sintered magnets with Ce/RE> 80

Up to now, the development of Ce/RE> 80% sintered permanent magnets still faces a challenge, owing to their seriously deteriorated microstructure. In this article, the Ce_25.5∼31.5Nd_0∼5Fe_bal.B_1.25M_1.15 (wt.%) sintered magnets with Ce/RE > 80% were investigated, the roles of minor Nd substitution were explored. The typical island-like phase common existed in the grain-boundary (GB) regions of Ce₂Fe₁₄B-based SC alloys (which destroys the continuity of RE-rich GB phase), was confirmed as a tetragonal B-rich phase (fct-RE₅Fe₁₈B₁₈), and probably generated by the peritectic reaction of "L + Ce₂Fe₁₄B → CeFe₂ + B-rich" at 797 °C. It was found that: the deteriorated microstructures of high Ce sintered magnets were hardly improved (Nd element was not enriched in the main phase as it was expected), and the coercivity increments were far below expectations by directly adding 3-5% Nd in alloy designs. However, the GB phase distribution was more uniform and continuous, Nd-rich shells formed, and magnetic properties were remarkably promoted by blending 2% NdH_x powders into the Ce_27.5Nd₃Fe_bal.B_1.25M_1.15 (Ce_27.5Nd₃) magnet during JM milling, compared with the Ce_25.5Nd₅Fe_bal.B_1.25M_1.15 (Ce_25.5Nd₅) magnet with similar composition. The good comprehensive magnetic properties of H_cj = 1.714 kOe, B_r = 9.395 kG, and (BH)_max = 11.16 MGOe have been reached in the Ce_27.5Nd₃+2% (NdH_x) magnet (Ce accounted for 84.5 wt% of total rare earth). The present work deepens our understanding of metallurgical behavior, process/microstructure designing for Ce/RE> 80% sintered magnets, and shed a light on the large-scale utilization of Ce element in a permanent magnet.