Achievement of high coercivity in sintered R-Fe-B magnets based on misch-metal by dual alloy method

TLDR

The R‑Fe‑B sintered magnets were fabricated by a dual‑alloy method using varying ratios of misch‑metal Fe‑B and Nd‑Fe‑B alloys. High misch‑metal content severely reduces coercivity, but maintaining MM/R ≤ 21.5 % yields practical Br ≥ 12.1 kGs, Hcj ≥ 10.7 kOe, and (BH)max ≥ 34.0 MGOe, with grain‑boundary diffusion further improving Hcj; the loss of magnetic performance is mainly due to microstructural deterioration and phase changes rather than matrix‑phase degradation, as evidenced by Ce segregation and CeFe₂ formation at high MM ratios.

Abstract

The R-Fe-B (R, rare earth) sintered magnets prepared with different ratio of alloys of MM-Fe-B (MM, misch-metal) and Nd-Fe-B by dual alloy method were investigated. As expected, the high ratio of MM-Fe-B alloy degrades the hard magnetic properties heavily with intrinsic coercivity lower than 5 kOe. When the atomic ratio MM/R ≤ 21.5% the magnetic properties can reach a practical level of Br ≥ 12.1 kGs, Hcj ≥ 10.7 kOe, and (BH)max ≥ 34.0 MGOe. And the effect of Hcj enhancement by the grain boundary diffusion process is obvious when MM/R ≤ 21.5%. It is revealed that the decrement of intrinsic magnetic properties of R2Fe14B matrix phase is not the main reason of the degradation of the magnets with high MM ratio. The change of deteriorated microstructure together with phase component plays fundamental roles in low Hcj. In high MM ratio magnets, (a) after annealing, Ce atoms inside main phase are inclined to be segregated in the outer layer of the main phase grains; (b) there is no thin layer of Ce-rich phase as an analogue of Nd-rich phase to separate main phase grains; (c) excessive Ce tends to form CeFe2 grains.