Comparative study of Pr/PrFe/PrAl grain boundary diffusion processed Nd–Ce–Fe–B sintered magnets with high Ce substitution: Evolutionary microstructure and magnetic responses

Abstract

Owing to high natural abundance, surplus stockpile and low price of Ce, the Nd–Ce–Fe–B magnet possesses outstanding advantages as a novel permanent alternative with reduced critical rare earth (RE). However, the wide application of Nd–Ce–Fe–B magnet with high Ce substitution is still impeded by the low-coercivity issue. Here we report a comparative study of Pr/PrFe/PrAl grain boundary diffusion processed (GBDP) Nd–Ce–Fe–B sintered magnets to unravel the evolutionary microstructure and magnetic responses towards higher coercivity. Via Pr diffusion-induced microstructural engineering, continuous REFe2 and RE-rich/Fe-lean intergranular phases coexist at a long diffusion depth, with concurrent formation of thick Pr-rich shell surrounding RE2Fe14B grains. The resultant magnetic performance with Hcj = 14.4 kOe, Br = 13.2 kG, and (BH)max = 41.0 MGOe reaches the highest level reported so far upon high Ce substitution of 40 wt%, revealing that Pr GBDP successfully manages to overcome the coercivity-remanence trade-off. For PrFe GBDP, significant quantities of Fe are redistributed to inevitably form Ce-rich REFe2 precipitates at the outmost diffusion surface, which retard the diffusion efficiency and limit the depth of continuous Pr-rich GBs/shells compared to pure Pr diffusion. For PrAl GBDP, extra Al induces ferromagnetic RE–Fe–Al phase to replace REFe2 phase at a deeper diffusion depth, which limits the coercivity increment. Further micromagnetic simulations coincide with the experimental results, revealing the priority of regulating non-ferromagnetic GB phase towards high-coercivity Nd–Ce–Fe–B. Above findings not only deepen our understanding on distinct metallurgical behaviors of Pr/PrFe/PrAl diffusion, but also delight future work on developing Ce-rich Nd–Ce–Fe–B magnets.