Substantial coercivity enhancement in Dy-free Nd-Fe-B sintered magnet by Dy grain boundary diffusion

Abstract

To date, the highest coercivity reported for Dy-free Nd–Fe–B sintered magnets under the Dy grain boundary diffusion (GBD) process is ∼2.4 T. Therefore, exploring various approaches for achieving higher coercivity is beneficial for eliminating the dependence on the scarce and expensive heavy rare-earth elements, Dy and Tb. In this study, we report record-breaking high coercivity by separately diffusing Dy70Cu30 and then Pr68Cu32 eutectic alloys through grain boundaries in fine-grained Dy-free sintered magnets. The two-step GBD-processed sample containing a low Dy content (only 0.9 wt%) exhibited a high coercivity of 2.8 T and an excellent temperature coefficient of coercivity of β = -0.447%/ °C. The second GBD process facilitated Dy infiltration from the bulk surface toward the center by chemically induced liquid film migration to form a uniform Dy-rich shell throughout the bulk. Pr was also enriched in the Dy-rich shell near the surface of Nd2Fe14B grains in the two-step GBD-processed sample. However, the Pr-enrichment in the shell did not cause the degradation of the coercivity at high temperatures. This unique shell structure strengthened the anisotropy field to achieve 2.8 T coercivity without alloying Dy to the initial sintered magnet.