Atomic scale insights into the segregation/partitioning behaviour in as-sintered multi-main-phase Nd-Ce-Fe-B permanent magnets

Abstract

As-sintered multi-main-phase Nd-Ce-Fe-B permanent magnets typically exhibit a higher coercivity than as-sintered single-main-phase Nd-Ce-Fe-B magnets with identical content of Ce. To clarify the microstructural features that are relevant to the coercivity, as-sintered multi-main-phase Nd-Ce-Fe-B permanent magnets are characterised by high-resolution magnetic force microscopy and atom probe tomography down to the atomic scale. Though inhomogeneous distribution of rare earth elements (Nd, Pr, and Ce) is observed inside 2:14:1 matrix grains, Ce atoms unexpectedly show similar partitioning behaviour (∼14–27% of rare earth element) at the edge of these grains in the vicinity of rare earth-rich grain boundary compared with as-sintered single-main-phase Nd-Ce-Fe-B magnets with a 20% Ce substitute of Nd and Pr. By contrast, the observation of stripe-liked magnetic domains pinned at the grain boundaries and a thin (Nd,Pr,Ce)-rich grain boundary with a composition of Nd9.9Pr2.6Ce19.0Fe65.3Co1.9Balance1.3 (at.%) jointly indicates the enrichment of Ce in grain boundaries benefit to the magnetic isolation of neighbouring matrix grains in the as-sintered state. Unusual Ce-rich clustering regions are also observed in the grain boundary, which also contribute to the coercivity enhancement. The segregation of Ce, Co, and Cu and rare-earth-Cu clusters are only observed at the interface between rare earth-Cu rich phase and matrix phase while no obvious elemental segregation and clusters are observed at the interface between the rare earth-Ga rich phase and matrix phase, suggesting the occurrence of rare earth-Cu rich phase may be one of the reasons for achieving high coercivity. These findings offer fresh insights into the as-sintered multi-main-phase Nd-Ce-Fe-B permanent magnets.