Correlated microstructure and magnetic properties of Ce-substituted sintered Nd-Fe-B magnets: Role of various phases at triple junctions and grain boundaries

We report the investigation of the microstructure and magnetic properties of a sintered (Nd,Pr)30.3-xCexFebalM1.4B0.92 (in wt.%, xCe = 0) magnet with varying Ce substitutions (xCe = 2, 4, 6, 8). Structure analysis revealed a lattice contraction with Ce addition of up to xCe < 6, and the further increase in the Ce content to 6 wt.% led to a suppression in lattice contraction. The magnetic properties showed a notable deviation from the linear trend in coercivity and remanent magnetization for the magnets with Ce content of xCe > 4. The detailed microstructure analysis revealed that the magnets with xCe > 4 exhibit the REFe2 phase at the expense of a reduced mass fraction of a REOx phase, suggesting that the generation of the REFe2 phase suppressed the formation of the REOx phase. The site occupancy and valence state of Ce, analyzed using atomic-resolution energy dispersive spectroscopy and electron energy loss spectrometry, established that the suppressed lattice contraction is accompanied by a preferential site occupation of Ce from a random case to a 4f-centric site, along with a valence change from a mixed state to a trivalent state, inferring that Ce3+ ions are rejected from the 4g site of the RE2Fe14B phase to occupy the 4f site with a subsequently reduced number of Ce4+ ions due to the magneto-volume and chemical bonding effects. These findings display a new phase formation pathway of the REFe2 phase from a valence state perspective, i.e., the part of Ce4+ ions is considered to be rejected from the RE2Fe14B phase to nucleate the REFe2 phase, which is later found to separate chemically into RE-rich and Fe-rich regions. Further, a bi-layer interfacial structure, composed of Fe-rich and RE-rich layers, is formed in between the RE2Fe14B and the REFe2 phases due to the weak wettability of the REFe2 phase and a slightly positive mixing enthalpy between Ce and Fe. The deviation in magnetic properties is discussed based on the structural and microstructural results. These findings provide structural insight for developing low-cost and high-performance Ce-substituted Nd-Fe-B magnets.