Magnetic properties and rare earth element diffusion behavior of hot-deformed nanocrystalline dual-main-phase Nd–Ce–Fe–B magnets

Abstract

To inhibit the magnetic dilution effect of Ce in Nd–Ce–Fe–B magnets, a dual-alloy method is employed to prepare hot-deformed dual-main-phase (DMP) magnets using mixed nanocrystalline Nd–Fe–B and Ce–Fe–B powders. A REFe₂ (1 : 2, where RE is a rare earth element) phase can only be detected when the Ce–Fe–B content exceeds 30 wt%. The lattice parameters of the RE₂Fe₁₄B (2 : 14 : 1) phase exhibit non-linear variation with the increasing Ce–Fe–B content due to the mixed valence states of Ce ions. Owning to inferior intrinsic properties of Ce₂Fe₁₄B compared to Nd₂Fe₁₄B, the magnetic properties of DMP Nd–Ce–Fe–B magnets almost decrease with the increase of Ce–Fe–B addition, but interestingly, the magnet with 10 wt% Ce–Fe–B addition exhibits an abnormally increased intrinsic coercivity H_cj of 1215 kA m⁻¹, together with the higher temperature coefficients of remanence (α = −0.110%/K) and coercivity (β = −0.544%/K) in the temperature range of 300–400 K than the single-main-phase (SMP) Nd–Fe–B magnet with H_cj = 1158 kA m⁻¹, α = −0.117%/K and β = −0.570%/K. The reason may be partly attributed to the increase of Ce³⁺ ions. Different from the Nd–Fe–B powders, the Ce–Fe–B powders in the magnet are difficult to deform into a platelet-like shape because of the lack of low melting point RE-rich phase due to the precipitation of the 1 : 2 phase. The inter-diffusion behavior between the Nd-rich region and Ce-rich region in the DMP magnets has been investigated by microstructure analysis. The significant diffusion of Nd and Ce into Ce-rich and Nd-rich grain boundary phases, respectively, was demonstrated. At the same time, Ce prefers to stay in the surface layer of Nd-based 2 : 14 : 1 grains, but less Nd diffuses into Ce-based 2 : 14 : 1 grains due to the 1 : 2 phase presented in the Ce-rich region. The modification of the Ce-rich grain boundary phase by Nd diffusion and the distribution of Nd in the Ce-rich 2 : 14 : 1 phase are beneficial for magnetic properties.