Magnetic properties and rare earth element diffusion behavior of hot-deformed nanocrystalline dual-main-phase...

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.