Microstructure of Nd-rich grain boundary phase in die-upset Nd-Fe-Co-Ga-B magnet

Microstructure of die-upset Nd-Fe-Co-Ga-B magnet was investigated with a scanning electron microscopy (SEM), high resolution and analytical transmission electron microscopy (HRTEM and ATEM). Inside the original melt-spun flakes in the magnet, the typical texture consists of fine platelet Nd ₂(Fe,Co) ₁₄B grains surrounded continuously by a thin grain boundary (GB) phase with 2-3nm in thickness. The GB phase has an amorphous structure of which the chemical composition is Nd ₄₄Fe ₃₉Co ₁₀Ga ₇. It was found that high fluidity of the Nd- and Ga-rich GB phase contributes to produce a high crystallographic c-axis alignment of Nd ₂(Fe,Co) ₁₄B grains. Near the interfaces of the original flakes, morphology of the Nd ₂(Fe,Co) ₁₄B grains changes from platelet to globular involving the deterioration of c-axis alignment. Many Nd-oxide (NdO and fcc-(Nd,Fe)O _x) grains and a large amount of fine Nd ₃Co precipitates are formed near the interfaces, and then the composition of GB phase changes to Nd ₃₆Fe ₅₀Co ₉Ga ₅. The insufficiency of Nd in the GB phase and the existence of Nd-oxide particles prevent the anisotropic growth of Nd ₂(Fe,Co) ₁₄B grains, and then lead to the inhomogeneous GB phase and the low c-axis alignment.