Microstructure Formation in Strip-Cast RE-Fe-B Alloys for Magnets

During the manufacturing of sintered NdFeB magnets, it is well known that the microstructure of the starting alloy has a strong influence on the processing and the magnetic properties of the product. In this study, we clarify the microstructure formation in strip-cast rare earth (R)-Fe-B alloys used to produce magnets. The microstructure of the alloy surface in contact with the cooling roll and its cross-section were observed using laser microscopy, field emission electron microprobe analysis, and transmission electron microscopy. The orientations of crystal grains were determined by X-ray diffraction and electron backscatter diffraction analyses. Petal-shaped structures were found to cover the alloy surface in contact with the cooling roll, each consisting of a central nucleation region and radially grown Nd₂Fe₁₄B dendritic structures. The nucleation region, consisting of a “disc” and “predendrites”, occurs in the super-cooled region of the contact area between the cooling roll and melt. In the disc region, spherical Nd₂Fe₁₄B particles <100 nm in size are formed in the amorphous R-rich phase, with the overall atomic composition of rare earth:Fe = 1:2. During the primary growth stage of the dendritic structure, each Nd₂Fe₁₄B rod with a different crystal orientation (around 2 to 3 μm in diameter) forms one radially extending crystal grain. As these structures extend to the free surface, crystal grains with orientations close to 〈100〉 in the thickness direction increase in volume. These discs and predendrites observed in the super-cooled area negatively influence the magnetic orientation and sinterability in the produced magnets. Therefore, it is important to avoid excessive super-cooling to obtain optimum magnetic properties.