Suppression mechanism of abnormal grain growth by Zr addition in pressless processed Nd-Fe-B sintered magnets

Microstructures of pressless processed (PLP) Nd-Fe-B sintered magnets with and without Zr addition were investigated by scanning electron microscopy and scanning/transmission electron microscopy. In the Zr-free magnet, huge polygonal abnormal grains (AGs) about 100 times larger than the normal grain size were generated by PLP sintering at 975 °C. Electron backscattered diffraction analysis revealed that the PLP sintered magnet had a biaxial oriented texture of fine Nd₂Fe₁₄B grains with the orientation of the AGs significantly deviated from the biaxial oriented grains. By adding a small amount of Zr, fine plate-like Zr-B precipitates with a long period stacking ordered structure derived from the ZrB₂ phase formed on the growth surfaces of Nd₂Fe₁₄B grains with a crystallographic orientation relationship. There were two Nd-rich grain boundary (GB) phases, namely, orthorhombic Nd₃(Co, Cu) and metastable CaF₂-NdO₂, which became liquid during sintering. The boron consumption associated with the formation of Zr-B precipitates increased the Nd-rich GB phases, resulting in a homogeneous distribution of the liquid phase around the Nd₂Fe₁₄B grains. Therefore, the addition of Zr effectively suppressed the abnormal grain growth during PLP sintering owing to the pinning effect of the Zr-B precipitates and the formation of a homogeneous liquid phase distribution.