A Phenomenological Analysis on the Anisotropic Shrinkage of Nd-Fe-B Compacts during Sintering Process

Magnetic compaction is a well-known and effective method to improve the coercivity of Nd-Fe-B sintered magnets. During magnetic compaction, the application of an external magnetic field causes the anisotropic alignment of Nd₂Fe₁₄B grains as a ferromagnetic phase. After the magnetic compaction, the sintering process is carried out to densify the Nd-Fe-B compacts. In this paper, to identify the relationship between the anisotropic alignment of Nd₂Fe₁₄B grains and the densification behavior during the sintering process, we phenomenologically analyzed the shrinkage of magnetically compacted Nd-Fe-B sintered magnets according to the directions of magnetic field that were perpendicular direction and parallel direction to the magnetic aligned direction. In addition, the shrinkage and the grain size were measured at various sintering temperatures. It was concluded that the difference in the shrinkage and grain growth rates between the directions that are the perpendicular and the parallel could be evaluated based on their anisotropic characteristics and the crystal structure of the Nd₂Fe₁₄B phase. Shrinkage analysis indicated that the perpendicular direction to the magnetic aligned direction yields a larger shrinkage compared with the parallel direction to the magnetic aligned direction. In addition, grain size analysis revealed that the grain size was ∼ 10 % larger in the perpendicular plane as compared with the grain size in the parallel plane.