Anisotropic Fracture Behavior of Sintered Rare-Earth Permanent Magnets

Sintered rare-earth permanent magnets (for example, Sm$_2$Co$_17$, SmCo$_5$, and Nd$_2$Fe$_14$B magnets) are quite brittle and easily crack in the course of fabrication, machining, and application. Here, we report on an investigation of fracture behavior and mechanical characteristics of sintered rare-earth permanent magnets and discuss the origin of rare-earth magnet brittleness. We studied three groups of bending specimens with different orientations, cut from the same block of magnet. The bending strength was measured. The fracture surfaces were carefully observed by scanning electron microscopy and showed that the fracture behavior and bending strength of sintered rare-earth permanent magnets obviously exhibit anisotropy. Sintered Sm–Co magnets tend to cleavage fracture in the close-packed ($0001$) plane or in the ($10bar11$) plane. The fracture mechanism of sintered Nd$_2$Fe$_14$B magnet appears to be mainly intergranular fracture. Our analysis indicates that the anisotropy of fracture behavior and mechanical strength of sintered rare-earth magnets is caused by the strong crystal-structure anisotropy and grain texture created by magnetic field alignment during the fabrication process.