2013 Volume 62 Issue 7 Pages 451-456
Bending strength in hot-deformed Nd-Fe-B magnets which is brittle materials was investigated by 4-point bending test and after that fracture surface was observed with a scanning electron microscope (SEM). Crystal orientation was evaluated using an electron backscatter diffraction (EBSD). Grains were disk-shaped and microstructural anisotropy was observed. Deformation temperature was set to 750°C, 800°C and 850°C in order to control the grain size. Specimens were cut out in four directions from deformed plates to investigate the relationship between bending strength and grain direction. By observing fracture surface, fracture origins can be divided into three groups - a single void, aggregation of voids and contaminant. The contaminant was located in normal grains region with microstructural anisotropy. The others, however, were fine abnormal grains region without microstructural anisotropy. Bending strength increased with increasing deformation temperature. Higher temperature brings about the increase in crystal growth and crack propagation resistance, because crack has to propagate in a zigzag manner along grain boundary. When the defect is large enough, the tip of the defect closes to the microstructural anisotropy region. Namely, the strength is affected by the direction of specimen. On the other hand, when a defect is small, the influence of specimen direction on the bending strength is small. Small defects are buried in a fine abnormal grain region without microstructural anisotropy.
