Orientation dependence of plastic deformation of sintered Nd-Fe-B magnets at high temperature

While the microstructure at the grain boundary of hot-deformed nanocrystalline Nd-Fe-B magnets is intensively studied, limited works have been devoted to the deformation behavior of the Nd ₂Fe ₁₄ B matrix grain, especially for micrometer scale grains and at high temperatures. In this study, the plastic deformation behavior of the Nd ₂Fe ₁₄ B matrix grains was studied by uniaxial compression from 1050°C to 1120°C using both anisotropic and isotropic sintered Nd-Fe-B magnets. The anisotropic magnets were orientated with the c-axis 0°, 45° and 90° to the loading axis. According to the Arrhenius-type power law relationship, the stress exponent n increased from 1.5 to 2.7 when the c-axis changed from 0° to 90° to the loading direction and the corresponding apparent activation energies Q increased from 384 to 437 kJ/mol. Although the observed n > 1 coincides with the interface-reaction controlled solution-precipitation creep mechanism, the determined Q is much larger than those reported in the nanocrystalline magnet. Our study conferred the first demonstration of short straight dislocations array on the (001) plane in the 45°-orientated anisotropic specimen after 10% height reduction. Grain elongation and local misorientation inside the grain were most obvious in the 45°-oriented specimen after compression. The extent of the local misorientation agreed well with the spacing corresponding to geometrically necessary dislocation. The results reveal that grain rotation via grain boundary sliding and dislocation-mediated deformation were operable depending on the loading direction when the micro-sized magnets were deformed above 1000°C.