Effects of hot pressing temperature on the alignment and phase composition of hot-deformed nanocrystalline Nd-Fe-B magnets

The magnetic properties, alignment and phase composition of nanocrystalline Nd-Fe-B magnets hot deformed (HD) at 830 degrees C employing different hot-pressed precursors fabricated at various hot pressing (HP) temperatures were investigated. Both the remanence and coercivity simultaneously increased first and then decreased with the HP temperature from 450 degrees C to 650 degrees C. The optimum magnetic properties of HD magnets were obtained at HP temperature of 550 degrees C with remanence and coercivity of 1.40 T and 924 kA/m, respectively. The usage of the HP temperatures lower than 500 degrees C was resulted in high porosity and low crystallization degree for hot pressed magnets, causing aggregation of the RE-rich phase and small amount of residual amorphous after hot deformation. This procedure increased the volume fraction of nonmagnetic phase and reduced magnetic properties. Furthermore, the platelet-shaped grains were separated by thick triple junctions, which deteriorated the alignment of grains. When HP temperature was 550 degrees C, the hot-deformed magnet gained the optimum alignment and the stacked platelet-shaped grains were separated by smooth thin grain boundary. However, at HP temperature of 650 degrees C equiaxed coarse grains were dominant instead of platelet-shaped grains in the hot-deformed magnet. The high HP temperature led to apparent grain growth, and then the nucleated grains further grew and were hard to be deformed, developing a poor alignment. A schematic model of the microstructure evolution for hot-deformed magnet is proposed.