Improvement of thermal stability in hot-deformed Nd-Fe-B magnets by grain refinement

The room-temperature coercivity of the hot-deformed magnet increased from 1.21 to 1.86 T and the coercivity at 180°C increased from 0.33 to 0.70 T by adding 8 wt.% Nd60Fe15Ga20Cu5 alloy powders. The temperature coefficient of coercivity increased from −0.469 to −0.402%/°C and is superior to that of most magnets. The microstructure revealed that the Nd2Fe14B grains were isolated by Nd-Ga-Cu-rich intergranular phases. Statistical analysis indicated that the average grain size was reduced by approximately 25%. Both the exchange decoupling between grains and the ultrafine grains were responsible for the improved thermal stability of the magnets without using heavy rare-earth elements.