Ultrafine nanocrystalline NdFeB prepared by cryomilling with HDDR process

In this work, the NdFeB ribbons (Nd13.5Fe73Co7·5B6) were first subjected to the hydrogenation disproportionation (HD) to prepare disproportionation NdFeB precursor and subsequently cryomilled for different times. The submicron NdFeB particles with ultrafine nanocrystalline were finally obtained after desorption recombination (DR) process. The results indicated that with the increase of the cryomilling time, Nd2Fe14B grains were refined to nanoscale, but the coercivity decreased due to the strong exchange coupling between the nano-grains. The optimized nanocrystalline NdFeB particles have a size of 40 nm or less and present a coercivity of 6.63 kOe after cryomilling for 10 h followed by DR treatment at 800 °C for 10 min. To further improve the magnetic properties, Ca was added into HD powders in the course of cryomilling process to protect the powders from oxidation and a noticeable coercivity of 9.49 kOe was obtained. Finally, the mechanism of magnetic interactions and coercivity enhancement were discussed.