Significant changes in the microstructure, phase transformation and magnetic properties of (Nd,Pr)2Fe14B/α-Fe magnets induced by Nb and Zr additions

The effects of Nb and Zr additions on the microstructure, phase transformation and magnetic properties of melt-spun (Nd,Pr)2Fe14B/α-Fe nanocomposite magnets have been investigated. The results show that, for (Nd0.4Pr0.6)8.5Fe85.5B6 ribbons, the metastable (Nd,Pr)3Fe62B14 phase precipitates after the initial crystallization of α-Fe and then decomposes into the final mixture of (Nd,Pr)2Fe14B and α-Fe. For (Nd0.4Pr0.6)8.5Fe84.5Zr0.5Nb0.5B6 ribbons, however, (Nd,Pr)2Fe14B and α-Fe phases precipitate simultaneously from the amorphous phase, which can avoid the formation of metastable phase and refine the grains. During crystallization, Nb and Zr atoms partly enter the 2:14:1 structure and replace rare earth atoms, which increases the volume content of hard magnetic phase. The intrinsic coercivity of (Nb,Zr)-free samples increases from 411 to 534 kA/m for (Nb,Zr)-doped samples, and the maximum energy product increases from 107 to 144 kJ/m3 correspondingly. In addition, due to the Nb and Zr addition, the coercivity mechanism for the ribbons is changed from nucleation of reverse domains to the pinning of domain walls.