Effects of Annealing Processes on Properties and Microstructure of Sintered Nd-Fe-B Magnet with Ga and Cu Additions

Abstract

In this work, the properties and microstructure of sintered Nd-Pr-Fe-Co-Zr-Ga-Cu-B magnet prepared by the single-step annealing, double-step annealing, and triple-step annealing processes were studied. The triple-step annealed magnet exhibits the highest intrinsic coercivity of 19.72 kOe, which is a 58.5% enhancement relative to the as-sintered magnets, and has the best temperature stability of coercivity. The best continuity of grain boundary (GB) phase and the highest content of R6Fe13Ga phase can be observed in the triple-step annealed magnet, along with relatively small grain size. After triple-step annealing process, the phase separation occurs at the triple-junction (TJ) region of the magnet, which are the Fe-rich phase identified as R6Fe13Ga and the Fe-poor phase identified as Ia3(—)-RE2O3. The Ia3(—)-RE2O3 phase located at the corner of the TJ region can extend between the grains of (Nd,Pr)2Fe14B main phase to form the GB phase with a Fe content of less than 15 at.%, thereby enhancing the magnetic isolation effect. The lattice misfit between the Ia3(—)-RE2O3 phase and the adjacent main phase is less than 5%, which is helpful to reduce defects at the edges of the main phase grains, thus reducing the nucleation of reverse domains.

Keywords: Nd-Fe-B, Sintered magnet, Annealing process, Microstructure, Magnetic properties