Effects of annealing processes on properties and microstructure of sintered Nd-Fe-B magnet with gallium and copper additions

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 magnet, and has the best temperature stability of coercivity. The best continuity of grain boundary (GB) phase and the highest content of RE 6 Fe 13 Ga 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 is the Fe-rich phase identified as RE 6 Fe 13 Ga and the Fe-poor phase identified as Ia -RE 2 O 3 . The Ia -RE 2 O 3 phase located at the corner of the TJ region can extend between the grains of (Nd,Pr) 2 Fe 14 B 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 Ia -RE 2 O 3 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. Among different annealing processes, the triple-step annealed magnet shows the highest coercivity and temperature stability. The best continuity of grain boundary phase and the highest content of RE 6 Fe 13 Ga phase can be observed in the triple-step annealed magnet. The separation of Fe-poor and Fe-rich phases occurs at the triple-junction region of the triple-step annealed magnet. • The TS annealed magnet has higher H cj than the SS and DS annealed magnets. • The value of α / N eff increases significantly for the TS annealed magnet. • The separation of Fe-poor and Fe-rich phases occurs at the TJ region of magnet. • The GB phase with low Fe content has a small lattice misfit with the adjacent grains.