Coercivity enhancement of Nd-Fe-B sintered magnet by grain boundary diffusion process using Pr-Tb-Cu-Al alloys

•Tb-rich shells can be optimized by adjusting diffusion source, temperature and time.•Magnetic isolation is enhanced by forming continuous and smooth Nd-rich GBP.•Crystal structure of TJP and GBP transforms from hcp-Nd2O3 to cubic-Nd2O3.•Coercivity of magnet is greatly enhanced with efficient utilization of Tb. Effect of Tb content and diffusion parameters (diffusion temperature and time) on boundary structure and magnetic properties of Nd-Fe-B sintered magnet was systematically studied by grain boundary diffusion process (GBDP) using Pr70−xTbxCu15Al15 (x = 5, 10, 15, 20, atom fraction) alloys. When x increases from 5 at% to 15 at%, coercivities of diffused magnets increase from the original 11.9 kOe to 22.4 kOe for 5 at% magnet (PTCA-5) and 23.5 kOe for 15 at% magnet (PTCA-15), with a slight decrease in remanence. With x increasing to 20 at% (PTCA-20), coercivity of diffused magnet is only 0.6 kOe higher than that of PTCA-15 magnet, with sharp decrease in remanence. So, Tb content in Pr-Tb-Cu-Al alloy should be limited. And when diffusion temperature and time increase from 600 °C and 2 h to 950 °C and 8 h, coercivities of diffused magnet linearly increase first, and then reach stable. The enhancement mechanisms for magnetic properties contribute to magnetic hardening by Tb-rich shells, magnetic isolation by continuous Nd-rich grain boundary phases and reduced lattice misfit between Nd-rich phases and Nd2Fe14B grains. Furthermore, distribution of Tb-rich shells along diffusion length can be optimized by adjusting Tb content in diffusion sources and diffusion parameters, contributing to coercivity enhancement of magnet.