Enhanced magnetic properties and thermal stability of spark plasma sintered multi-main-phase Nd-Ce-Fe-B magnet via co-adding DyF3 and Cu

•Thermally-stable high-performance nanocrystalline MMP Nd-Ce-Fe-B magnet is fabricated by grain boundary engineering.•Hcj and thermal stability are obviously improved with simultaneously enhanced (BH)max.•Microstructure evolution is revealed, and the Dy diffusion model is proposed.•The underlying mechanisms for enhanced magnetic properties and thermal stability are systematically analyzed. Low coercivity and poor thermal stability are still the main challenges for practical application of low-cost Nd-Ce-Fe-B magnet. To solve these challenging problems, a strategy to simultaneously improve the coercivity and thermal stability of Nd-Ce-Fe-B magnet through combining the nanocrystalline multi-main-phase (MMP) structure and grain boundary engineering is demonstrated. In this study, nanocrystalline MMP Nd-Ce-Fe-B magnet was grain boundary engineered by co-adding DyF3 and Cu powders. The results show that the intrinsic coercivity Hcj is significantly improved, and meanwhile optimum maximum energy product (BH)max is achieved at 2 wt% DyF3-Cu. Moreover, the superior reversible temperature coefficients of Hcj (β) and Br (α) are both achieved, implying the improved thermal stability for the DyF3-Cu added magnet. Microstructural and compositional characterizations indicate that the modified RE-rich phase, the inhibited abnormal grain growth near the flake boundary, and the diffused Dy towards main phase grains are the main reasons for simultaneously enhanced magnetic properties and thermal stability. Thus, combining the nanocrystalline MMP structure and grain boundary engineering will be a promising approach for preparing thermally-stable high-performance Nd-Ce-Fe-B magnet.