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• There is a substantial 2.86 kOe increment of the intrinsic coercivity when B content is reduced from 0.98 wt.% to 0.90 wt.%. • After post sinter annealing, the coercivity has an increment of 65.3% for samples with 0.90 wt.% B. • We have explored a new path to develop NdFeB-based sintered magnets with high comprehensive properties by novel approaches through varying B content. In traditional Nd-Fe-B-based sintered magnets, the composition, role of each element and microstructures have been extensively investigated globally since they were invented in 1983. However, the effects of boron (B) content and post-sinter annealing (PSA) on the microstructure and magnetic properties have been least studied so far and the relative mechanisms are not yet clear. In this paper, we investigated the influence of B on the magnetic performance and microstructure of Nd-Fe-B sintered magnets originally containing copper (Cu), gallium (Ga) and titanium (Ti). It is shown that the intrinsic coercivity has a substantial increment of 2.86 kOe and the remanence has a slight reduction of 0.16 kGs when B content is reduced from 0.980 wt.% to 0.900 wt.%. Moreover, there is a coercivity increment of 27.3% and 65.3% for samples with 0.980 wt.% and 0.900 wt.% B content after PSA, respectively. It is shown that the impacts of B content and PSA are significant and their regulation mechanisms are worthwhile to be studied systematically. Furthermore, it is revealed by microstructural analysis that high coercivity of the sample with 0.900 wt.% B after PSA results from the uniform distribution of Ga, Cu, Nd, and the formation of RE 6 (Fe, M) 14 (RE=Pr, Nd, M=Cu, Ga) compound in triple junction phases. The dilution of Fe content in grain boundary phases (GB phases) also plays an important role. It is found out that decrease of the remanence is mainly due to reduction of the matrix phase and c -axis alignment degree. In this study, we explored a new path to develop Nd-Fe-B-based sintered magnets with high comprehensive properties by novel approaches through varying B content, PSA technique and co-adding trace elements. [ABSTRACT FROM AUTHOR]

• Both the remanence and coercivity are improved after the 2nd annealing process. • The coercivity is hugely increased from 10.09 kOe for the as-sintered sample to 17.19 kOe for the 2nd annealed magnet, with a significant increment of 70.37%. • The matrix grains are enveloped by a (Pr,Nd,Cu,Ga)-enriched layer after the 2nd annealing process. • HRE-free Nd-Fe-B sintered magnets with excellent comprehensive magnetic properties were manufactured by Pr-Fe-Ga boundary addition. Post-sinter annealing process plays an important role in the microstructures and magnetic properties of the Nd-Fe-B sintered magnets. In this paper, systematically investigated are the magnetic properties and microstructures of the as-sintered and post-sinter annealed Nd-Fe-B magnets with Pr-Fe-Ga boundary addition. Two choice consecutive annealing methods are adopted at high and low temperatures, namely the 1st annealing at 880 ºC for 2 h and then the 2nd annealing at 440 ºC for 3 h. It is exceptional to find out that both the remanence and coercivity are improved after 2nd annealing process for this type of magnet. The coercivity is hugely increased from 10.09 kOe for the as-sintered sample to 17.19 kOe for the 2nd annealed magnet, with a significant increment of 70.37% in coercivity. The extraordinary magnetic properties of B r =14.44 kGs, H cj =17.19 kOe and (BH) max =51.08 MGOe are obtained for the designated Nd-Fe-B sintered magnets without heavy rare earth (HRE) elements manufactured by dual alloy method. The Curie temperature is monotonically decreased from 634 K to 602 K while the c -axis alignment degree is optimized after annealing. Microstructural observation and analysis indicate that the elemental distribution patterns are altered after the 2nd annealing. The diffusion of the aggregate (Pr,Nd,Cu,Ga)-rich phase from triple junctions into the grain boundary regions is ascribed to the formation of thin and continuous grain boundary layer, which is critical to improve the microstructures and magnetic properties. [Display omitted] [ABSTRACT FROM AUTHOR]