Effects and regulation mechanisms of post-sinter annealing treatment on magnetic properties and microstructures of Nd-Fe-B sintered magnets with varying boron contents

Abstract

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 RE6(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.