Optimized microstructure and improved magnetic properties of sintered Nd-Fe-B magnets induced by co-doping high and low melting point elements

Abstract

The control of microstructure provides a significant avenue for enhancing the magnetic properties of sintered Nd-Fe-B magnets. In this work, the grain size of the sintered Nd-Fe-B magnets is reduced by 33 %, the average size of triple-junction phase (TJP) is decreased by 50 %, and the main phase (MP) content remains constantly by co-doping the high melting point element (HMPE) Ti and the low melting point element (LMPE) Ga. It is worth noting that while maintaining a high remanence about 14 kGs, the coercivity of the magnet increased from 13.5 to 17.4 kOe (an increment of 29 %), and both the temperature coefficients of remanence and coercivity were significantly reduced, thereby improving the performance of commercial heavy rare earth (HRE)-free sintered Nd-Fe-B magnets. Detailed microstructure characterization has revealed that the precipitates containing the HMPE Ti effectively refined the grain size of as-cast strips and sintered magnets, while the LMPE Ga promoted the formation of the low melting point (Nd, Cu, Ga)-rich phase. Ultimately, the distribution of the grain boundary (GB) phase around the grains is optimized. The optimization of the microstructure by co-doping trace HMPEs and LMPEs provides a novel research approach for the development of HRE-free sintered Nd-Fe-B magnets with high remanence and coercivity.