Tb–Cu grain boundary diffusion effects on single- and multi-main-phase Nd–Fe–B based magnets

Abstract

Grain boundary diffusion (GBD) has been widely employed in single-main-phase (SMP) Nd–Fe–B magnets for coercivity enhancement. However, for cost-effective multi-main-phase (MMP) magnets, the GBD process has not been well developed. Here, we carry out a systematic study to compare the different element diffusion and demagnetization behaviors in Tb–Cu diffused SMP and MMP magnets. The SMP magnets contain only (Nd,Pr)-based hard magnetic grains, whereas MMP magnets contain both (Nd,Pr,Dy)-based and (Ce,La,Nd,Pr,Dy)-based hard magnetic grains. The results suggest that although the infiltration of Tb is deeper in MMP magnets than in SMP magnets, the coercivity enhancement for MMP magnets is much less than that for SMP magnets. After GBD treatment, the magnetic hysteresis loop squareness of MMP magnets is seriously destroyed, resulting in a low energy product. The deep diffusion of Tb in MMP magnets is mainly attributed to the low melting point of the Ce-rich intergranular phase, which can act as the diffusion channel for the Tb–Cu alloy. However, the strong inter-diffusion between the two main phases in the MMP magnets that occurred during the GBD treatment results in composition homogenization, which is not beneficial to the coercivity and the hysteresis loop squareness. The micromagnetic simulations reveal that the large difference in H_A between the surface and the center of the MMP magnets after composition homogenization by GBD is the main reason for the non-concurrent magnetic reversal and the poor loop squareness. The different GBD effects on the microstructure and magnetic properties of SMP and MMP Nd–Fe–B based magnets obtained in this work may help in further optimizing their GBD process.