Enhancing the temperature and chemical stability of sintered Nd-Fe-B magnets by Al and Ni assisted Tb grain boundary diffusion

The temperature stability and chemical stability of Nd-Fe-B permanent magnets need to be improved to extend their service life. In this work, Tb75Al25, Tb75Al10Ni15 and Tb75Ni25 (at%) alloys were employed for grain boundary diffusion (GBD) of a sintered Nd-Fe-B magnet with intrinsic coercivity (Hcj) of 1171 kA/m, and Tb75Al10Ni15 alloy exhibits the best performance. After Tb75Al10Ni15 alloy diffusion, the Hcj of the magnet has been enhanced to 2030 kA/m due to the formation of core-shell structured hard magnetic grains with high surface anisotropy field. As a result, the improved temperature stability with coercivity temperature coefficient of -0.564 %/degrees C in the temperature range from 25 degrees C to 150 degrees C has been obtained. The enhanced corrosion resistance by Tb75Al10Ni15 alloy diffusion is manifested by the reduced corrosion current density from 12.48 to 0.22 mu A & sdot;cm- 2 in 3.5 wt% NaCl solution, due to the optimized elemental distribution in the grain boundary (GB) phase. In Tb75Al10Ni15 diffused magnet, microstructure characterizations show that Al mainly stays in the GBs, increasing its wettability with the grains and the magnetic isolation between neighboring grains. Ni diffuses into the magnet through GBs, and the diffusion of Ni improves the chemical stability of GB phase and reduces the potential difference between the hard magnetic grain and GB phase. The deep diffusion of Ni has been verified by electrochemical analysis of the magnet from surface to center region. The distributions of Al and Ni atoms in the magnets have been demonstrated by first-principles calculation of substitution energies. The present work suggests that by fully understanding and utilizing the advantages of different elements in Nd-Fe-B magnets, excellent temperature and chemical stability can be obtained.