Effects of Post-sintering Annealing on (NdLa)-(FeCo)-B Magnets

A common limitation of La substitutions into (Nd, La)₂Fe₁₄B magnets is the reduction of the anisotropy field (H_a), which decreases magnet coercivity (H_cj). Adding a small amount of Pr-Cu to a La-containing neo magnet may modify the grain boundary (GB), which can help recover a fraction of the degraded H_cj. An optimal GB modification requires a carefully designed post-sinter heat treatment, which is the focus of this work on characterizing the effects of multi-step post-sintering annealing on the evolutions of microstructure and magnetic properties. We find that Nd and Cu concentrations at GBs and triple junctions (TJs) increase when the annealing temperatures are lowered. Annealing temperatures of 580°C and 480°C (near rare-earth-rich eutectic temperatures) enabled the development of a thick and continuous GB phase, which helps to magnetically decouple the grains, and, thus, enhance H_cj. For the alloy with 25% Nd replaced by La, and with the addition of 7.5 wt.% of Pr₆₈Cu₃₂ as GB modifier, the optimized multi-step post-sinter annealing improved the H_cj from 4.8 kOe to 9 kOe.