Surface modification of Nd-Fe-B feedstocks by Cu nanoparticles to refine the microstructure after melting and rapid solidification

Nanoparticle (NP) surface modification of powder feedstocks has been demonstrated to achieve finer, more equiaxed grains, unlocking potential for the application field of functional materials. However, for high-performance Nd-Fe-B hard magnets, where microstructure, composition, and grain boundary control are crucial, this nano-modification strategy is underexplored. Accordingly, our study investigates the influence of surface loadings (0.1–2.5 wt%) of monomodal 8 nm Cu NPs on the microstructure and hard magnetic properties of the commercially available Nd-Fe-B-based powder feedstock MQP-S processed via the melting and rapid solidification process of suction casting (SC). To circumvent undesirable reactions with the highly reactive rare-earth elements during sample processing, Cu NPs were produced surfactant-free via laser ablation in acetone. Our findings reveal that the microstructure of unmodified samples is composed of dendritic Nd-Fe-B-rich grains and intergranular Zr-Ti-rich grains caused by the melting and rapid solidification process. Notably, the addition of Cu NPs results in a finer and more equiaxed grain microstructure. But we also found the formation of a surface loading-dependent amount of α-Fe and Zr-Ti-rich phases, gradually diminishing the overall magnetic properties, in particular at NP loadings ≥ 1.0 wt%. However, low surface modification of 0.1 and 0.5 wt% yielded 15 % and 13 % higher coercivity than the unmodified feedstock processed via SC. Overall, our study underscores the potential of minute amounts (≤ 0.5 wt%) of Cu NP additions in improving the microstructure and magnetic properties, warranting further research on their transferability to other thermal powder processing techniques like laser powder bed fusion.