Coercivity improvement in Nd-Cu infiltrated Nd-Fe-B hot-deformed magnets by controlling microstructure of initial HDDR powders

We have studied influence of the microstructure of initial hydrogenationdisproportionation-desorption-recombination (HDDR) treated isotropic powders on the magnetic and microstructural properties of Nd-Cu infiltrated Nd-Fe-B hot-deformed magnets. To control the microstructure of the initial HDDR powders, the starting strip-cast alloys were heat-treated prior to the HDDR process (HDDR powder A). Compared to the HDDR powders prepared from the un-treated strip-casts (HDDR powder B), non-uniform and discontinuous Nd-rich intergranular phases were formed in the HDDR powder A. Therefore, most of the Nd-rich phases in the HDDR powder A were agglomerated at the triple junctions after the hot-deformation (Hot-deformed magnet A). In contrast, homogeneous Nd-rich intergranular phases enveloping the laterally elongated grains are observed in the hot-deformed HDDR powder B (Hot-deformed magnet B). As a result, the coercivity of hot-deformed magnet A is lower than that of the hot-deformed magnet B. Interestingly, after the Nd-Cu infiltration, the coercivity of hot-deformed magnet A increases more rapidly than that of the hot-deformed magnet B because the faster diffusion of liquid Nd-Cu via the c-plane intergranular phases occurs in the hot-deformed magnet A, while the agglomeration of Nd-Cu into the triple junctions occurs in the hot-deformed magnet B. High resolution transmission electron microscopy showed that the Nd-Cu infiltration on the hot-deformed magnet A leads to the formation of thicker and non ferromagnetic intergranular phases on the (001) planes of Nd2Fe14B grains, resulting in the magnetic isolation of individual grains and increase of pinning force against the domain wall motion. (C) 2021 The Authors. Published by Elsevier B.V.

Automated summary

The microstructure of initial hydrogenation-disproportionation-desorption-recombination (HDDR) treated isotropic powders significantly influences the magnetic and microstructural properties of Nd-Cu infiltrated Nd-Fe-B hot-deformed magnets. The presence of non-uniform and discontinuous Nd-rich intergranular phases in HDDR powder A leads to lower coercivity compared to HDDR powder B. Nd-Cu infiltration in hot-deformed magnet A results in faster increase in coercivity due to thicker and non-ferromagnetic intergranular phases on the (001) planes of Nd2Fe14B grains, enhancing magnetic isolation and pinning force against domain wall motion.