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 hydrogenation-disproportionation-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.