Detailed SEM studies of the HDDR behaviour of direct-reduced Nd15Fe77B8 and Nd15Fe77-xB8Gax powders

Direct-reduced (DR) Nd15Fe77B8 and Nd15Fe77-8B8Gax powders with x = 0.5, 1, 2 and 4 were treated by the HDDR process at various temperatures and processing times. SEM samples of the as-received, disproportionated (cooled under H-2 atmosphere to prevent recombination) and HDDR treated powders were investigated with a high-resolution (held emission) scanning electron microscope. It was found that the DR powders show different microstructures compared with those of induction melted (IM) Nd-Fe-B type alloys, probably due to faster kinetics and diffusion processes during the HDDR treatment of DR powders. Ga additions were found to inhibit the disproportionation reaction. Increasing Ga content in the samples leads to an increased amount of undecomposed Nd2Fe14B phase after the disproportionation reaction.