Abstract
Microstructural changes in Nd-Fe-B alloys during the hydrogenation, disproportionation, desorption, and recombination (HDDR) phenomena were studied by XRD and TEM. These phenomena are caused by a heat treatment in hydrogen (H-treatment), in which the disproportionation occurs, and a subsequent heat treatment in vacuum (V-treatment), in which the recombination reaction occurs. The disproportionated mixture is formed of submicron sized colonies, which consist of NdH 2 rods embedded in an α-Fe matrix, at the first stage of the disproportionation reaction. Some relationship in crystallographic orientation was observed among the NdH 2 , α-Fe, and the undecomposed Nd 2 Fe 14 B phases. H-treatment at higher temperatures causes the grain growth of the disproportionated phases, but the relationship in crystallographic orientation remains between the NdH 2 and α-Fe phases. The recombined Nd 2 Fe 14 B grains in the alloys V-treated at temperatures higher than 800°C, exhibit the same texture as the original, untreated alloys, and this grain alignment appears to begin as the recombination reaction occurs. The alloys V-treated at temperatures lower than 800°C show isotropic characteristics.
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