Microstructural changes in Nd₂Fe₁₄B permanent magnet alloys during HDDR phenomena

Microstructural changes in Nd-Fe-B alloys during the hydrogénation, 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 NdH₂ rods embedded in an a-Fe matrix, at the first stage of the disproportionation reaction. Some relationship in crystallographic orientation was observed among the NdH₂, α-Fe, and the undecomposed Nd₂Fe₁₄B 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 Nd₂Fe₁₄B 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.