Detailed TEM analysis of solid-HDDR Nd16Fe76B 8 magnetic materials

Detailed microstructural characterization was carried out by means of transmission electron microscopy on a Nd₁₆Fe₇₆B ₈ alloy during various stages of a S(Solid)-HDDR process (T=800°C, p(H₂)=0.7 bar). The disproportionated, colony-type matrix consists of parallel aligned NdH_-2 cylinders of submicron size, embedded in an α-Fe matrix. There is a crystal orientation relationship within each colony: NdH₂ {100}||α-Fe{100}, NdH₂<001>||α-Fe<001>. Grains of Fe₂ B with an average size of 100 nm show planar crystal defects and are randomly distributed within the matrix. The recombination starts with a coarsening process in each colony. In an early recombined Nd₂Fe₁₄B matrix, remaining spherical Nd-rich and α-Fe regions were found. Further vacuum annealing leads to a refined Nd₂Fe₁₄B matrix with some remaining α-Fe inclusions. Due to Nd diffusion from the grain boundaries into the disproportionated regions, some Nd-rich material can be found at the triple points of the recombined Nd₂Fe₁₄B grains