Detailed TEM analysis of solid-HDDR Nd16Fe76B8 magnetic materials

Detailed microstructural characterization was carried out by means of transmission electron microscopy on a Nd16Fe76B8 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 NdHapproximately 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 Fe2B 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.