CHARACTERIZATION OF SOLID-HDDR PROCESSED ND16FE76B8 ALLOYS BY MEANS OF ELECTRON-MICROSCOPY

Systematic microstructural characterisations were carried out on a solid-HDDR processed (T = 800 degrees C, p(H-2) = 0.7 bar) Nd16Fe76B8 alloy by means of SEM, HRSEM and TEM. The advancement of the disproportionation and recombination processes were investigated in detail. In-situ electrical resistivity measurements have been used to monitor the progress of the S-HDDR reaction. It was found that the disproportionated matrix exhibits a colony-type structure consisting of NdH2+/-x and alpha-Fe. Within each colony, rods of NdH2+/-x are embedded in an alpha-Fe matrix. The crystal orientation relationship between the two phases can be described as NdH2 {100} parallel to alpha-Fe {100} and NdH2 [001] parallel to alpha-Fe [001]. Grains of Fe2B with an average size of 100 nm and displaying planar faults could be detected within the disproportionated mixture. Prolonged exposure to hydrogen also results in the decomposition of the Nd1+epsilonFe4B4 phase. After short desorption times, corresponding to an initial coarsening of the disproportionated mixture, the recombination reaction starts by diffusion of alpha-Fe into the Nd-rich regions. Spherical regions of NdH2+/-x or Nd and alpha-Fe can be observed subsequently within a homogeneous, multi-crystalline Nd2Fe14B matrix in the centre of the sample. Within the fully recombined material, Nd-rich phase can be found occasionally decorating the now submicron sized grains. Large, faceted grains grow with prolonged recombinations times, as a result of a solid-liquid reaction when in contact with Nd-rich phase.