AIP Digital Archive
A high-resolution scanning electron microscope was used to study the microstructural changes occurring during the desorption and recombination stages in a cast Nd14Fe79B7 alloy HDDR processed at 780 °C. Phase identification was based on backscattered electron contrast. In the early stages of desorption, the disproportionated structure in the matrix coarsens to produce a mixture of Nd-rich (NdH2 and/or Nd) and Fe-rich (αFe and/or Fe2B) clusters. At the center of the specimen, regions of recombined multicrystalline Nd2Fe14B phase are observed associated with Nd- and Fe-rich clusters. When desorption of hydrogen is almost complete, the microstructure consists of a Nd2Fe14B matrix with a few remaining Nd- and Fe-rich clusters. The subsequent completion of recombination results in a multigrained Nd2Fe14B structure, some grains being separated by Nd-rich material. The magnetic properties of the Nd14Fe79B7 alloy have been measured at significant stages during the desorption/recombination process at 600 °C, the stages being deduced from electrical resistivity measurements. Initially, the material is noncoercive with a high magnetization at 1100 kA/m, reflecting the high proportion of αFe in the sample. As recombination proceeds, the magnetization drops and the coercivity increases as the fine-grained Nd2Fe14B develops and the αFe disappears.
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