Development of a Control Techique for Microstructures in Nd-Fe-B Magnetic Materials produced by HDDR Process

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The changes in microstructure of Nd-Fe-B and Nd-Fe-Co-B-Zr alloy powders during the hydrogenation-decomposition-desorption-recombination (HDDR) process have been investigated by X-ray diffraction measurements and transmission electron microscope (TEM) observations, in order to clarify the formation of submicron grains of Nd_2Fe_<14>B and the appearance of the magnetic anisotropy in the HDDR powder. It was found that the microstructural changes in the HDDR process start with the formation of cellular and spherical NdH_2 embedded in alpha-Fe matrices. The hydrogenation-decomposition (HD) reaction in the Nd-Fe-Co-B-Zr alloy progresses slower than the one in Nd-Fe-B ternary system. There is no specific crystallographic orientation relationship between the original Nd_2Fe_<14>B and the decomposed phases ; NdH_2 and alpha-Fe. A large amount of boron is contained in the NdH_2 grain in the early stage of the HD process. As the HD process advances, the NdH_2 grain spews out the boron and consequently Fe_2B grains appear at relatively latter stage. The magnetic anisotropy decreases with the formation of the Fe_2B grains in the decomposed texture. In the desorption-recombination (DR) stage, the rim-like Nd_2Fe_<14>B phase is formed by the recombination at the boundaries between the NdH_2 particles and the alpha-Fe matrix. The rim phase is composed of some fine Nd_2Fe_<14>B grains whose crystallographic orientations are strongly correlated with one another. After further hydrogen desorption, the rim phase grows up to form the recombined Nd_2Fe_<14>B fine grains with a strongly correlated c-aixs. The grain size of the recombined Nd_2Fe_<14>B crystal is related to the size and the number density of the NdH_2 particles. It is thus expected to obtain superfine Nd_2Fe_<14>B crystals if the NdH_2 particle before the HD stage can be controlled smaller in size and dispersed more density in the alpha-Fe inatrix.