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Influence of milling time on the mechanism of phase structure formation in mechanically alloyed Nd-Fe-B magnets.

Kaszuwara W., Leonowicz M.

Archives of Materials Science

2004

Vol. 25, nr 4

523-529

Mechanical alloying is one of the major methods for the processing of nanocrystalline Nd-Fe-B alloys. Long-time milling of elementary powders leads to formation of phase structure consisting of nanocrystalline iron particles embedded in an amorphous matrix. Synthesis of the hard magnetic Nd2Fe14B phase occurs in the course of low-temperature diffusion anealing of the milled material. Milling time appears to be one of the crucial parameters, determining the final phase structure and the material properties. The DSC and Moessbauer spectroscopy studies showed that the milling time affects the mechanism of phase formation. The materials obtained with short milling times contain substantial amount of the Nd2Fe17 phase. One can conclude that the Nd2Fe14B phase is formed by transformation of the Nd2Fe17 phase accompanied by decomposition of the Fe2B phase. The degree to which the transformation occurs is controlled by the milling time. One can neither exclude the apperance of the liquid phase during annealing of the material within a short milling time. Keeping other processing parameters constant, the milling time also affects the microstructure of magnets, especially the distribution of the grain size. The mean grain size decreases substantially with extension of the milling time.