X-ray diffraction and calorimetric study of crystallization behavior of melt-spun amorphous alloys of Fe_3B/Nd_2Fe_14B-type nanocomposite permanent magnets

摘要

Effects of small amounts of addition of Cu, Ga, and Nb on kinetics of crystallization reactions in melt-spun amorphous alloys of Fe_3B/Nd_2Fe_14B nanocomposite compositions were investigated by means of powder X-ray diffraction and differential scanning calorimetry (DSC). Impact of small amount of Cu, Ca, and Nb on activation energy of crystallization was clearly observed in the ln(T_I~2/(dT/dt)) vs. 1T plot, where T_I and dT/dt are, respectively, DSC peak temperatures and heating rate of a DSC measurement. Addition of cu was found to cause considerable decrease in crystallization temperaures (T_x) of both Fe_3B and Nd_2Fe_14B. In isothermal DSC measurements slightly below T_x of Fe_3B, a clear two-stage crystallization reaction was observed in the case of Cu-doped specimens in the sequence of Fe_3B followed by Nd_2Fe_14B, the same sequence as was detected by heating runs at a constant heating rate. Nb addition was found to cause increase in T_x of Nd_2Fe_14B which forms simultaneously with Fe_23B_6. The importance of kinetics in designing optimal heating scheme for production of the nanocomposite permanent magnets is discussed in connection with a recent X-ray diffraction experiment using 6 GPa ultra-high pressure performed on a Nd-Fe-Cr-Co-B Fe_3B/Nd_2Fe_14B nanocomposite.