MICROSTRUCTURAL EVOLUTION OF Fe_3B/Nd_2Fe_14B NANOCOMPOSITE MAGNETS MICROALLOYED WITH Cu AND NB

摘要

The microalloying effect of Cu and Nb on the microstructure and magnetic properties of an Fe_3B/Nd_2Fe_14B nanocomposite permanent magnet has been studied by transmission electron microscopy (TEM) and atom probe field ion microscopy (APFIM). Additions of Cu are effective in refining the nano- composite microstructure and the temperature range of the heat treatment to optimize the hard magnetic properties is significantly extended compared with that of the ternary alloy. Combined addition of Cu and Nb is further effective in reducing the grain size. Optimum magnetic properties obtained by annealing a melt-spun Nd_4.5Fe_75.8B_18.5Cu_0.2Nb_1 amorphous ribbon at 660℃ for 6 min are Br = 1.25 T, H_cJ = 273 kA/m and (BH )_max = 125 kJ/m~3. The soft magnetic Fe_23B_6 phase coexists with the Fe_3B and Nd_2Fe_14B phases in the optimum microstructure of the Cu and Nb containing quinternary alloy. Three- dimensional atom probe (3DAP) results show that the finer microstructure is due to the formation of a high number density of Cu clusters prior to the crystallization reaction, which promote the nucleation of the Fe_3B phase. The Nb atoms appear to induce the formation of the Fe_23B_6 phase when the remaining amorphous phase is crystallized.