Microstructural evolution of Fe3B/Nd2Fe14B nanocomposite magnets microalloyed with Cu and Nb

The microalloying effect of Cu and Nb on the microstructure and magnetic properties of an Fe₃B/Nd₂Fe₁₄B 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 nanocomposite 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_0.2Nb₁ amorphous ribbon at 660°C for 6 min are B_r=1.25 T, H_cJ=273 kA/m and (BH)_max=125 kJ/m³. The soft magnetic Fe₂₃B₆ phase coexists with the Fe₃B and Nd₂Fe₁₄B 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 Fe₃B phase. The Nb atoms appear to induce the formation of the Fe₂₃B₆ phase when the remaining amorphous phase is crystallized.