The microalloying effect of Cu and Nb on the microstructure andmagnetic properties of an Fe3B/Nd2Fe14B nanocomposite permanent magnethas been studied by transmission electron microscopy (TEM) and atom probefield ion microscopy (APFIM). Additions of Cu are effective in refining thenanocomposite microstructure and the temperature range of the heat treatment tooptimize the hard magnetic properties is significantly extended compared withthat of the ternary alloy. Combined addition of Cu and Nb is further effective inreducing the grain size. Optimum magnetic properties obtained by annealing amelt-spun Nd4.5Fe75.8B18.5Cu0.2Nb1 amorphous ribbon at 660?C for 6 minare Br = 1.25 T, HcJ = 273 kA/m and (BH)max = 125 kJ/m3. The soft magneticFe23B6 phase coexists with the Fe3B and Nd2Fe14B phases in the optimummicrostructure of the Cu and Nb containing quinternary alloy. Three-dimensionalatom probe (3DAP) results show that the finer microstructure is due to theformation of a high number density of Cu clusters prior to the crystallizationreaction, which promote the nucleation of the Fe3B phase. The Nb atoms appearto induce the formation of the Fe23B6 phase when the remaining amorphousphase is crystallized.
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