The effect of niobium alloying additions on the crystallization of a Fe-Si-B-Nb alloy

The effect of Nb alloying additions on the crystallization behavior of a Fe-Si-B-Nb alloy was studied. The selected alloy compositions are Fe77.5Si13.5B9 and Fe74.5Si13.5B9Nb3. By comparing the crystallization temperatures, activation energy for crystallization, phases and microstructures formed after annealing, the effect of Nb alloying addition in the Fe-Si-B-Nb alloy was determined. It was found that Nb alloying addition changed the crystallization process from primary crystallization in the case of the Fe77.5Si13.5B9 alloy to eutectic crystallization in the Fe74.5Si13.5B9Nb3 alloy. Nb alloying addition also induced complex phase formation including the Fe23B6, Fe3B and Fe3Si phases. From HRTEM the lattice parameter of the Fe23B6 phase was measured to be 1.073 nm and the Fe23B6 phase was found to have an ordered fcc structure. Interestingly, Nb alloying addition changed the microstructure from the dendritic morphology in the case of Fe77.5Si13.5B9 alloy to equiaxed precipitates in the Fe74.5Si13.5B9Nb3 alloy. In the case of the Fe74.5Si13.5B9Nb3 alloy, the morphology showed little change as the annealing time was increased from 30 min to 24 h at 550 °C. Nb alloying addition decreased the saturation magnetization.