Effects of B addition on the microstructure and magnetic properties of Fe-Co-Mo alloys

The microstructure, precipitation hardening and magnetic properties of melt-spun Fe78Co11Mo11 and Fe74Co11Mo11B4 alloys were studied. It is shown that B addition increases the precipitation reaction temperature of the supersaturated matrix from 605 degrees C for the Fe78Co11Mo11 alloy to 640 degrees C for the Fe74Co11Mo11B4 alloy. In the Fe78Co11Mo11 alloy, a cotton-shaped (Fe, Co)(7)Mo-6 precipitate is observed inside the grains of the matrix phase. In the Fe74Co11Mo11B4 alloy, in addition to the (Fe, Co)(7)Mo-6 phase, spherical MoB2 precipitates are also observed. Compared to the Fe78Co11Mo11 sample, the Fe74Co11Mo11B4 sample exhibits a finer and more homogeneous microstructure with more equiaxed grains. This finer microstructure explains the enhanced hard magnetic properties in the Fe74Co11Mo11B4 sample (M-r = 60.4 Am-2/kg, H-i(c) = 20.8 kA/m) compared to those of the Fe78Co11Mo11 sample (M-r = 56.5 Am-2/kg, H-i(c) = 17.3 kA/m). Furthermore, the Fe74Co11Mo11B4 sample shows better antioxidant ability than the Fe78Co11Mo11 sample, especially when the temperature exceeds 400 degrees C. (C) 2018 Elsevier B.V. All rights reserved.