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 °C for the Fe78Co11Mo11 alloy to 640 °C for the Fe74Co11Mo11B4 alloy. In the Fe78Co11Mo11 alloy, a cotton-shaped (Fe,Co)7Mo6 precipitate is observed inside the grains of the matrix phase. In the Fe74Co11Mo11B4 alloy, in addition to the (Fe,Co)7Mo6 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 (Mr = 60.4 A m2/kg, iHc = 20.8 kA/m) compared to those of the Fe78Co11Mo11 sample (Mr = 56.5 A m2/kg, iHc = 17.3 kA/m). Furthermore, the Fe74Co11Mo11B4 sample shows better antioxidant ability than the Fe78Co11Mo11 sample, especially when the temperature exceeds 400 °C.