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

Abstract The microstructure, precipitation hardening and magnetic properties of melt-spun Fe 78 Co 11 Mo 11 and Fe 74 Co 11 Mo 11 B 4 alloys were studied. It is shown that B addition increases the precipitation reaction temperature of the supersaturated matrix from 605 °C for the Fe 78 Co 11 Mo 11 alloy to 640 °C for the Fe 74 Co 11 Mo 11 B 4 alloy. In the Fe 78 Co 11 Mo 11 alloy, a cotton-shaped (Fe,Co) 7 Mo 6 precipitate is observed inside the grains of the matrix phase. In the Fe 74 Co 11 Mo 11 B 4 alloy, in addition to the (Fe,Co) 7 Mo 6 phase, spherical MoB 2 precipitates are also observed. Compared to the Fe 78 Co 11 Mo 11 sample, the Fe 74 Co 11 Mo 11 B 4 sample exhibits a finer and more homogeneous microstructure with more equiaxed grains. This finer microstructure explains the enhanced hard magnetic properties in the Fe 74 Co 11 Mo 11 B 4 sample (M r  = 60.4 A m 2 /kg, i H c  = 20.8 kA/m) compared to those of the Fe 78 Co 11 Mo 11 sample (M r  = 56.5 A m 2 /kg, i H c  = 17.3 kA/m). Furthermore, the Fe 74 Co 11 Mo 11 B 4 sample shows better antioxidant ability than the Fe 78 Co 11 Mo 11 sample, especially when the temperature exceeds 400 °C.