Phase separation and ordering in the Fe-Co system

The Fe50Co50, Fe65Co35, and Fe35Co65 alloys heat-treated between 500 and 1300°C were characterized by x-ray diffraction and transmission electron microscopy. The surface layer of the alloys was found to phase-separate in the range 900–1300°C, leading to the formation of alternating α and γ zones on the order of 0.5 μm in thickness. Phase separation in the bulk of the alloys led to precipitation of an fcc phase, with a volume fraction no greater than 0.1. In the ordering region, the volume fraction of the B2 phase was also no greater than 0.1. The phase-separation region was shown to be divided from the ordering region by a solid-solution phase. Below 550°C, the alloys undergo a phase transformation accompanied by a slight increase in lattice parameter. Comparison with a similar transformation of the Fe60Cr30Co10 alloy suggests that, under certain conditions, this transformation may lead to a significant rise in coercivity. The results are represented in a modified Fe-Co phase diagram.