Ordering and phase separation in alloys of the Fe-Co system

Fe50Co50, Fe65Co35 and Fe35Co65 alloys heat treated at temperatures from 500 to 1300 deg. C have been studied by means of the X-ray diffraction phase analysis and transmission electron microscopy. Phase separation of a solid solution has been found at 900-1300 deg. C. That phase separation takes place in all the surface layer in the shape of alternating α and γ layers of width about 0.5 μm each. Phase separation in the interior bulk of the alloys occurs in the form of precipitation of the Co-enriched fcc particles occupied no more than 10% of the bulk. The B2 phase precipitated in the temperature region of ordering also occupies no more than 10% of the bulk of an alloy. Ordering and phase separation regions are divided on the Fe-Co phase diagram by the narrow region of a solid solution. At lowering temperature below 550 deg. C the phase transformation accompanied with a small increase of the lattice parameter takes place. Comparing this phase transformation with the phase transformation in the Fe60Cr30Co10 alloy at the same temperature the conclusion has been made that this is precisely the transformation which at the conditions of the existence of the α1 + α2 cell microstructure brings about the increase of coercive force. The results obtained are shown in the updated Fe-Co phase diagram