Atomic ordering of Fe3Co and its impact on the mechanical properties of low-cobalt (18–24 wt%) iron-cobalt alloys

Abstract

B2 ordering behavior and its impact on mechanical properties of Fe–Co alloys with Co content range of 29–50 at% have already been thoroughly clarified. For Fe-25 at% Co alloy, L60 Fe3Co have been predicted by first-principles calculations to be energetically stable, but lack of experimental evidence. In this work, L60 Fe3Co ordering was experimentally demonstrated in slowly cooled Fe-23.7 wt% Co alloy by X-ray diffraction analysis. Different cooling conditions of water quenching (WQ), air cooling (AC), furnace cooling (FC) and slow cooling (SC) were set to determine the effect of ordering behavior on mechanical properties. Fe3Co ordering significantly causes the poor tensile plasticity and impact toughness of Fe-23.7 wt% Co alloy, and a faster cooling rate (e.g. water quenching) is required to suppress the ordering behaviors to improve ductility. For Fe-21.5 wt% Co, Fe-19.8 wt% Co and Fe-18.4 wt% Co alloys, we surprisingly found that Fe3Co ordering has a negligible effect on tensile plasticity and yield strength, but prominently brings about impact brittleness. The average width of paired dislocations configurations in SC Fe-23.7 wt% Co alloy was determined via TEM observation to be ∼29 nm, while ∼8.8 nm for SC 1J22 alloy due to its higher antiphase boundary energies. Our experimental and theoretical widths display a satisfied consistency within a certain error range, further confirming the atomic ordering of L60 structure in slowly cooled Fe-23.7 wt% Co alloy. For these low-cobalt alloys, the suppression of Fe3Co ordering by increasing the cooling rates can significantly improve the mechanical properties.