Ordering transformations in Fe-50Co based alloys

Abstract

The rates of isothermal ordering in the temperature range 300–550°C of Fe-50Co with additions of 0.4, 1, 1.2, and 2% of vanadium are determined after disordering at 800° and 1100°C followed by quenching to 0°C. Long-range order parameters are measured by X-ray diffraction and relative magnetic saturation; antiphase domain (APD) sizes and configurations are studied by X-ray diffraction and dark-field transmission electron microscopy. Two mechanisms of ordering are apparent for alloys containing less than 1%V: (1) continuous ordering to a foam structure of APDs occurs above 425°C, to be followed by domain coalescence. The latter with the increase in linear dimensions obeying a t ^1/2 law. The measured activation energy for this transformation is 170 kJ mol⁻¹. Quenching from the fcc stability range (1100°C) reduces the rate of ordering; this effect is related to the energies of vacancy formation for the fcc and bcc phases of FeCo. (2) At temperatures of below 510°C alloys containing less than l%V will order by a discontinuous transformation. Nucleation is rapid along all grain boundaries and growth of ordered blocks occurs at the high-angle interfaces. Disordering from higher temperatures reduces the observed rate of ordering; this effect is related to the effect of grain size on sites for discontinuous ordering. The activation energy for discontinuous ordering in FeCo is 105 kJ mol⁻¹. The ordered blocks contain columnar antiphase domains of width about 500 Å increasing to 0.5 μm or greater after about 1 μm of growth. The habit plane of the order/disorder interface is usually {011} of the superlattice. In the temperature range 425–510°C both mechanisms of ordering occur concurrently for the binary FeCo alloy, but increasing amounts of vanadium up to 1% will progressively inhibit the discontinuous transformation. This effect is interpreted in terms of the effect of vanadium on the mobility of the order/disorder interfaces during the early stages of growth.