The Disorder → Order Transformation in Iron–Cobalt-Based Alloys

Abstract

The rates of ordering and antiphase domain coalescence during isothermal annealing of quenched equiatomic iron–cobalt and iron–cobalt–vanadium alloys have been studied in the range 400–600° C. Experimental techniques included X-ray diffraction (superlattice line intensity and breadth, and lattice parameter) and measurement of magnetic saturation moment. Amounts of vanadium up to 2.5% do not significantly affect the ordering kinetics in these alloys. It is suggested that the transformation bcc → L2₀ in this system occurs by a homogeneous process rather than by nucleation and growth of ordered domains. Rates of antiphase domain coalescence are affected by vanadium. Coalescence is very rapid in FeCo below 550°C but is relatively slow in FeCo and FeCo-V above 550°C, when a linear (domain size) vs. time relationship is obeyed. The effect of vanadium on the equilibrium degree of order in the range 300–900°C has been assessed on the basis of specific-heat, lattice-parameter, and magnetic-saturation measurements. Vanadium reduces the critical temperature for order by 11 degrees per percent. There is no significant change in the enthalpy (4.4 kJ/mol) of the order → disorder transformation between 0 and 2.5% V.