Enthalpies of formation of ordered iron-cobalt alloys

Liquid tin solution calorimetry has been used to measure the enthalpies of formation of iron-cobalt alloys covering the composition range of 7.6 to 91.0 at.-% iron. Measurements at two temperatures (475 and 775 K) allowed extrapolation to 923 K at which temperature free-energy measurements are available for combination with the enthalpies to determine entropies of formation. The low values of these lead to speculation about the possible sources of substantial negative contributions and point to the necessity for low-temperature heat capacity measurements to resolve the problem. The compositional dependence of the enthalpy results indicates a maximum at the non-stoichiometric composition of 53 at-% iron. This, and deductions concerning the value of the order/disorder transformation, are in keeping with previous work on the system.