Composition dependence of the order-disorder transition in iron-cobalt alloys

The order-disorder phase transition in iron-cobalt alloys has been investigated by neutron-powder diffraction techniques for a composition range from 30- to 70-at.% cobalt. There appears to be a critical order-disorder phase transition throughout the composition range. The plot of the transition temperature against composition shows a peak at 47.9 ± 0.3-at.% cobalt; this is unexpected since simple theories predict a peak at the stoichiometric 50% composition and such a result is not compatible with our data. To within error, the critical index $\beta$ of the order parameter is independent of composition; its weighted mean value is 0.303 ± 0.004.