Lattice dynamics and migration enthalpies in iron-rich Fe-Al alloys and ordered DO₃ and B2 compounds

The phonon dispersion curves of iron-rich Fe-Al alloys have been studied using inelastic neutron scattering. Measurements were performed on three Fe_1-xAl_x single crystals at three temperatures, 300 K, 800 K and 1250 K, corresponding to the bcc solid solution for x = 0.2 and to three different states of order DO₃, B2 and A2 for x = 0.25 and 0.30. The migration enthalpies (H_M) are deduced from the phonon density of states through a model proposed by Schober. They decrease with increasing temperature and have a minimum value at the Fe₃Al composition. A comparison of the present determination of H_M in Fe-Al with similar H_M -measurements on Fe-Si confirms the contribution of the effective pair interaction energies to the migration enthalpy, as described in models of Monte Carlo simulations of ordering kinetics in B2 and DO₃ compounds. In the Fe₃Al DO₃ phase, the three separate determinations of H_M (present work), of the formation enthalpy of the vacancies (H_F), and of the activation enthalpy of the ordering kinetics (H_A^{O K}), which are all average quantities, satisfy the addition law: H_A^{O K} = H_F + H_M.