LATTICE-DYNAMICS AND MIGRATION ENTHALPIES IN IRON-RICH FE-AL ALLOYS AND ORDERED DO3 AND B2 COMPOUNDS

qThe phonon dispersion curves of iron-rich Fe-Al alloys have been stud ied using inelastic neutron scattering. Measurements were performed on three Fe1-xAlx single crystals at three temperatures, 300 K, 800 K an d 1250 K, corresponding to the bcc solid solution for x = 0.2 and to t hree different states of order DO3, B2 and A2 for x = 0.25 and 0.30. T he migration enthalpies (H-M) are deduced from the phonon density of s tates through a model proposed by Schober. They decrease with increasi ng temperature and have a minimum value at the Fe3Al composition. A co mparison 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 model s of Monte Carlo simulations of ordering kinetics in B2 and DO3 compou nds. 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(OK)), which a re all average quantities, satisfy the addition law: H-A(OK) = H-F + H -M.