Experimental determination of migration enthalpies in iron-rich Fe-Al alloys and DO3 and B2 compounds

The phonon dispersion curves of iron-rich Fe-Al alloys have been studied by inelastic neutron scattering. Measurements were performed on three Fe1-xAlx 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 al the DO3 stoichiometry. A better understanding of the atomic mobility in DO3 intermetallic compounds is obtained through a comparison of the present results with similar H-M measurements in iron-rich Fe-Si DO3 compounds confirming the contribution of the effective pair interaction energies to the migration enthalpy, as found in Monte Carlo simulations of ordering kinetics in DO3 compounds. In the Fe3Al DO3 phase, the three separated determinations of the migration enthalpy (H-M, present work), of the formation enthalpy of the Vacancies (H-F) and of the activation enthalpy of ordering kinetics (H-A(OK)), that are ail average quantities, satisfy the addition law : H-A(OK) = H-F+H-M.