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

Abstract

The phonon dispersion curves of iron-rich Fe - Al alloys have been studied using inelastic neutron scattering. Measurements were performed on three 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 , B2 and A2 for x = 0.25 and 0.30. The migration enthalpies 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 composition. A comparison of the present determination of in Fe - Al with similar -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 compounds. In the phase, the three separate determinations of (present work), of the formation enthalpy of the vacancies , and of the activation enthalpy of the ordering kinetics , which are all average quantities, satisfy the addition law: .