PHONON-DISPERSION CURVES FOR ORDERED, PARTIALLY-ORDERED AND DISORDERED IRON ALUMINUM-ALLOYS

Neutron inelastic scattering determination of the phonon dispersion curves in high symmetry directions for iron-aluminium alloys are presented. The alloys have compositions near 25 atomic percent aluminium, and were prepared in three states of ordered BCC, B2 and D0(3) structures. The data were analysed by fitting sixth-nearest neighbour Born-von Karman models. The phonon frequencies follow a consistent trend in the ordering sequence BCC to B2 to D0(3). A similar trend is apparent in the values of the atomic force constants for the nearest neighbours but is less apparent for the force constants for more distant neighbours. Of the low frequency modes, the soft [qq0] transverse mode is the most sensitive to changes in the atomic force constants. High frequency modes exhibit large changes in frequency as a result of ordering because of the large difference in the masses of iron and aluminium atoms, as well as the changes in force constants.