LATTICE-DYNAMICS AND RELATED DIFFUSION PROPERTIES OF INTERMETALLICS .1. FE3SI

The phonon dispersion of iron-rich Fe-Si alloys of D0(3) structure (sp ace group Fm - 3m) has been studied by inelastic neutron scattering. T he measurements were carried out on two crystals of different composit ion: Fe75Si25 at 20 and 930 degrees C, and Fe80Si20 at 20, 930 and 110 0 degrees C. The respective degree of order was determined by powder d iffraction measurements. A general decrease of phonon frequencies with increasing temperature is found, the temperature dependence being str ongest for transverse phonons with [xi xi 0] propagation. With decreas ing order, phonon gaps close and the highest optical band degenerates into a broad distribution of inelastic intensity due to disorder scatt ering. The dependence of the dispersion on the alloy composition is no t very pronounced. The migration enthalpies as well as several other t hermodynamic quantities have been calculated from the densities of sta tes. Although the low frequencies of the transverse phonons indicate p articularly low migration barriers, they cannot explain the pronounced change of the diffusivity with composition. This composition dependen ce is tentatively explained by high vacancy concentrations.