Elastic inhomogeneity and acoustic phonons in Pd-, Pt-, and Zr-based metallic glasses

We have investigated acoustic-phonon behavior of Pd-, Pt-, and Zr-based metallic glasses with the combination of inelastic x-ray scattering and ultrasonic (US) measurements. In the dispersion of the longitudinal acoustic modes at small wave number Q, the phase velocity of phonon tends to exceed the US velocity with a wavelength of macroscopic scale for Pd- and Pt-based glasses, whereas the former goes close to the latter for Zr-based glasses. Furthermore, in all the cases, the mean free path L of phonon apparently increases steeply at a certain Q value. These phenomena on the acoustic phonon behavior can be qualitatively explained by the elastic-wave-scattering theory with a simple two-phase model.