Structural inhomogeneity of metallic glass observed by ultrasonic and inelastic X-ray scattering measurements

The structural stability of metallic glasses is frequently deteriorated under ultrasonic perturbation at relatively low temperatures, e.g., near the glass transition temperature T_g, even for thermally stable Pd- and Zr-based metallic glasses. By a mechanical spectroscopy analysis, it is suggested that such an instability, i.e., crystallization, is caused by atomic motions associated with the (slow) βrelaxation, that are resonant with the ultrasonic-strain field. Furthermore, such atomic motions below T_g are considered to occur at weakly bonded regions in a nanoscale inhomogeneous microstructure of glass, which was intuitively inferred from a partially crystallized microstructure obtained by annealing a Pd-based metallic glass just below T_g under ultrasonic perturbation. On this basis, we proposed a structural model of metallic glasses that consists of strongly bonded regions surrounded by weakly bonded regions. To reveal the validity of the model, we have also employed the inelastic X-ray scattering technique to measure the sound velocity of nanometer wavelength of longitudinal acoustic phonons. We have found in a completely frozen Pd-based metallic glass that the velocity of nanometer wavelength exceeds ultrasound velocity of millimeter wavelength, which suggests that elastically harder nanoscale regions exist in the glass matrix.