Does the diffusion mechanism change at the caloric glass transition of bulk metallic glasses?

The glass transition in bulk metallic glasses is of considerable interest from the technological and fundamental point of view. Employing isotope effect measurements by means of a radiotracer technique and positron annihilation lifetime spectroscopy we have investigated changes in the diffusion mechanism and in the local structure of bulk amorphous alloys at the caloric glass transition. The temperature dependence of the average positron lifetime indicates a strong increase in the thermal expansion coefficient of the local "free" volume at the caloric glass transition temperature Tg and thus clearly reflects structural changes. In contrast, the isotope effect measurements do not show any indication of a different diffusion behavior in the deeply supercooled liquid and the glassy state. In both states very low isotope effects suggest diffusion to proceed via coordinated hopping of many atoms. The isotope effect remains constant in amorphous Zr47Ti8Cu7.5Ni10Be27.5 in the investigated temperature range up to about 120 K above Tg. The results confirm the predictions of the mode coupling theory on the nature of the glass transition. The observed structural changes in the supercooled liquid state, particularly the marked increase in "free" volume, show that the increase in slope of Arrhenius plots of diffusion above Tg is not due to an increase in the activation energy.