Self-Diffusion in Bulk Metallic Glasses

We have studied self-diffusion in the bulk metallic glasses Zr46.75Ti8.25Cu7.5Ni10Be27.5 and Zr65Cu17.5Ni10A17.5 by means of the radiotracer method. Diffusion of 63Ni has been investigated as a function of temperature in both alloys and also as a function of hydrostatic pressure in Zr46.75Ti8.25Cu7.5Ni10Be27.5. With the isotope 95Zr diffusion studies of the major component Zr were performed in Zr46.75Ti8.25Cu7.5Ni10Be27.5. The diffusivity of 95Zr is much smaller than that of 63Ni. The temperature dependence of 63Ni self-diffusion into Zr46.75Ti8.25Cu7.5Ni10Be27.5 and into Zr65Cu17.5Ni10A17.5 cannot be described by a single set of Arrhenius parameters, breaks in the Arrhenius curves are observed. We attribute the non-linear Arrhenius behaviour to the transition from the glassy to the supercooled liquid state. For the first time activation volumes of diffusion in a supercooled melt have been determined. From the pressure dependence of 63Ni diffusion in Zr46.75Ti8.25Cu7.5 Ni10Be27.5 we get activation volumes around one mean atomic volume favouring a diffusion mechanism via vacancy-like defects.