Effect of atomic size on diffusion in metallic glasses

This thesis research is to study diffusion in amorphous alloys. Systematic investigations of atomic transport in metallic glasses are reported. Direct measurements of diffusion coefficients were performed by using secondary ion mass spectroscopy, by which the diffusion coefficients less than 10$\sp{-20}$ cm$\sp2$/sec can be measured with reasonable accuracy and annealing times. Amorphous Ni-Zr was used as a model system for these studies.It was shown that atomic size plays an important role in the mechanisms of diffusion in this metallic glass. Differences in the diffusion behavior between small and large tracer diffusion atoms in amorphous Ni-Zr suggested that the diffusion mechanisms of the two constituent elements of the alloy are different. The dependence on alloy composition of the tracer diffusion coefficient of smaller atoms was fundamentally different than that of large atoms. The trends of the pre-exponential factors D$\sb{0}$ and activation enthalpies, $\Delta$H, support the idea that small atoms in metallic glass diffuse by localized defects and that large atoms require the collective motions of many atoms. Moreover, the strong correlation of the diffusion coefficient of small atoms and the formation enthalpy required to create a hole of the given diffusion atoms provide the direct evidence that point defect type mechanism dominates small atom diffusion in amorphous Ni-Zr. However, diffusion of large atoms can not be correlated to the hole formation enthalpies. The relevance of diffusion behavior to crystallization kinetics of amorphous Ni-Zr and to the kinetic of solid state amorphization transformation is discussed.U of I OnlyETDs are only available to UIUC Users without author permissio