ON THE SIZE DEPENDENCE OF THE DIFFUSION-COEFFICIENT IN METALLIC GLASSES

Literature data on the diffusion of various diffusants in the metallic glasses Pd40Ni40P20, Fe40Ni40B20 and Pd78Cu6Si16 is analysed. It is shown that the observed activation energy of diffusion, Q*, strongly depends on whether the free volume in the glass is in thermal equilibrium or not. It is concluded that only for PdNiP the diffusion coefficients were measured under thermal equilibrium conditions, and that the observed Q* is in excellent agreement with the prediction based on the free volume model. The diffusion coefficients in the other two metallic glass systems were obtained in what is often called ''the well relaxed state''. It is shown that this corresponds to a state of a nearly constant, non-equilibrium concentration of ''diffusion defects''. Finally it is demonstrated that in these glasses in general Q* is proportional to the volume of the diffusing atom.