Quantitative explanation of the enhancement of surface mobility of the metallic glass Pd40Cu30Ni10P20 by the Coupling Model

Surface diffusion coefficient D-S(T) measured for the canonical metallic glass, Pd40Cu30Ni10P20, by the method of surface-grating decay was found to be more than eight orders of magnitude larger than the bulk diffusion coefficient. So far no theory has attempted to account quantitatively for this spectacular enhancement of diffusion at the surface. The objective of the research reported in this paper is the application of the Coupling Model (CM) to predict the size of the surface diffusion found in Pd40Cu30Ni10P20. The prediction compares well with the size of the enhancement obtained by experiment. There is yet another prediction of the CM, which is D-s(T) having the same Arrhenius activation energy as the bulk diffusion coefficient D-v(Co) of the tracer Co-57 atoms in the same metallic glass. This prediction is also verified by the experimental data, and it provides additional support of the theoretical explanation of the enhancement of diffusion at the surface. A successful application of the CM to account for the decoupling of diffusion coefficient of P from that of Pd in the bulk of a similar metallic glass, Pd43Cu27Ni10P20, is recalled because the physics involved therein is similar to the present problem.