Highly Cooperative and Defect-Assisted Diffusion in Metallic Glasses

Measurements of pressure dependence and isotope effect of Co diffusion in melt-spun amorphous Co sub 76.7 Fe sub 2 Nb sub 14.3 B sub 7 ribbon were made. In samples preannealed for structural relaxation the resulting activation volume of )0.06 plus/minus 0.1 atomic volume and the isotope effect of E = 0.1 plus/minus 0.03 allow us to rule out diffusion via quasivacancies. The extremely small E value rather indicates a highly cooperative mechanism. Measurements of the isotope effect at different stages of structural relaxation show a strong decrease of E from E > 0.5 in the as-quenched glass to E = 0.1 in the relaxed state. The large isotope effect in the as-quenched ribbon points to the prevalence of single-atom jumps, which require the presence of vacancylike holes of excess volume.