Atomic transport and structural modifications during shear banding of a bulk metallic glass

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At temperatures significantly below the glass transition temperature, plastic deformation of bulk metallic glasses (BMGs) is well known to localise within shear bands. The resulting structural changes as well as related signatures of calorimetric, kinetic and mechanical responses of the deformed glasses have been thoroughly analysed by post-mortem approaches, i.e. after the applied mechanical stress has ceased. However, the processes during shear banding have scarcely been investigated up to now and the few (quasi-) in-situ studies available are controversially discussed. Preliminary in-situ diffusion experiments performed concurrently to plastic shear deformation indicated an abnormally large enhancement of atomic transport. No such enhancement has been observed for self-diffusion of crystalline Ni deformed under similar conditions. The overarching goal of the present proposal is thus to uncover the mechanisms governing the abnormally fast atomic transport and the related structure modifications by a strategic approach based on complementing in-situ and quasi-in-situ experiments. These analyses shall provide deep and new insight into the mechanisms of shear band activation and propagation. In particular, we shall address a number of unresolved problems which are critical for a comprehensive understanding of the deformation response of BMGs, such as a dependence of the abnormal enhancement of atomic transport rates on the atom size of the diffusing species or the deformation protocol. We shall analyze the interrelation of the transport enhancement with specific chemical re-distributions or structural changes within the shear bands. For these purposes, both radiotracer diffusion experiments and TEM-based investigations need to be performed. Finally, it is the combination of complementing methods and expertise based on dedicated radiotracer diffusion analyses and advanced structural analyses methods, which shall be applied in a concerted way, that shall provide significant advances in the basic understanding of deformation properties of bulk metallic glasses.