Shear bands in metallic glasses: atomic mobility, relaxation and excess volume

Deformation in glasses proceeds differently than in crystalline materials due to the absence of defined lattice planes and due to the absence of line defects with discrete Burgers vectors. Experiments have shown that deformation exceeding the elastic range is mostly localized in plate-like mesoscopic defects, so-called shear bands. Although the occurrence of shear bands during plastic deformation of metallic glasses is well known, their actual physical properties remain fairly unknown. Here, experimental data on the rate of atomic diffusion within shear bands has been obtained using the radiotracer method on post-deformed specimens. The experimental results indicate unambiguously that the diffusivity is largely enhanced as compared to volume diffusion in the same metallic glass at identical temperatures. This result is also discussed with respect to nanocrystal formation in shear bands.

In order to analyze the local properties of glassy matter within the shear band regions, a new approach based on analytical transmission electron microscopy methods has been developed. In fact, this approach allows to quantitatively determining the local mass density, composition and structural states with nanometer resolution. Thus, shear bands and the surrounding matrix can be analyzed separately and comparatively, including also the local structures that are analyzed by fluctuation electron microscopy. For further property characterization, low-temperature heat capacity measurements that measure the Boson-peak contribution have also been performed on as-quenched, deformed and partially relaxed samples. The experimental results are discussed with respect to the underlying mechanism during the early stages of shear band activation and on the properties characterizing these “defects” in deformed metallic glasses.