Nanoindentation study of size effect on shear transformation zone size in a Ni-Nb metallic glass

An apparent reduction in both size and amount of shear bands was observed, as the sample converted from 45 μm ribbon to 1.8 μm film in a Ni-Nb metallic glass. Such size effect on the macroscopic plastic characteristic could be intrinsically related to the scale variation of the shear transformation zone (STZ). Statistical analysis of the maximum shear stress, which is based on the distribution of first pop-in events in nanoindentation, was employed to estimate the STZ volume and atoms it contains. The result indicates that the STZ size decreases from 83 atoms in the ribbon to 36 atoms in the film. This phenomenon was consistent with the result by another experimental method based on the strain rate sensitivity (SRS). Upon nanoindentation creep test, STZ sizes with 44 atoms and 18 atoms were calculated for the ribbon and film, respectively. The variation trend of the STZ size was discussed in terms of free volume content combined with internal stress which arises during the film deposition and the local atomic structure changes recognized by EXAFS measurement.