Structure relaxation effect on hardness and shear transformation zone volume of a Ni–Nb metallic glassy film

The annealing effect below glass transition temperature (T_g) on nanoindentation hardness and shear transformation zone (STZ) volume was studied in a Ni–Nb metallic glassy thin film. The indentation size effect (ISE) and loading rate effect on hardness were systematically investigated. It was found that ISE was less pronounced and strain rate sensitivity (SRS) was higher in the sample suffered longer annealing time. As annealing time reached a critical value, hardness value, ISE and SRS changed quite limited which could be due to full structure relaxation in samples. The hardness spatial distribution was more scattered with increasing annealing time, indicating a more pronounced mechanical heterogeneity at microscale. Based on SRS values and cumulative probability distribution of hardness, STZ volumes were estimated upon two distinct methods, respectively. A tight correlation between STZ volume and structure state was revealed. As increasing annealing time, STZ volume was continuously decreased and approached down to a stable value.