Effect of loading rate on creep behavior and shear transformation zone in amorphous alloy thin films, and its correlation with deformation mode transition

The effects of loading rate on creep behavior and shear transformation zone (STZ) in magnetron sputtered La-Co-Al and Zr-Cu-Ni-Al amorphous alloy thin films were characterized through instrumented nanoindentation with a spherical indenter. It was revealed that with an increase in loading rate, both thin films became harder with a narrower distribution of mechanical response, and exhibited more pronounced creep displacement with higher creep strain rate. Based on cooperative shear model, both STZ volume and activation energy were calculated by measuring the strain rate sensitivity during creep, showing an increasing tendency with loading rate. However, the strain rate sensitivity was found to decrease with loading rate. Furthermore, structural heterogeneity density, estimated by analyzing the stressed volume at yielding, decreased with loading rate. Therefore, with increasing loading rate, the STZ event can only be activated at detectable structural heterogeneities with relatively larger volume, and the number of available fertile sites was decreased, resulting in postponed shear band formation and suppressed deformation mode transition.