Investigation on the structural characteristics of metallic glasses based on the first displacement excursion behavior in nanoindentation

The first displacement excursion (pop-in) behavior of bulk metallic glasses (BMGs) with different casting diameters and annealing conditions is studied by nanoindentation test using a spherical indenter. It is found that the load, displacement and the well-predicted shear stress corresponding to the first pop-in initiation distinctly increase with the sample diameter and annealing temperature. By using differential scanning calorimetry, it is revealed that the free volume decreases with the sample diameter and annealing temperature, indicating that the first pop-in initiation is greatly affected by the free volume amount in BMGs. Based on the distribution of free volume and cluster derived from a series of BMGs' models, it is revealed that the first pop-in initiation reflects the breakdown of cluster bonds, viz., the cluster bonding characteristic in general, which plays a significant role in understanding of the mechanical properties of BMGs.