Plastic flow softening in a bulk metallic glass

An experimental investigation into the role of the excess free volume that is created during plastic deformation in strain softening of amorphous metals was conducted. A well-defined and large plastic zone was created through the spherical indentation of a bonded interface of a Zr-based bulk metallic glass (BMG). Elastic modulus and hardness mapping of the deformation zone was conducted through nanoindentation. Experimental observations show that the load, P, vs. depth of penetration, h, curves obtained from the deformation zone are decorated with discrete displacement jumps, which are otherwise absent in the undeformed material. The prior-deformed zone underneath the large indenter was also found to be softer than that far away from the indenter. A simple and approximate analysis shows that the strain softening of the BMG is related to the excess free volume that is created during prior deformation. Contrary to general expectation, differential scanning calorimetry of the deformed material indicates a reduced free volume. These results can be explained by postulating the formation of nanovoids due to the coalescence of the excess free volume. These nanovoids, in turn, lower the stress required for plastic deformation through shear bands, which leads to the observation of reduced hardness.