Rate dependence of serrated flow during nanoindentation of a bulk metallic glass

Plastic deformation of Pd-40Ni-20P bulk metallic glass (BMG) was investigated by instrumented nanoindentation experiments over a broad range of indentation strain rates. At low rates, the load-displacement curves during indentation exhibited numerous serrations or pop-ins, but these serrations became less prominent as the indentation rate was increased. Using the tip velocity during pop-in as a gauge of serration activity, we found that serrated flow is only significant at indentation strain rates below about 1-10/s. This result suggests a transition in plastic flow behavior at high strain rates, in agreement with prior studies of BMGs under different modes of loading.