Effects of loading rate on development of pile-up during indentation creep of polycrystalline copper

Nanoindentation tests with loading rates spanning three orders of magnitude were carried out on annealed polycrystalline copper. In addition to the hardness increasing with loading rate, the formation and development of pile-up around the indentation sites were also found to be strongly rate-dependent. The development of pile-up with increased time at peak load was found to be sensitive to the prior loading rate, being much larger for tests at 50 mN/s than at 0.05 mN/s. The underlying mechanisms were investigated in terms of the kinetic aspects of the nucleation and interactions of dislocations, and can be well explained by the activation volume and the strain gradient plasticity theory.