Time dependent plasticity at real nanoscale deformation

Nanoindentation creep test has been carried out in displacement of 4-100 nm regime at single grained (SG) Cu and ultrafine grained (UFG) Cu. The strain rate sensitivity m has been determined and exhibits strong indentation size effect. The magnitudes of m in UFG and SG Cu are comparable and can be simulated by the same curve below a critical depth. The authors experimental findings and analysis support that the tip-sample diffusion is the dominant mechanism below a critical indentation depth and the effect of grain boundary media mechanisms will emerge as the indenter penetrates deeper.