Strain rate sensitivity of unequal grained nano-multilayers

Nanoscaled bimetallic Cu/Ta multilayers were fabricated and their deformation behaviors characterized under nanoindentation. The individual Cu and Ta layers had equal thickness (∼30 nm) but quite different grain sizes. By evaluating the hardness of the bi-metal system at various strain rates, a transitional point of its strain rate sensitivity at the strain rate of 10−3 s−1 was observed. Contributions from dislocation and grain boundary (GB) motions to plastic deformation are found to be strongly dependent upon strain rate as well as grain size in alternative constituent layers. Whilst dislocation-mediated motions take up the majority of deformation in a Cu/Ta multilayer at high strain rates, GB motions occurring mainly in the Ta layers take over at low strain rates.