Layer thickness-dependent hardness and strain rate sensitivity of Cu-Al/Al nanostructured multilayers

Cu-Al/Al nanostructured metallic multilayers with Al layer thickness hAl varying from 5 to 100 nm were prepared, and their mechanical properties and deformation behaviors were studied by nanoindentation testing. The results showed that the hardness increased drastically with decreasing hAl down to about 20 nm, whereafter the hardness reached a plateau that approaches the hardness of the alloyed Cu-Al monolithic thin films. The strain rate sensitivity (SRS, m), however, decreased monotonically with reducing hAl. The layer thickness-dependent strengthening mechanisms were discussed, and it was revealed that the alloyed Cu-Al nanolayers dominated at hAl B 20 nm, while the crystalline Al nanolayers dominated at hAl ≤ 20 nm. The plastic deformation was mainly related to the ductile Al nanolayers, which was responsible for the monotonic evolution of SRS with hAl. In addition, the hAl-dependent hardness and SRS were quantitatively modeled in light of the strengthening mechanisms at different length scales.