The quantitative evaluation of the plasticity of Nb/amorphous CuNb nanolayered thin films by micro-pillar compressions and micro-indentations as well as their correlation

Micro-indentation (MI) tests have been widely used to investigate the deformation of nanolayered metallic films (NMFs) due to the convenience, simplicity and low cost. However, MI is unable to directly provide a quantitative information on the plasticity of the NMFs because of the complex 3-D stress state. Here, a combinational approach is proposed to address the above critical issue, in which systematic micro-pillar (MC) tests has been first conducted to investigate the strength and plasticity of Nb/amorphous CuNb NMFs with layer thicknesses of 100 nm, 40 nm and 5 nm. Then, an effective strain based theoretical model has been developed to derive a homogeneous deformation strain (HDS) by distinguishing the shear banding-induced strain localization region from the non-localized one for the MI-induced 3-D stress state. The MI-derived HDS can be directly compared with the MC-measured one that is determined as the maximum applied strain without causing shear banding and micro/nano-cracks in the deformed pillars. The results show that the MI-evaluated HDSs are in quantitatively agreement with the MC-measured ones, revealing the best plasticity (i.e., with HDS of 48.5 %) in the 40 nm sample. The enhanced plasticity in the 40 nm sample is attributed to the deformation twinning in the Nb layers as revealed by the transmission electron microscopy analysis and molecular dynamics simulations. The above findings demonstrated that the plasticity of NMFs can be quantitatively evaluated by several simple MI tests with the aid of the developed combinational approach, in which the time-consuming and costly MC tests could be avoided.