Strain gradient effect in nanoscale thin films

Compared to uniform deformation, gradient-dominant deformation experiments at the micro-scale have consistently shown remarkable strengthening effect. It is proposed in the literature that pronounced strain gradient, on the order of the inverse of a characteristic length scale, adds to the materials strength. The literature contains several formulations that account for the strain gradient in the constitutive equation. However, the role of microstructures in these formulations remains to be investigated. Due to the difficulties in micro/nano scale experimentation, attempts to investigate this mechanism so far have been confined to specimens with dimensions more than 10 microns, or to nano-indentation experiments. In this study, we use MEMS-based testing techniques to explore the effect of strain gradient in 100, 150, 200 and 485 nm thick freestanding Aluminum specimens with average grain size of 50, 65, 80 and 212 nm respectively. Strain gradient plasticity analysis show that the characteristic length scale for Aluminum is about 4.5 μm, which is similar to the values for copper and nickel reported in the literature. Experimental results suggest that the strain gradient effect is fundamentally related to dislocation-based mechanisms, and is absent at extremely small length scales (< 100 nm for Aluminum), where accommodation of dislocations in the crystal grains is not energetically favorable.