Length-scale effects on nano-scale materials behavior

The objective of the present study is to investigate the length-scale effects on mechanical properties of nano-scale thin films. To perform in-situ studies in SEM and TEM for simultaneous qualitative and quantitative information, we have developed a MEMS-based tensile testing technique with the following unique features, (1) The freestanding thin film specimens are co-fabricated with force and displacement sensors. (2) The experimental set up fits in both SEM and TEM chambers. (3) Gripping and alignment of the specimens are automatically accomplished during fabrication.; For the first time, we have performed in-situ, quantitative tests on 100 nm thick aluminum films in TEM. We also have performed tensile (both low and high temperature) and bending tests on 30–485 nm thick aluminum and 75–350 nm thick gold specimens. Followings are the observations of this study. (1) The Young's modulus of aluminum deviates from the bulk values for average grain size below 50 nm (70 nm for Gold). For the first time, we observe strong non-linear elastic behavior for grain sizes below 20 nm. (2) Texture and anelastic effects cannot explain the elastic behavior at the nano-scale and we propose a mechanism based atomic-scale interactions. (3) In-situ TEM tests show that below a transitional length-scale (50 nm grain size for aluminum), dislocations cease to exist in the grain interiors. Absence of both dislocation and diffusion-based deformation mechanisms, and the increasing influence of the native oxide give rise to the resemblance to the reverse Hall-Petch behavior. (4) For the first time, we observe absence of permanent strain hardening in aluminum for grain sizes below 50 nm. We propose that this is due to the absence of dislocations at interior of grains at this length-scale. (5) For the first time, we observe grain size dependence of the strain gradient plasticity in aluminum. For grain sizes below 50 nm, the absence of dislocations results in no appreciable strain gradient strengthening. However, as the grain size increases, dislocations appear to dominate deformation characteristics, and strong strain gradient effects are observed. The characteristic length scale for aluminum is found to be about 4 microns.