DEFORMATION MECHANISMS IN NANOCRYSTALLINE ALUMINUM THIN FILMS: AN EXPERIMENTAL INVESTIGATION
Johns Hopkins University
Materials consisting of grains or crystallites with sizes below a hundred nanometers have exhibited unprecedented physical and mechanical properties in comparison to their coarse-grained counterparts. As a result, nanocrystalline materials have garnered considerable interest and a quest to uncover the new deformation mechanisms that give rise to this superior response has revealed that nanoscale behavior is quite different from that described by continuum plasticity. While the production of nanocrystalline materials with reasonable sizes for structural applications remains a challenge, thin metallic films used in next-generation MEMS and NEMS devices can be nanostructured by virtue of their limited dimensions. Ultimately, the reliability and lifetime prediction of these devices will hinge on the accurate modeling of their mechanical response. This dissertation describes efforts to elucidate the deformation mechanisms operating in nanocrystalline aluminum freestanding submicron thin films. Results obtained from these films demonstrate unique mechanical behavior, where discontinuous grain growth results in a fundamental change in the way in which the material deforms. In contrast to the low tensile ductility generally associated with nanocrystalline metals, these nanocrystalline films demonstrate extended tensile ductility. In situ X-ray diffraction and post-mortem transmission electron microscopy point to the importance of stress-assisted room temperature grain growth in transforming the underlying processes that govern the ii mechanical response of the films; nanoscale deformation mechanisms give way to microscale plasticity. The findings highlighted in this work emphasize that the microstructure and the attendant properties are dynamic; they evolve as the nanocrystalline material is being deformed. Experiments designed to address the role of impurities in stabilizing the microstructure against an applied stress are used to demonstrate that a critical concentration of impurities can effectively pin the grain boundaries from any motion. A detailed comparison of the characteristics of grain growth with traditional driving forces for grain boundary migration reveals the need for an alternative description. Measurements of surface topography evolution indicate that shear stresses directly couple to grain boundaries, induce motion, and result in grain growth that dramatically changes the mechanical behavior of these films. Finally, comparison with recently published theoretical formulations and molecular dynamics simulations is shown.