Atomistic and Continuum Studies of Diffusional Creep and Associated Dislocation Mechanisms in thin Films on Substrates
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ATOMISTIC AND CONTINUUM STUDIES OF DIFFUSIONAL CREEP AND ASSOCIATED DISLOCATION MECHANISMS IN THIN FILMS ON SUBSTRATES Markus J. Buehler, Alexander Hartmaier and Huajian Gao Max Planck Institute for Metals Research, 70569 Stuttgart, Germany ABSTRACT Motivated by recent theoretical and experimental progress, large-scale atomistic simulations are performed to study plastic deformation in sub-micron thin films. The studies reveal that stresses are relaxed by material transport from the surface into the grain boundary. This leads to the formation of a novel defect identified as diffusion wedge. Eventually, a crack-like stress field develops because the tractions along the grain boundary relax, but the adhesion of the film to the substrate prohibits strain relaxation close to the interface. This causes nucleation of unexpected parallel glide dislocations at the grain boundary-substrate interface, for which no driving force exists in the overall biaxial stress field. The observation of parallel glide dislocations in molecular dynamics studies closes the theory-experiment-simulation linkage. In this study, we also compare the nucleation of dislocations from a diffusion wedge with nucleation from a crack. Further, we present preliminary results of modeling constrained diffusional creep using discrete dislocation dynamics simulations. INTRODUCTION Thin films are frequently deposited on substrate materials to build complex devices. In past years, an ever increasing trend to miniaturization of technology is observed. The mechanical properties, behavior and reliability of devices containing polycrystalline thin films are of critical importance to technological innovations. In this work, we focus on polycrystalline thin metals films, as plotted schematically in Figure 1 (a). In many applications and during the manufacturing process, thin films are subject to external loading because of thermal mismatch between the film material and the substrate. This is known to have a significant effect on the performance and reliability of the devices [1]. Different inelastic deformation mechanisms are relaxing such thermal stresses. In thin films, dislocation activity within the grains and diffusional flow of matter from the surface and the grain boundary (GB) into the film are known mechanisms for inelastic deformation [2]. In-situ electron transmission microscopy observations in sub-micron thin films suggest that dislocation glide occurs preferably on glide planes parallel to the surface and close to the substrate-film-interface [4], in contrast to classical threading dislocation mechanisms for thicker films [5]. Recent investigations of nanostructered bulk material suggest breakdown of classical dislocation mechanisms [3] at the nano-scale, causing diffusion controlled dislocation mechanisms to dominate. A continuum model for diffusional creep in thin films has been developed by Gao et al. [6] successfully explaining experimental observations. Diffusion is of fundamentally different nature in thin films than in bulk materials, sin
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