In-Situ TEM Study of Plastic Stress Relaxation Mechanisms and Interface Effects in Metallic Films
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In-Situ TEM Study of Plastic Stress Relaxation Mechanisms and Interface Effects in Metallic Films Marc Legros1, Gerhard Dehm2, T. John Balk3 1 CEMES-CNRS, 29 rue J. Marvig, 31055 Toulouse - France 2 Erich Schmid Institute for Materials Science and University of Leoben, Department Materials Physics, Jahnstr. 12, 8700 Leoben - Austria 3 Dept. of Chemical and Materials Engineering, University of Kentucky, Lexington, KY, USA ABSTRACT To investigate the origin of the high strength of thin films, in-situ cross-sectional TEM deformation experiments have been performed on several metallic films attached to rigid substrates. Thermal cycles, comparable to those performed using laser reflectometry, were applied to thin foils inside the TEM and dislocation motion was recorded dynamically on video. These observations can be directly compared to the current models of dislocation hardening in thin films. As expected, the role of interfaces is crucial, but, depending on their nature, they can attract or repel dislocations. When the film/interface holds off dislocations, experimental values of film stress match those predicted by the Nix-Freund model. In contrast, the attracting case leads to higher stresses that are not explained by this model. Two possible hardening scenarios are explored here. The first one assumes that the dislocation/interface attraction reduces dislocation mobility and thus increases the yield stress of the film. The second one focuses on the lack of dislocation nucleation processes in the case of attracting interfaces, even though a few sources have been observed in-situ. INTRODUCTION Stresses are known to increase in small-scale systems such as nanocrystalline materials or thin films. This strengthening is qualitatively well understood in term of constrained dislocation motion. For fine-grained materials, the Hall-Petch relation, based on dislocations piling up against boundaries, states that the yield stress varies as one over the square root of the grain size. In the case of thin films on rigid substrates, the so-called Nix-Freund [1, 2] model, itself adapted from the work of Matthews and Blakeslee [3], describes the well-established experimental fact that the strength of thin films varies as one over their thickness [4-6]. In this model, a threading dislocation must deposit an interfacial dislocation as it shears the film. The stress necessary to extend this interfacial dislocation varies as one over the film thickness. For polycrystalline thin films, Thompson combined both models by accounting for the deposition of a dislocation on grain boundaries and also derived a stress dependence that varies as one over the grain size [7]. The need for new concepts was recognized from experimental observations in very fine-grained materials and ultra thin metallic films (less than 100 nm) that the strength started to level off and eventually decreased, at variance from what is expected from the models. This signifies the activation of new deformation mechanisms and their eventual dominance of dislocation-bas
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