Atomistic Simulations of Dislocation-Interface Interactions in Thin Films
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Atomistic Simulations of Dislocation-Interface Interactions in Thin Films R. W. Leger and Y.-L. Shen Department of Mechanical Engineering, University of New Mexico Albuquerque, NM 87131, U.S.A. ABSTRACT Atomistic simulations are carried out to study the effect of atomic sliding capability at the interface between a plastically deforming film and a stiff substrate. Molecular statics modeling is utilized to corroborate the overall film response and the nano-scale defect mechanisms. A free-sliding interface is shown to be able to cause “reflection” of oncoming dislocations and enhance film plasticity. A rigidly bonded interface, on the other hand, is seen to resist approaching dislocations. Partial sliding results in a transitional behavior between the two extremes, as revealed in our parametric analysis. The sliding capability of interface atoms is also seen to dictate the overall film response. INTRODUCTION This study focuses on small-scale plasticity and the interaction of dislocations with film-substrate interfaces. Our recent study has aimed at providing an atomistic picture on the interface-mediated plasticity in thin metal films [1]. Through two-dimensional (2D) molecular statics simulations it was shown that the elimination of a free surface facilitated by a stiff substrate enhances the yield strength of the film by restricting dislocation activities. In particular, when atoms at the interface have the capability of unrestricted tangential slide, an oncoming dislocation tends to be “reflected” by the interface by way of a dislocation reaction at the interface. On the other hand, when the interfacial atoms are not allowed to slide, the oncoming dislocation tends to be pinned near the interface and becomes immobile. The “noslide” case leads to much greater strengthening than the “free-slide” case. In a follow-up work [2], the effects of controlled partial sliding along the interface were examined. When the interface atoms are endowed with certain capabilities of tangential slide, the film displays a yield behavior in between those of a perfectly bonded film and a freely sliding film. A maximum allowable sliding distance of 1% of the current interatomic spacing along the interface results in a film response only slightly different from the unrestricted free-sliding case. With a sliding capability greater than about 0.075%, the interface is seen to have sufficient flexibility to interact with the oncoming dislocation, forcing it to reflect from the interface and to slip toward the free surface of the film. While the above studies provided insight into the dislocation-interface interaction in thin crystals, the influence caused by the sample size is still unclear. In the present work, we extend the analyses to feature an approximately 50% increase in film thickness compared to the previous studies [1,2], with the purpose of investigating both the qualitative and quantitative features affected by the problem size. APPROACH Figure 1 shows the model system, which is a close-packed planar crystal (containing 227
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