An Atomistic View of Interface-Mediated Dislocation Plasticity in Thin Metal Films
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An Atomistic View of Interface-Mediated Dislocation Plasticity in Thin Metal Films E. S. Ege and Y.-L. Shen Department of Mechanical Engineering, University of New Mexico Albuquerque, NM 87131, U.S.A. ABSTRACT Atomistic simulations using molecular statics are carried out to study dislocation plasticity in thin metal films attached to stiff substrates. The analysis utilizes a sample two-dimensional crystal, with an embedded initial point defect used for triggering dislocation activities in a controlled manner. The existence of an interface between the film and the substrate is shown to delay plastic yielding and lead to film strengthening. The capability of atoms to slide along the interface plays a crucial role in determining the macroscopic stress-strain response and the microscopic dislocation activities. Within the modeling framework we examine the quantitative interfacial sliding behavior and the resulting dislocation-interface interactions and their consequences. INTRODUCTION Polycrystalline metallic films attached to stiff substrates can carry higher plastic flow stresses compared to their free-standing counterparts [1,2]. Existing theories for this substrate strengthening effect have been largely centered around the concept of misfit dislocations left behind at the interface between film and substrate, caused by gliding of threading dislocations inside a crystal [1,3-6]. The need for balancing the work done by the film stress and the stored energy of the interfacial misfit dislocation gives rise to the strengthening. The difficulty of such model, however, can be understood by considering the fact that the metal films in question are typically adjacent to an amorphous layer (such as oxidized silicon or silicon nitride), so a “misfit” is not likely to exist. Recent microscopy studies have indeed shown the lack of evidence of interface misfit dislocations in such systems [7,8]. A recent simulation study has aimed at providing an atomistic picture on the interface-mediated plasticity in thin films [9]. Through two-dimensional (2D) molecular statics simulations it was shown that the elimination of a free surface facilitated by the 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 “no sliding” case leads to much greater strengthening than the “free sliding” case. In the present work, we extend the previous analyses to include the effect of controlled partial atomic sliding along the interface. Attention is devoted to the transitional behavior between the two extremities. APPROACH Figure 1 shows the model system, which is a close-packed planar crystal with one of its close-packed directions parallel to
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