Atomistic Simulations of Steps in Bimetallic Interfaces as Barriers to Interface Slip Transmission
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Atomistic Simulations of Steps in Bimetallic Interfaces as Barriers to Interface Slip Transmission Charles H. Henager, Jr.*, Howard L. Heinisch, Jr, Richard J. Kurtz, and Richard G. Hoagland Pacific Northwest National Laboratory1 Richland, WA 99335-0999 ABSTRACT Atomistic models of coherent interfaces in the CuNi system with and without (111)-steps were used to study slip transmission across interfaces in CuNi metallic bilayers. The lattice mismatch of the CuNi system results in large coherency stresses at the interface. The (111)-steps afford a larger barrier to slip than the flat, coherent interface. The coherent flat interface dislocation barrier is largely due to the large compressive stresses in the Cu layer that must be overcome by applied tensile stresses. Additional Koehler forces are present as the dislocation in the elastically softer Cu approaches the stiffer Ni layer. The step, however, possesses a small residual edge dislocation with a Burgers vector equal to the difference of bCu and bNi times the height of the (111)-step in (111)-layers. We find that these steps are potent slip barriers, which suggests that homogeneous slip is preferred in such systems. INTRODUCTION Nanolayered materials can have strengths approaching theoretical strengths and empirical laws, such as the Hall-Petch relation, can be used to estimate these strengths as a function of layer spacing. However, the Hall-Petch formalism neglects important mechanisms that begin to become important at small sizes for which single dislocation motion is critical and, therefore, strength estimates may be erroneous. These mechanisms include glide dislocation interactions with coherency strains, Koehler forces due to elasticity differences, and dislocation core structure changes in passing from one layer to another. The mechanical behavior of these materials suggests the term “interface strengthening” as a better description than that given by the HallPetch model. Interface strengthening not only describes the yield behavior but subsequent deformation as well since the onset of deformation can change the nature of the interface, which, in turn, can have significant effects on continued deformation processes. Deformation of cast CuCr eutectics provides indirect evidence that interface steps play a significant role in deformation of interface strengthened materials. Drawn samples of CuCr material containing Cr fibers exhibit rather homogeneous deformation such that the fibers undergo rather uniform shape changes [1]. Extracted Cr fibers exhibit surface slip steps but also show uniform elongation and homogeneous shape changes. These observations suggest that slip is occurring uniformly along the fiber length with dislocations shuttling between the CuCr interfaces [1]. This suggests that surface steps inhibit continued deformation and prevent fiber necking and local shear banding within the Cr fibers. The size scale of nanolayered materials allows detailed atomistic modeling of deformation behavior. We determine the force required for a dislocation to t
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