Dislocation Confinement and Ultimate Strength in Nanoscale Metallic Multilayers
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Dislocation Confinement and Ultimate Strength in Nanoscale Metallic Multilayers Qizhen Li and Peter M. Anderson Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43210, U.S.A. ABSTRACT In nanostructured metallic multilayers, hardness and strength are greatly enhanced compared to their microstructured counterparts. As layer thickness is decreased, three different regions are frequently observed: the first region shows Hall-Petch behavior; the second region shows an even greater dependence on layer thickness; and the third region exhibits a plateau or softening of hardness and strength. The second and third regions are studied using our discrete dislocation simulation method. This method includes the effects of stress due to lattice mismatch, misfit dislocation substructure, and applied stress on multilayer strength. To do so, we study the propagation of existing threading and interfacial dislocations as the applied stress is increased to the macroyield point. Our results show that in region 2, dislocation propagation is confined to individual layers initially. This "confined layer slip" builds up interfacial content and redistributes stress so that ultimately, the structure can no longer confine slip. The associated macroyield stress in this region depends strongly on layer thickness. In region 3, layers are so thin that confined layer slip is not possible and the macroyield stress reaches a plateau that is independent of layer thickness. INTRODUCTION An increase in strength is observed frequently in multilayered structures with a decrease of layer thickness. However, when the layer thickness is on the order of several nanometers, many experimental observations [1] indicate that there is a strength plateau or even softening with a further decrease in layer thickness. For example, Misra et al [2] reported the softening effect in Cu/Ni multilayer system; Kim et al [3] presented the similar effect in Ag/Al system, and Tixier et al [4] reported this effect in Ni3Al/Ni system. The mechanism behind this phenomenon is not clear. To explain this phenomenon and predict the plateau strength of a multilayer system, we use our triangular discrete dislocation model to simulate interfacial and threading dislocation propagation in multilayers. The effects from lattice mismatch and misfit dislocation on the interfaces are studied. In the second section, theoretical background is given, and different configurations are defined. The third section reports the simulation result of the critical resolved shear stress to propagate interfacial and threading dislocations for different layer thickness. This critical resolved shear stress corresponds to macroyield stress. The last section gives the conclusions. MODELING Theoretical background The triangular discrete dislocation model [5] is used in the following simulations. This model provides instantaneous local stress to activate each newly slipped triangle. These local stresses are averaged to get the critical resolved shear stress. The averaging methods
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