Peak/Plateau Strength in Nanoscale Multilayer Thin Films: Constrained vs Unconstrained Dislocation Nucleation
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1177-Z09-34
Peak/Plateau Strength in Nanoscale Multilayer Thin Films: Constrained vs Unconstrained Dislocation Nucleation Qizhen Li(a) and Peter M. Anderson(b) (a) Chemical and Metallurgical Engineering, University of Nevada, Reno, NV 89557 (b) Materials Science and Engineering, The Ohio State University, Columbus, OH 43210 ABSTRACT
The peak/plateau strength of multilayer thin films is analyzed in terms of the stress to bow out a dislocation loop from an interface. Comparison of approximate analytic models to experimental data suggests that the bow out is “constrained” by nearby interfaces, at least for eNbN/Mo and e-Ni/Cu films. Estimates of the interfacial pinning distance to form the bow out are ~20b for e-NbN/Mo and ~70b for e-Ni/Cu around peak/plateau strength (b is the magnitude of Burgers vector). INTRODUCTION Many A/B metallic multilayer systems deviate from a Hall-Petch relation [1, 2] when the bilayer period (Λ, the summation of the layer thickness h1 of A and the layer thickness h2 of B) is on the order of 10 to 30nm or less. In this regime, several nanoindentation investigations and a limited number of tensile tests [3-5] show that hardness and yield strength reach a peak or plateau [6-9]. This phenomenon is often explained in terms of the discrete interaction of single dislocation pile-ups with interfaces [10-13]. Recently, Fang and Friedman [33] introduced a source activation stress τs and interfacial pinning stress τp to model the nucleation and subsequent transmission of dislocations across interfaces. That work concluded that both parameters are poorly understood, particularly τs. It also focused on a 2D geometry with dislocations that are infinitely long and parallel to the interfaces. Figure 1: (a) multilayer thin film geometry showing an inclined slip plane p-q-r-s and (b) projection onto the slip plane p-q-r-s showing an unconstrained dislocation bow out geometry on the left and a constrained dislocation bow out geometry on the right.
A Ni/Cu nanolaminate shows dislocation bowing out from an interface by transmission electron microscope image [10]. The bow out process occurs repeatedly during in-situ straining, sometimes at persistent interfacial sites. Two possible bow-out configurations might control the onset of profuse plasticity throughout the multilayer thin film, especially in the vicinity of peak hardness or strength. Figure 1b (lower image, left) depicts an unconstrained scenario in which the strength controlling event is the expansion of interfacial loop of dimension L to a critical semicircular shape. After this critical event, the loop is assumed to expand across the multilayer thin film without further increase in stress. In contrast, Figure 1b (lower image, right) depicts a
constrained scenario in which the expanding loop encounters a nearby interface prior to achieving a semi-circular shape. Here, the strength controlling event is the transmission of a compressed, distorted bow out across the interface into phase 2. Several factors determine whether an unconstrained or constr
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