Dislocation Confinement and Ultimate Strength in Nanoscale Polycrystals
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Dislocation Confinement and Ultimate Strength in Nanoscale Polycrystals Qizhen Li1, Peter M. Anderson1, Michael Mills1, and Peter Hazzledine2 1 Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43210, U.S.A. 2 Universal Energy Systems Inc., Dayton, OH 45432 USA ABSTRACT Nanoscale polycrystalline metals typically exhibit increasing hardness with decreasing grain size down to a critical value on the order of 5 to 30 nm. Below this, a plateau or decrease is often observed. Similar observations are made for nanoscale multilayer thin films. There, TEM observations and modeling suggest that the hardness peak may be associated with the inability of interfaces to contain dislocations within individual nanoscale layers. This manuscript pursues the same concept for nanoscale polycrystalline metals via an analytic study of dislocation nucleation and motion within a regular 2D hexagonal array of grains. The model predicts a hardness peak and loss of dislocation confinement in the 5 to 30 nm grain size regime, but only if the nature of dislocation interaction with grain boundaries changes in the nanoscale regime. INTRODUCTION Typically, the room temperature strength of polycrystalline metals increases with decreasing grain size according to a Hall-Petch relation [1, 2]. This trend continues until a plateau in strength on the order of GPa is observed in the range of 5 to 30 nm grain size. For example, Sanders et al [3] report that the hardness of high-density, high-purity Cu polycrystals follows a Hall-Petch prediction down to approximately 15 nm grain size, below which a plateau occurs. Masumura, Hazzledine, and Pande [4] show that for many metallic systems, a Hall-Petch description is appropriate above about a 30nm grain size and below this, the Hall-Petch relation typically overestimates hardness. A physical understanding of a hardness peak stems from both experimental and modeling results. Sanders et al note that the density of compacted polycrystals can decrease in the nm regime and thereby contribute to a drop in yield strength with decreasing grain size [3]. Transmission electron microscopy (TEM) of deformed, nanocrystalline, electrodeposited Ni samples with a 30 nm grain size indicates that grain boundaries prevail as sources and sinks for dislocations [5], so that at this scale, macroscopic behavior is expected to be dominated by grain boundary structure and interaction with dislocations. Several large-scale molecular dynamics (MD) simulations [6-11] employing the embedded atom method suggest that indeed, grain boundaries serve as the source and sink for dislocations, provided grain size is greater than 10 nm. At smaller grain size, grain boundary sliding and/or diffusion intervenes as the dominant plastic deformation mechanism and dislocation motion is not observed. This manuscript explores the possibility that dislocation motion may still be an important strength-controlling deformation mechanism in the nm grain size regime. Although MD simulations suggest the absence of dislocations
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