Fracture Resistance of Nanocrystalline Ni
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Fracture Resistance of Nanocrystalline Ni D. FARKAS Atomic level simulations are used to study crack propagation mechanisms in nanocrystalline Ni. Digital samples with a mean grain size of 5 and 8 nm containing 125 grains were used. For both grain sizes, the mechanism of crack propagation involves the formation of nanocracks along grain boundaries in the vicinity of the main crack. Crack resistance curves for the two grain sizes indicate that the smaller grain sizes are more ductile, requiring higher stress intensities for crack propagation. This result is consistent with softer behavior for smaller grain sizes in the inverse Hall–Petch regime, where deformation is accommodated by grain boundary mechanisms. The present simulations specifically show that grain boundary sliding also plays an important role in crack blunting observed in these materials. In many cases, the crack is arrested as it encounters grain boundaries in its path, showing increased resistance to propagation. Increased ductility for smaller grain sizes in this regime indicates that there is a minimum in ductility as a function of grain size in these materials, located around the 10- to 12-nm grain size.
I. INTRODUCTION
NANOCRYSTALLINE metals with grain sizes on the nanometer scale, typically from 5 to 50 nm, are of technological interest because their strength and hardness are far above what they are in coarse-grained metals. This is believed to be due to the grain boundaries acting as barriers to dislocation motion: as the number of grain boundaries increases, dislocation motion becomes harder, leading to a harder material. These materials are not only interesting from a technological point of view, but also from a theoretical point of view, because, for the smallest grain sizes, particular behavior is observed that is typical of the nanoscale and different from the coarse grain counterpart material. In particular, grain boundary accommodation of plasticity becomes very important and results in a maximum of the strength with grain size.[1,2] It is of great interest that for the smallest grain sizes, it becomes possible to model the deformation process directly using atomic-scale computer simulations. Large-scale atomic level simulations have been used to provide insight into the atomic-scale processes that occur during plastic deformation.[1–4] The effect of grain size on strength has been studied using full three-dimensional simulations of nanocrystalline digital materials.[1] Simulation has shown that an external applied stress can be accommodated by both grain boundary sliding and the emission of dislocations from the grain boundaries.[1,2,3] The interplay between these two basic mechanisms controls the strength of these materials and results in a maximum in the strength as a function of grain size.[1,2,3] The simulations show that nanocrystalline grain boundaries act as sources and sinks for D. FARKAS, Professor, is with the Department of Materials Science and Engineering, Virginia Tech, Blacksburg, VA 24061-0237. Contact e-mail: [email protected]
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