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nanocrystalline aluminum by molecular dynamics simulation
Cooperative Research Team on
Microstructural Effects on the Mechanics of Materials
Note. The following is adapted from Nature Materials 1, 45-49 (2002).
Figure 1. Successive snapshots of the vicinity of the triple junction connecting grains 2, 3 and 4, demonstrating the mechanism by which the new grain A was formed. a) Plastic strain = 6.09%; b) 6.19%; c) 6.76%; and d) 8.71%. Four distinct processes labeled 1-4 are revealed; these are described in the text. Red dots denote atoms in an h.c.p. environment. Blue dots denote atoms that are 'defected,' that is, neither in an f.c.c. nor h.c.p. environment, including atoms with broken nearest-neighbor bonds. Scale bar, 10 nm.
The mechanical behavior of nanocrystalline materials (that is, polycrystals with a grain size of less than 100 nm) remains controversial. Although it is commonly accepted that the intrinsic deformation behavior of these materials arises from the interplay between dislocation and grain-boundary processes, little is known about the specific deformation mechanisms. Here we use large-scale molecular-dynamics simulations to elucidate this intricate interplay during room-temperature plastic deformation of model nanocrystalline Al microstructures. We demonstrate that, in contrast to coarse-grained Al, mechanical twinning may play an important role in the deformation behavior of nanocrystalline Al. Our results illustrate that this type of simulation has now advanced to a level where it provides a powerful new tool for elucidating and quantifying - in a degree of detail not possible experimentally - the atomic-level mechanisms controlling the complex dislocation and grain-boundary processes in heavily deformed materials with a submicrometer grain size.
The sequence of snapshots in the Figure captures in detail the underlying GB and dislocation processes involved in the formation of the new grain, A. The nucleation of the new grain starts by the emission of a complete 1/2[110] dislocation from the grain boundary (GB) between grains 3 and 4 (process 1 in the first panel). The complex core structure labelled 1' was formed by two such dislocations, emitted, however, onto different slip planes. As seen in panel (b), this new, almost immobile complex core structure subsequently begins to continuously emit partials, producing a growing twin lamella by "partial-dislocation breakaway" [1] (process 2). This lamella grows further by absorbing additional 1/2[110] dislocations emitted from the same GB (process 1 in panel b), leading to its increased size in panel (c). The development of the new grain also involves the emission of another twin lamella (labeled 3 in panel c) together with extrinsic stacking faults terminated by double-Shockley partials (labeled 4). Finally, the rather complex deformation substructure thus formed subsequently coalesces to form the final grain A in panel (d).
References
1. Hirth, J.P. and Lothe, J. Theory of Dislocations Ch. 10-3 (Wiley, New York, 1992).
Contact: Dieter Wolf, Idaho National Laboratory,
Phone:(208) 526-8394, Fax:(208) 526-5327
Email:dieter.wolf@inl.gov
