In general, mechanical behavior of nanomaterials is closely related with the nucleation and movement of discrete defects, which might span several temporal and spatial scales. To connect the deformation processes in different scales with each other seamlessly, the concept of multiscale simulation is proposed，and becomes more and more attractive. For analyzing the elastic and plastic deformation processes of some kinds of nanomaterials, multiscale methods have multiple advantages over traditional simulation methods. For example, the grain sizes in molecular dynamics simulations are usually in nanoscale, while the Quasicontinuum multiscale method can be easily used to simulate the deformation process of grain with the size over 1μm3.

Recently, a series of multiscale simulations on the mechanical behavior of nanocrystalline metals mediated by grain boundaries are performed by the research group leaded by Prof. Wang in Shenyang national laboratory for materials science. Some striking results are obtained. The nanoindentation process on nanocrystalline Al is simulated via the Quasicontinuum method. A fivefold deformation twin is observed. According the concept of the coarse-grained local stress, the local stress states within grains under indenter tip are presented. The stress calculation clearly shows that the variation of high stress orientation is required to form a fivefold deformation twin.  Particularly, a low index asymmetric grain boundary plays a very important role in the deformation mechanism, since this grain boundary not only confines high hydrostatic stress within several grains underneath indent tip, but also causes dislocation reflection. Then, the first twin boundary in the fivefold twin is nucleated. The formation process of fivefold deformation twin structure can be completely understood by the present Quasicontinuum result. This kind of particular grain boundary has been studied systematically in a bicrystal model, but not related with any experimental phenomenon yet. The important effect of microstructure on the mechanical behavior of nanomaterials is clearly confirmed by the formation process of fivefold deformation twin mediated by this particular grain boundary.

Besides that, the intrinsic fracture behavior of nanocrystalline Ni is studied via the Quasicontinuum method. The simulating results show that the there are intense tensile stresses in the regions around nanovoids in the grain boundaries in front of crack tip. The nucleation and propagation mechanisms of crack mediated by grain boundaries might result in a limited tensile strain of nanocrystalline Ni. Lately, the atomistic mechanism of ductility improvement is investigated by adopting two different embedded-atom-method potentials for Ni. The most striking result is the brittle–to-ductile transition in the fracture behavior. It is shown that a reduction in the unstable stacking fault energy can prompt emission of partial dislocations from grain boundaries, then the ductility of nanocrystalline Ni can be improved effectively.

Several research results have been published on SCI-cited journals, e.g. Scripta Materialia(Y. F. Shao, S. Q. Wang, Scripta Mater. 62, 419 (2010)). This research project was supported by the National Basic Research Program of China.