Molecular dynamics is used to examine the deformation and failure of Al-Si interfaces and of nanocomposites consisting of Al and Si nanograins loaded in tension. The modified embedded atom method, is used to describe the forces and energies in the system and accounts for the directional dependence of Si bonding. Various Al/Si interfaces with a common  axis parallel to the interface and polycrystals of eight hexagonal grains of 5 nm diameter with -oriented columnar grains with zero, one, or two Si particles are examined to understand the effect of Si additions on the deformation and failure of the material. Three main features are demonstrated. First, imperfectly-matched Al/Si interfaces have relatively high tensile strengths and fracture energies that are larger than the ideal Griffith fracture energies. Second, different mechanisms dominate the early stages of non-linear deformation in the Al polycrystal and Al-Si nanocomposites: the Al polycrystal shows a mix of grain boundary deformation and dislocation activity while the Al-Si polycrystals show significant grain boundary sliding/shearing only at the Al-Si interfaces. The grain boundary sliding/shearing in the Al-Si nanocomposite is also reduced with increasing Si content, leading to higher yield stress. Third, the failure modes differ between the polycrystal and the nanocomposite: failure in the Al polycrystal initiates at a grain boundary and propagates transgranularly whereas failure in the Al-Si composites occurs by void damage accumulation at an Al/Si interface. The composite failure stresses are lower than the strengths of the Al-Si bimaterial interfaces due to stress concentrations in the composites and non-ideality of the as-fabricated composite interfaces but the normal stresses at the particle interfaces do reach values comparable to those of the bimaterial interfaces. These results suggest that Al-Si nanocomposites can be engineered for high hardness strength. (c) 2005 Elsevier Ltd. All rights reserved.