The mechanisms of deformation and failure in Al-Si nanocomposites, formed by adding Si particles to Al nanocrystalline materials, are investigated using molecular dynamics. The deformation and fracture mechanisms are found to be different in the Al-Si composites as compared to single-phase all-Al materials. The plastic deformation of all-Al polycrystals is associated with a mix of grain boundary deformation and dislocation activity while the deformation in the Al-Si nanocomposites is associated with predominantly grain boundary sliding/shearing at the Al/Si interfaces and little deformation elsewhere. The Al-Si nanocomposites also have a higher yield stress than the all-Al nanocrystals, consistent with recent experimental data. The failure of the Al polycrystals occurs by crack initiation at triple junctions and grain boundaries accompanied by localized shearing, leading to either trans- or intergranular cracks, while failure in the Al-Si nanocomposites occurs by void damage accumulation, culminating in crack formation, at an Al/Si interface. The tensile strength in the Al-Si nanocomposites correlates well with the fundamental Al/Si interface strength when the interface stress concentration caused by the modulus mismatch is considered. In spite of very different failure modes, the tensile strengths of the Al-Si and all-Al materials are similar. These results show that Al/Si interfaces control the mechanical behavior in the nanocomposites, reducing the role of bulk metal deformation modes, indicating that Al-Si nanocomposites can be engineered for enhanced hardness over all-Al nanocrystals of the same grain size. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.