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We investigate a new generation of aluminium alloys, Al-Si-Mg/TiB2 nanocomposites, with both high stiffness (Young's modulus 94 GPa) and high yield strength (322 MPa), designed for automotive applications. A significant weight reduction (>30%) can be achieved by these nanocomposites compared with conventional Al alloys in components designed for stiffness. The crystallography and distribution of TiB2 particles and β″ precipitates, as well as the chemistry and structure of Al/TiB2 interfaces, have been characterized at atomic scales. The TiB2 nanoparticles (average particle size ∼450 nm), the main cause of stiffness improvement, were homogeneously distributed within the Al-Si-Mg matrix. The interfaces between Al and TiB2 nanoparticles were mainly parallel to dense planes of the TiB2, including basal {0001}, prismatic {11¯00} and pyramidal {011¯1} planes. All facets show a transitional zone in the alloy matrix, roughly 1 nm in thickness. We also find a Cu-rich layer (∼2 at.%) on pyramidal {011¯1} facets. The β″ precipitates are the main cause of high yield strength and work in conjunction with TiB2 nanoparticles to produce an alloy with outstanding mechanical properties.