What's behind aluminum alloys' strength

When spiced with silicon and magnesium, aluminum transforms from its soft, pliable self into alloys as strong as steel but only half the weight. Some alloys even take on a welcome but poorly understood "quick-bake hardening" property: They first can be formed into useful shapes such as automotive panels and then strengthened with a 30-minute bake at kitchen-oven temperatures. Using high-resolution electron microscopy, Jianghua Chen and his colleagues in the Netherlands Institute for Metals Research at Delft University of Technology have chronicled in atomic detail what happens inside AlMgSi alloys during this annealing step (Science 2006, 312, 416). First, pillars of silicon approximately 2 nm long assemble into skeletons on which particles with a rough composition of Mg₂Si₂Al₇ form. These nuclei then evolve into nanoscale Mg₅Si₆ precipitates (visible in the micrograph shown). It is these that squelch the kinds of motions within crystal grains that make pure aluminum so soft. "These nanoparticles are dynamic objects in the annealing process," Chen says. This refined scenario of the hardening process could lead to aluminum alloys that are more workable and stronger than existing ones, he adds.