Decagonal Phases: Non-Quasi-Crystalline Microcrystalline State in an Al-Cu-Co-Si Alloy

We present high-resolution electron microscopy and x-ray diffraction studies of the Al 63 Cu 17.5 Co 17.5 Si 2 alloy at room temperature. The sample is found periodic in one direction c and presents tenfold symmetry in the l = 0 diffraction plane perpendicular to this direction. The two complementary techniques show that the sample is not in a 2D quasi-crystalline state but in a microcrystalline state, which restores the overall tenfold symmetry in diffraction pattern. On the basis of our present results, we discuss some relationships that could exist between microcrystalline and quasi-crystalline phases.