CRYSTALLINE AND QUASI-CRYSTALLINE STRUCTURES IN AN Al-Li-Cu-Mg ALLOY

Abstract
We have studied by means of transmission electron microscopy the particular properties of an icosahedral phase (I-phase) formed in equilibrium conditions in an Al-Li-Cu-Mg alloy. For instance this I-phase is observed as long faceted precipitates embedded in the f.c.c. Al matrix and exhibits definite orientation relationships which were unexpected considering the natural orientation between a cube and an icosahedron. The I-phase is observed together with a crystalline phase (C-phase) which is strongly related to it, since it gives pseudo fivefold symmetry diffraction patterns fitting almost perfectly the fivefold symmetry pattern of the I-phase. First results on this unknown phase are reported.