TWINNED STRUCTURE IN AL75CU10CR15 EXHIBITING DECAGONAL SYMMETRY

A twinned structure has been discovered for the first time in as-cast Al75Cu10Cr15 alloy. The selected-area electron diffraction patterns ex hibit apparent decagonal symmetry, but the corresponding lattice image s display a twinned structure. It is shown that the observed electron diffraction patterns from this structure can be simulated by the super position of single-crystal patterns in twin orientations. The structur e of the twinned phase can be described as a periodic close packing of decagonal stuctural units with a radius of about 1.0 nm. This allows 72-degrees (fivefold) twinnning with coherent twin interfaces. Since t here is a diad axis of the crystalline phase parallel to the twin rota tion axis, the overall symmetry is decagonal.