CONVERGENT-BEAM ELECTRON-DIFFRACTION AND ELECTRON-MICROSCOPE STUDY ONDECAGONAL QUASI-CRYSTALS OF AL-CU-CO AND AL-CO ALLOYS

Decagonal quasicrystals of melt-quenched Al-Cu-Co and Al-Co alloys hav e been investigated by convergent-beam electron diffraction (CBED) and transmission electron microscopy. The CBED method has revealed that t he space group of the alloys is non-centrosymmetric P10m2BAR. Al73Co27 is the first non-centrosymmetric binary quasicrystal. Dark-field micr oscopy has revealed the existence of inversion domains with an antipha se shift of c/2 at the domain boundaries. These results are analogous to those for the melt-quenched alloy Al70Ni15Fe15 (Saito et al., Jpn. J. Appl. Phys., 31 (1992) L109). However, structural phase transformat ion as a function of the transition metal composition, as observed for the Al-Ni-Fe system, does not occur in the present system, and two ty pes of pentagonal atom cluster images are observed in high-resolution electron microscope images, but only one type in the Al-Ni-Fe alloys.