TRANSMISSION ELECTRON-MICROSCOPY STUDY OF SMALL-ANGLE GRAIN-BOUNDARIES, DISLOCATIONS AND MODULATED PHASES IN AL-LI-CU QUASI-CRYSTALS

Quasicrystalline samples of the alloy Al-Li-Cu prepared by a conventio nal melting method have been studied by transmission electron microsco py. In conventional dark-field and in high-resolution images, modulati ons of the image contrast along threefold and fivefold axes with modul ation periods of 2.7 and 0.99 nm have been found. This modulated icosa hedral phase can be considered as an intermediate state between the qu asicrystalline and a crystalline state and the image contrast modulati ons result probably from a local variation in the chemical composition . In our samples a very large density of small-angle grain boundaries have been found. From their image contrast it can be concluded that th ey are of twist boundary type. Single dislocations that occur in the s ame sample area have been studied with high-resolution microscopy. The ir dislocation lines are situated in planes with fivefold symmetry and their Burgers vectors lie in planes with threefold symmetry.