Transmission electron microscopy study of small-angle grain boundaries, dislocations and modulated phases in Al-Li-Cu quasicrystals

Quasicrystalline samples of the alloy Al-Li-Cu prepared by a conventional melting method have been studied by transmission electron microscopy. In conventional dark-field and in high-resolution images, modulations of the image contrast along threefold and fivefold axes with modulation periods of 2.7 and 0.99 nm have been found. This modulated icosahedral phase can be considered as an intermediate state between the quasicrystalline and a crystalline state and the image contrast modulations 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 they are of twist boundary type. Single dislocations that occur in the same sample area have been studied with high-resolution microscopy. Their dislocation lines are situated in planes with fivefold symmetry and their Burgers vectors lie in planes with threefold symmetry.