HRTEM STUDY OF ALPHA-ALMNSI CRYSTALS INCLUDING NON-CRYSTALLOGRAPHIC PROJECTION AXES

The structure of alpha-AlMnSi is examined by atomic resolution high-re solution transmission electron microscopy (HRTEM) and computer-based i mage matching techniques. Six distinct zone axes are examined; includi ng both normal crystallographic and non-crystallographic zone axes of the structural motifs, which have m (3) over bar 5 icosahedral symmetr y. The results provide a sound basis for understanding HRTEM images of crystalline and quasicrystalline alloys of AlMnSi; thus we examine to what extent the requirements for obtaining so-called structure images of complex alloy structures may be met experimentally and define when the images may be reliably interpreted on the basis of computer simul ation and image-matching at about 0-17nm resolution. Most difficulty w as experienced in obtaining the experimental images, especially for th e non-crystallographic zones, which are very sensitive to slight chang es in orientation off the desired zone axis or projection, the rate at which the crystal thickness is increasing (wedge-angle) and the orien tation of the surfaces of the specimen. Surface amorphous layers due t o oxidation and/or electron-induced irradiation damage also limit the efficiency of the HRTEM analysis. For the thin specimens used for HRTE M, both the electron diffraction patterns and the HRTEM images are cha racteristic of Im (3) over bar space group symmetry. It is suggested t hat this Im (3) over bar symmetry may be an example of a statistical s ymmetry, where the local symmetry is close to Pm (3) over bar but the average symmetry is Im (3) over bar. The transition from Pm (3) over b ar to Im (3) over bar may be understood in terms of an analysis of sma ll changes in the outer shells of the large icosahedral structural ele ments which are located at the corners and body-centres of the cubic u nit cell.