HRTEM Study of α-AlMnSi Crystals including Non-crystallographic Projection Axes

Abstract

The structure of α−AlMnSi is examined by atomic resolution high-resolution transmission electron microscopy (HRTEM) and computer-based image matching techniques. Six distinct zone axes are examined; including both normal crystallographic and non-crystallographic zone axes of the structural motifs, which have m35 icosahedral symmetry. 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 simulation and image-matching at about 0·17nm resolution. Most difficulty was experienced in obtaining the experimental images, especially for the non-crystallographic zones, which are very sensitive to slight changes in orientation off the desired zone axis or projection, the rate at which the crystal thickness is increasing (wedge-angle) and the orientation of the surfaces of the specimen. Surface amorphous layers due to oxidation and/or electron-induced irradiation damage also limit the efficiency of the HRTEM analysis.

For the thin specimens used for HRTEM, both the electron diffraction patterns and the HRTEM images are characteristic of Im 3 space group symmetry. It is suggested that this Im 3 symmetry may be an example of a statistical symmetry, where the local symmetry is close to Pm 3 but the average symmetry is Im 3. The transition from Pm 3 to Im 3 may be understood in terms of an analysis of small changes in the outer shells of the large icosahedral structural elements which are located at the corners and body-centres of the cubic unit cell.