Electron microscopy of decagonal phases in Al-Cu-Co-Si system

Abstract

The present work started with synthesis of single crystals of decagonal (D) phases by slow cooling from liquid condition, Scanning electron microscopy (SEM) has shown the decagonal prismatic morphology and Si segregation in prismatic surfaces. X-ray diffraction patterns confirmed the structures of these decagonal needles to be decagonal quasicrystals. Thin slices normal to the 10-fold directions were prepared for transmission electron microscopy (TEM). The diffuse streaks have been recorded in the 10-fold diffraction patterns, The bright field images and dark field images on 2-beam condition showed the presence of discontinuous lines or fringes which are perpendicular to the direction of diffuse streaking. The existence of disordered B2 structure (0.29nm) has been established and attributed to ion milling during sample preparation. The superimposition of B2 and D phase causes the appearance of non-circular diffraction spots in the 10-fold diffraction pattern. However, the diffuse streaking will be interpreted as characteristic features in disordered decagonal structures. The disordering in some places leads to the evolution of microcrystalline (MC) phases. The coexistence of MC and D phases may cause the diffuse streaks and spot splitting is the diffraction patterns and line or fringe contrast in the images.