Loop- and band-shaped defects observed in quasicrystals by X-ray topography

Degree of structural perfection of icosahedral quasicrystalline grains investigated by synchrotron X-ray diffractometry and imaging techniques

“…Influence of mechanical polishing As already reported (Gastaldi et al 1995b), band defects were observed in synchrotron white-beam X-ray topographs of both Al-Pd-Mn and Al-Cu-Fe grains. They were related to some of the scratches generated on the external surfaces during mechanical polishing.…”

“…A tight correspondence between the faceted pores and the two-lobe contrasts observed in X-ray topographs was established as general. X-ray topographs showed in addition that subboundaries are found in all large grains and the peculiar defects giving loop-shaped contrasts previously reported (Gastaldi et al 1995b) are encountered in large subgrains (figure 2 (a)); these are, in other respects, not so perfect (table 2). However, in the small perfect region of grain I, no loop-shaped contrast was seen but only linear black and white contrasts looking like direct and dynamic images of dislocations (Authier 1996) were distinguished (figure 3).…”

“…Concerning band-shaped contrasts, after having established in a previous study (Gastaldi et al 1995b) by synchrotron white-beam XRT that they were related to scratches generated on the external sample surfaces by mechanical polishing, we expected that synchrotron monochromatic XRT would allow us to determine whether they correspond to strain at the level of scratch sides or to traces left by dislocations moving transverse planes (Gastaldi et al 1995b). In fact, only synchrotron white-beam and monochromatic X-ray topographs recorded at the wings of Bragg peaks displayed band-shaped contrasts but, as they reveal also that perpendicular streaks are spread in between these contrasts (figure 10), we have to check by recording synchrotron monochromatic X-ray topographs again after polishing to see whether the streaks remain in order to know whether they correspond to bulk defects.…”

“…Owing to their size (millimetres) we recorded only phase contrast radiographs and synchrotron white-beam X-ray topographs from Al-Cu-Fe as-grown grains (Gastaldi et al 1995b). Nevertheless it was possible to realize that their degree of perfection was comparable with that of Al-Pd-Mn as-grown grains.…”

“…Also, many structural defects such as those present in crystals (subboundaries, dislocations, stacking faults, etc.) were identified by transmission electron microscopy (TEM) (Devaud-Rzepski et al 1989, Singh and Ranganathan 1996, Wang et al 1998), but others giving peculiar contrasts on X-ray topographs (two lobes and loop shaped) (Gastaldi et al 1995b show resistence to characterization. Therefore it can be supposed that quasicrystalline grains still contain unidentified defects, either single or grouped, which could help to verify more distinctly theoretical considerations (Socolar 1991, Joseph and Elser 1997, Janot et al 2000.…”