On the twinning of alloys of near CoAl₃ stoichiometry exhibiting decagonal pseudosymmetry

Binary alloys Co25Al75...Co22Al78 were investigated by means of scanning electron microscopy, polarized light metallography as well as X-ray powder and single crystal diffraction methods. Needle-like decagonal prisms were observed in the surface habit of these as-cast alloys by scanning electron microscopy. Metallographic investigations in polarized light as well as ion beam etched specimens showed that the decagonal prisms are not true single crystals. Moreover, they show complex microstructure built by two different kinds of grain, twinned and untwinned, indicating two different phases. Weissenberg photographs of decagonal prisms showed splitting of diffraction reflections in five partial spots as a result of polysynthetic twinning. The diffraction motif of Weissenberg photographs with the rotation axis perpendicular to the basal plane of decagonal prisms exhibits decagonal pseudosymmetry. Guinier photographs of alloys rapidly quenched from the liquid state in the composition range Co27.5Al72.5...Co24Al76 showed reflections of a new phase. Its diffraction pattern is very similar to that of the decagonal phase Co20Cu15Al65. Comparable X-ray diffraction data were also found in the Gandolfi photographs of decagonal prisms