Electron microscope study of the symmetry of the basic atom cluster and a structural change of decagonal quasicrystals of Al-Cu-Co alloys

The symmetry of the basic atom cluster and the space group of the decagonal quasicrystal of an annealed Al65Cu15Co20 alloy have been investigated by high-resolution electron microscopy and convergent-beam electron diffraction. It has been revealed that the symmetry of the clusters projected along the c-axis (decagonal axis) is pentagonal and that the space group is P10(5)/mmc. Decagonal quasicrystals of melt-quenched Al-Cu-Co alloys, which belong to the space group of <P(10)over bar m2>, were previously found to have pentagonal clusters (Saitoh et al. 1994, Mater. Sci. Engng A, 181-182, 805). Therefore, the symmetry of the atom clusters projected along the c-axis is pentagonal in the Al-Cu-Co quasicrystals irrespective of specimen preparation methods. The space group of melt-quenched Al-Cu-Co alloys changes from <P(10)over bar m2> to P10(5)/mmc at about 600 degrees C. In the <P(10)over bar m2> phase, the inversion domains are several tens of nanometres in size and all the clusters in one domain have the same sense of polarity. Thus, either the domain size suddenly decreases down to the size of the diameter of the cluster at the above temperature, or the clusters with positive and negative senses are completely intermixed. The space-group change is explained by the formation of 10(5) screw axes and c-glide planes between the clusters with the two senses.