X-ray diffraction study of decaprismatic Al-Co-Ni crystals as a function of composition and temperature

The phase transformations of a series of decaprismatic Al-Co-Ni crystals were studied in situ by X-ray diffraction between ambient temperature and 1573 K. Five samples in the composition range Al74—70Co22—6Ni4—23 were investigated. In a small range at high temperatures, all specimens showed the diffraction pattern of the basic decagonal phase which has a 4 Å translation period. At lower temperatures and depending on the chemical composition, transformation to oriented nanodomain structures and a number of different superstructures were observed. The translation period of the superstructures is twice the translation period of the basic decagonal phase. Characteristic for these superstructures is a correlation length which is large in the periodic direction and small in directions perpendicular to it. The correlation length decreases with increasing temperature.