SHAPE AND INTERNAL STRUCTURE OF GUINIER-PRESTON ZONES IN AL-AG

The early stages of decomposition of Al-Ag single crystals with 0.3, 3 and 5 at.% Ag were followed by small-angle X-ray scattering (SAXS). The composition, form and structure of the Guinier-Preston (GP) zones were characterized for ageing temperatures ranging from 20 to 250-degrees-C. The composition of the GP zones (approximately Ag2Al) was found to be nearly independent of the ageing temperature. Below 170-degrees-C homogeneous (eta-)zones are formed. Above 170-degrees-C the (epsilon-)zones have a silver depleted core and a shell of nearly pure silver. The GP zones are spheres faceted mainly along {111} planes. Model calculations allowed the degree of faceting to be determined from the experimental scattering cross sections. For higher ageing temperatures, the result agrees well with predictions based on the modified regular solution model of Alexander et al.