Study of the behaviour of Al-Zn alloys by SAXS in the temperature range of the instable solid solution

During cooling from the homogeneous region the structural changes of Al-Zn alloys in the range from 10 at% up to 60 at% Zn are observed by means of SAXS. Using a special method of investigation typical courses of the matrix scattering (background scattering) with respect to the temperature are obtained with characteristic points and ranges. The behaviour of the matrix scattering intensity shows that already about 30 K above the maximal temperature of the miscibility gap at about 39 at% Zn an 'anomalous' concentration fluctuation is formed. The fluctuations increase with decreasing temperature and obviously represent a gradual transition into the inhomogenous state. The characteristics of the matrix scattering allow to define a line in the Al-Zn phase diagram which is due to the starting temperature of homogeneous nucleation. They also deliver points of the metastable as well as stable phase boundaries. Calculations based on well known models prove the concept that the behaviour of the matrix scattering is influenced by the concentration fluctuations depending on temperature. (author)