Influence of germanium-clustering on the precipitation behaviour of rapidly solidified AlGe solid solutions

Abstract

Differences in precipitation behaviour in liquid and solid quenched Al1.3at.%Ge solid solutions were investigated by means of transmission electron microscopy and electrical resistivity measurements. Precipitation behaviour in rapidly quenched Al(Ge) alloys is assumed to depend on the existence of germanium clusters that act as nucleation sites, thus leading to a strong correlation between cluster density and the density of germanium precipitates after annealing. The observed precipitation morphology indicates that such clusters exist in thermodynamically stable solid solutions as well as in slightly superheated melts. At higher temperatures the number of clusters in the superheated melt is drastically reduced. Appropriate high quenching rates have been found to result in the formation of supersaturated solid solutions with a low density of clusters, leading to a distinctly different precipitation morphology and retardation of the precipitation reaction upon ageing compared with solid quenched alloys.