Electron-Microscopic Investigation on Precipitation Phenomena in Aluminium-Zinc Alloys

Abstract

The precipitation process, its relation with lattice defects and the effect of cold working on precipitation and grain boundary reaction in the aluminium alloys containing ll.2, 17.4, 28.3 and 40.8% zinc respectively were investigated by means of X-ray diffraction, hardness measurements and transmission electron microscopy. It was found that face-centered cubic transition precipitates were formed in granular shape in ll.2% or 17.4% zinc alloys and were plate-like in 28.3% zinc alloys. The plate-like transition precipitates were formed on {110} matrix planes, perhaps preferentially on the prismatic dislocation loops. The results are shown to be consistent with the hypothesis that in the quenched alloys precipitation occurs on prismatic dislocation loops that result from the condensation of quenched-in vacancies. In Al-28.3 and 40.8% Zn alloys, the grain boundary reaction is predominant at lower aging temperatures.