Mechanism of growth of G. P. zones in aluminum-silver alloys

Abstract

The mechanism of growth of G. P. zones in Al-20%Ag, Al-5%Ag, and Al-5%Ag-0.04%Sn alloys was examined by electric resistivity measurements and tensile tests.The existence of two kinds of G. P. zones, ε and η, was confirmed in these alloys, and their ageing characteristics were satisfactorily explained in terms of the metastable miscibility gap given by Baur and Gerold. However, it was shown that the ε-η transition temperature in low silver concentration was much higher than the value expected from the metastable miscibility gap.It was found that ε was made coarser according to the Ostwald ripening mechanism in later stages of ageing. The activation energy of growth of ε was determined to be 21.7kcal/mol.Tin retarded the clustering during quenching and in the earliest stages of ageing, but accelerated the G. P. zone development in the next stages. After prolonged ageing, the effects of tin on the growth of ε were very little.Effects of the quenched-in vacancies in later stages of ageing were also discussed.