THE INFLUENCE OF THE MG CONTENT AND THE QUENCHING CONDITIONS ON THE DECOMPOSITION KINETICS OF AL-ZN-MG ALLOYS

By means of resistivity measurements, TEM and microhardness measurements the decomposition kinetics of A1‐3 at.% Zn‐1.5 at.% Mg and A1‐4.5 at.% Zn‐x at.% Mg (x = 0.2; 0.5; 1.0; 1.5; 2.25; 3.0) were investigated. For Ta < Ta, crit < Th (Th: temperature of the rapid homogenous nucleation) in A1‐3 at.% Zn‐1.5 at.% Mg and A1‐4.5 at.% Zn‐x at.% Mg (x = 1.0; 1.5; 2.25; 3.0) an inversion of the decomposition kinetics for Tq > Tinv: (Tinv: inversion temperature) was established. This result can be explained with reference to the binding of excess vacancies in dislocation loops. Proceeding from the discrepancy between the experimentally estimated concentrations of vacancies bound in dislocation loops and the concentrations of free vacancies at Tq, calculated by means of the Lomer equation, a model is proposed, which explains the high vacancy concentration of the loops. In terms of this model the values of the migration energy, obtained for A1‐3 at.% Zn — 1.5 at.% Mg in the Ta‐range 23 °C ≦ Ta ≦ 100 °C, and the shift of the inversion temperatures Tinv towards higher values with increasing Ta are interpreted. The same applies to the influence of the Mg content on the decomposition kinetics in A1‐4.5 at.% Zn‐x at.% Mg alloys. Copyright © 1990 WILEY‐VCH Verlag GmbH & Co. KGaA