Study of formation and reversion of Guinier-Preston zones in Al-4.5 at%Zn-x at%Mg alloys by positrons

Abstract

Measurements of the positron lifetime, τ, and the shape parameter,S, of the 2γ-angular correlation curve are reported for the Al-4.5 at%Zn-x at%Mg alloys (0 ⩽x ⩽ 2.5). In the alloys aged at room temperature after quenching two types of Guinier-Preston (GP) zones coexist both of which are able to trap positrons. The first type of zone is rich in zinc and completely free of any vacancy-type defects. The second type contains vacancies surrounded by zinc and magnesium atoms. From the comparison of τ andS, the composition of the surroundings of vacancies as a function of the magnesium concentration of the alloys is concluded. By increasing the magnesium content and the ageing time of the alloys the second type of GP zone increasingly dominates over the first one. The stability of both types of zone are studied in a reversion experiment. In an alloy with 2.5 at%Mg the formation of the η′ and η-phases are observed. The positron annihilation results are compared with the results obtained by calorimetry, small angle X-ray scattering and electron microscopy on the same alloy series.