Secondary ageing in an Al-Cu-Mg alloy with high Cu/Mg ratio

The mechanisms governing secondary ageing (structural transformations occurring at low temperature after a heat treatment at higher temperature) were investigated by combined measurements of positron lifetimes, Vickers microhardness measurements and Differential Scanning Calorimetry (DSC) on a laboratory alloy (Al-4.5 wt.% Cu-0.56 wt.% Mg). The results show that hardening occurs at a much slower rate than in case of primary ageing at RT. The positron lifetime data suggest that the hardening rate is controlled by slow release of vacancies from Cu-rich clusters formed during the initial high temperature treatment. The hardening stage is concomitant with an increase of the positron lifetime, and has probably the same origin, which is the formation of solute clusters containing vacancies and Mg as essential components. The formation at low temperature of new structures is also demonstrated by DSC.