Mechanisms of age-hardening in two Al-Cu-Mg alloys studied by Positron Annihilation Spectroscopy

Age-hardening at 150 C has been studied in an Al-Cu-Mg alloy microalloyed with 0.1 at.% Ag and the commercial alloy 2024. Positron Annihilation Lifetime Spectroscopy (PALS) was combined with measurements of microhardness changes during ageing at 150 C. Both alloys are known to harden in two stages separated by an extended plateau. Combined PALS and hardness data shows that the effect of Ag is to enhance vacancy trapping at Mg atoms in competition with the formation of mobile vacancy-Cu pairs. Thus, the addition of Ag gives a reduction of Cu atoms mobility, leading to slower initial hardening in comparison with a silver-tree alloy with similar Cu:Mg ratio. In both alloys, the second stage of hardening begins before increases occur in positron lifetimes that indicate the formation of misfit interfaces (i.e. semi-coherent precipitates). This observation supports the view that this second stage of hardening in alloys based on the Al-Cu-Mg system is associated mainly with the growth and transformation of coherent GP(Cu,Mg) zones. The PALS data are consistent with the hypothesis that the X' phase, previously observed to be present in Al-Cu-Mg-Ag at peak ageing, develops misfit interfaces with the matrix.