Co-evolution of vacancies and solute clusters during artificial ageing of Al-Mg-Si alloys

Al-Mg-Si alloys with total solute contents ranging from 0.8 to 1.4 wt % were solutionized, quenched, and then artificially aged (AA) at 1800.16em0ex^∘C, after which positron annihilation lifetime spectroscopy was applied to obtain information about precipitation and vacancy evolution during the preceding ageing step. Hardness and electrical resistivity measurements were carried out to complement these measurements. AA was carried out in four different heating media, which allowed for varying the average heating rate from 2.4 to 1700.16em0exK0.16em0exs^-1. The main result of the study is that there is a competition between vacancy losses and precipitation. Any precipitation taking place during quenching or during heating to the AA temperature helps to prevent vacancies from going to sinks and allows them to assist in solute clustering. Higher solute content, slower heating to 1800.16em0ex^∘C, and natural preageing before AA were found to have a comparable effect.