Abstract
Al-Mg-Si alloys with a low solute content (0.4 %Mg, 0.4 %Si) and higher contents (Mg+Si=1.4%, all in wt.%) were solutionised, quenched and then artificially aged (AA) at 180 °C, after which positron annihilation lifetime spectroscopy was applied to obtain information about precipitation and vacancy evolution during ageing. Hardness measurements were carried out to complement the measurements of positron lifetime. AA was carried out in four different heating media and allowed for varying the heating rate from 2.4 to 170 K/s. The main result of the study is that there is a competition between vacancy losses and precipitation. The more precipitation takes place before and during AA, the more vacancies are retained and act back on clustering. Higher solute content, slower heating to 180 °C, natural pre-ageing before AA and slower quenching after solutionising were found to have a comparable effect explainable by pre-precipitation before AA.
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