During quenching of aluminium alloys from the solutionising temperature vacancies are partially conserved as excess vacancies, partially lost to vacancy sinks, the exact fractions depending on the cooling rate. Positron lifetime measurements in samples from interrupted quenching experiments reveal that vacancies are lost during cooling down to 200 degrees C, after which solute atoms start to form clusters down to 20 degrees C. Slow cooling leads to 1 to 2 orders of magnitude lower excess vacancies than fast cooling. Since quenched-in vacancies are crucial for natural ageing (NA) in Al-Mg-Si alloys it is surprising to find just small differences between the NA hardening kinetics after different quenches. Specifically, hardening rates differ only in the initial stage (<100 min), after which they are almost identical for NA up to similar to 1 year. This suggests that interactions between vacancies and early-stage solute clusters help equalising the free vacancy fractions in differently quenched samples. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The influence of quench rate after solution heat treatment on the microstructure in the as-quenched state and subsequent ageing kinetics of alloy AA6014 was investigated by means of transmission electron microscopy, positron annihilation lifetime spectroscopy and hardness measurements. Various ageing temperatures and stages were taken into consideration. Consistent with previous studies, we found that solute and vacancy supersaturation decrease during slow quenching due to precipitation and annihilation, respectively. Additionally, we observed cluster formation during cooling below 200 °C. As for the influence on ageing behaviour we observe different behaviour for high and low ageing temperature: Artificial ageing is more affected than pre-ageing and natural secondary ageing. The detrimental effect of natural ageing on paint-bake hardening also depends on the quench rate. Possible interpretations are associated with cluster formation during natural ageing and also during quenching. The influence of pre-ageing at different temperatures on subsequent ageing kinetics is similar for slower industrial-type quenching and for fast quenching, thus allowing to apply the findings from idealised quenching conditions to situations closer to real application.

Precipitation hardening involves solutionising, quenching and annealing steps, the latter often at various temperatures. The phenomena observed in Al-Mg-Si alloys are very complicated and partially not well understood. During and after quenching, solute atoms diffuse through the lattice assisted by vacancies and form atom clusters that gradually grow. These act back onto vacancies, which complicates the situation. We apply positron annihilation techniques in addition to traditional hardness, resistivity and thermal measurements to clarify what happens in various stages of thermal treatment: The quenching process can be divided into a stage of vacancy loss and of precipitation. Very short artificial ageing treatments after heating at different rates show that there is a competition between vacancy losses and cluster formation as the temperature increases. The difference between natural ageing and artificial ageing can be defined based on the importance of excess vacancies. Based on such results the behaviour of “invisible” objects such as vacancies and small clusters can be better understood but some open question remain such as the kinetics of secondary ageing or the details of the negative effect of natural ageing on artificial ageing.

The deleterious effect of natural ageing (NA) on subsequent artificial ageing (AA) of Al-Mg-Si alloys can be markedly reduced by adding small amounts of Sn owing to the strong interaction between Sn atoms and excess vacancies. The retarding effect of Sn on clustering after quenching and during NA was verified in this study. The largest effect was found in a solute-lean Al-Mg-Si alloy containing 70 ppm Sn and solutionised at 570 °C. Employing the same strategy, the delayed clustering kinetics was also observed in alloys in which Sn was replaced with In. Based on the data obtained from positron annihilation lifetime, hardness and electrical resistivity experiments，we introduce some concepts describing the mechanisms of NA cluster formation in the presence of Sn (In) based on vacancy-solutes interactions.

The effect of pre-ageing (PA) time and temperature on subsequent natural secondary ageing (NSA) and paint bake (PB) hardening of an AA6014 Al-Mg-Si alloy was systematically investigated, especially when both parameters change. A wide range of PA conditions was covered with temperatures ranging from 80 °C to 160 °C and times from several minutes to several days depending on the PA temperature. Hardness data for such pre-treatments measured by Brinell method are given. Hardness data measured during NSA are fitted by various functions. This dataset might be reused for further kinetic analysis of the clustering in Al-Mg-Si alloys or for the determination of the optimal PA tactics for industrial production.