Influence of interrupted quenching on artificial aging of Al–Mg–Si alloys

In this study the influence of interrupted quenching (IQ) in the temperature range 150–250°C for periods of 15–1080s on artificial aging after long-term natural pre-aging was studied for the Al–Mg–Si alloy AA6061 by atom probe tomography, transmission electron microscopy, electrical resistivity and hardness measurements, and differential scanning calorimetry. Compared with a standard quenching procedure, the results showed that hardening kinetics and the age hardening response were enhanced for IQ at low temperatures but reduced at high temperatures. Quenched-in vacancies were shown to be of particular importance for the nucleation of precipitates occurring during IQ at the lower end of the temperature range, finally leading to the formation of a dense distribution of β″ during artificial aging. For standard water quenching and subsequent natural aging, nucleation is hindered by a low concentration of quenched-in vacancies in the matrix. IQ at high temperatures affects subsequent artificial aging via the formation of precipitates which do not contribute to hardening but consume a significant amount of solute.