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In this study natural aging of an Al–Mg–Si alloy was investigated using various quenching processes. Atom probe tomography and electrical resistivity measurements reveal that solute clustering during natural aging can be suppressed by interrupting quenching for 120 s at 160 °C. This phenomenon is elucidated by simulating the excess vacancy annihilation. Reduced frozen-in excess vacancy concentration after the interrupted quenching can explain this experimentally observed suppression of natural aging. [ABSTRACT FROM AUTHOR]

The influence of the thermal route on the peak-aged microstructures in an Al–Mg–Si aluminum alloy was investigated by atom probe tomography and transmission electron microscopy. Direct artificial aging induced a monomodal size distribution of β″-precipitates, while for artificial aging after natural pre-aging a bimodal size distribution was found. This difference can be explained by the impact of lattice defects on the β″-nucleation. β″ is supposed to contain Al and exhibits a high Mg/Si ratio in the studied alloy. [ABSTRACT FROM AUTHOR]

Abstract: In this study, the interdependent effect of chemical composition and thermal history on artificial aging was investigated for the aluminum alloy AA6061. Based on thermodynamic calculations, including Al, Fe, Cr, Zn, Ti, Mg, Si and Cu, model alloys exhibiting a maximum variation of the reachable solute super-saturation of elements relevant for precipitation hardening within the compositional limits of AA6061 were produced. The artificial aging behavior of these alloys at 175°C was studied by tensile testing for two thermal histories, including very short- and long-term room temperature storage after quenching. Precipitation during artificial aging was investigated by an analysis of yield strength data. As generally expected, precipitation kinetics was found to depend strongly on the solute super-saturation in the case of very short room temperature storage. For artificial aging after long-term room temperature storage the kinetics showed almost no dependence on the chemical composition. This seems to be an exception from simple precipitation kinetics and can be explained based on the complex role of quenched-in vacancies in artificial aging of AA6061. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: In this study the artificial aging behavior of the Al–Mg–Si alloy AA 6061 was investigated in the temperature range 150–250°C using atom probe tomography, hardness and resistivity measurements for various thermal histories. It was found that the precipitation kinetics and age-hardening response of artificial aging at temperatures below 210°C are lowered by prior natural aging but enhanced above this temperature. An analysis of hardness data was used to evaluate the temperature dependence of precipitation kinetics and dissolution processes. Supported by theoretical considerations, it is assumed that artificial aging of Al–Mg–Si alloys is controlled via the concentration of mobile vacancies. The “vacancy-prison mechanism” proposed determines the mobile vacancy concentration in the case of natural pre-aging by temperature-dependent dissolution of co-clusters and solute–vacancy interactions. [Copyright &y& Elsevier]

Abstract: This study investigates the effect of starting temperature on differential scanning calorimetry measurements of an Al–Mg–Si alloy. Linear heating experiments starting from 294K to 423K were performed at different heating rates directly after quenching. It is shown that the peak for the major strengthening phase β″ is shifted to unexpected high values at low heating rates for a starting temperature of 294K. The effect is supposed to be comparable to the well-known influence of room temperature storage after quenching on the precipitation of β″. As a consequence it is shown that methods to determine effective activation energies based on Kissinger-like analysis are restricted to measurement conditions where transformation mechanisms are not changed by the heating rate in Al–Mg–Si alloys. [Copyright &y& Elsevier]

This study first provides a concise review of natural aging in Al–Mg–Si alloys and its effect on artificial aging. The second part investigates prolonged natural aging at different temperatures for > 500 days of commercial and trace element added alloys. Together, the two parts improve the picture of underlying mechanisms and refine suggestions regarding the five stages of natural aging. Trace Sn- or Sn + In-added alloys show a trend towards higher activation energies of clustering, and a higher temperature dependency of natural aging than commercial alloys. This is attributed to an additional contribution of a thermally activated vacancy release from Sn- and In-vacancy pairs. Sn and In additions are suggested to decrease cluster number density while increasing cluster size. Prolonged natural aging increasingly retards artificial aging kinetics. This is interpreted according to increasingly slower cluster dissolution kinetics and slower preferential growth of β´´ needles. The reachable strength however seems to strongly depend on the ratio and size of coarse β´´ needles, preferentially grown, and the number of fine re-precipitated β´´ needles. Artificial aging after prolonged natural aging at 45 °C increases the artificial aging peak hardness due to a lower density of larger clusters than at lower temperatures. [ABSTRACT FROM AUTHOR]

This study presents a design strategy for Al–Mg–Si alloys to control natural aging. Recently, trace addition of Sn was shown to suppress natural aging for up to two weeks, which was explained by the strong trapping of vacancies to Sn atoms. Here we explore the effect of solution treatment temperature, the combination of trace elements such as Sn and In, and the composition of main hardening elements Mg, Si and Cu on natural aging. The results are discussed based on the dissolvable amount of trace elements and their effect on diffusion retardation, and solute clustering mechanisms in Al–Mg–Si alloys. Thermodynamic calculations using the CALPHAD approach show that maximum retardation of natural aging is achievable at the highest trace element solubility, which exists at significantly different solution treatment temperatures for Sn or In. The effects of Mg, Si and Cu content on natural aging kinetics are interpreted via their influence on the Sn solubility and clustering mechanisms. It is proposed that Sn additions reduce the concentration of excess vacancies, which is most important for early Si clustering, and that the effect of Cu is comparable to the effect of Sn, but less pronounced. Based on the investigated parameter space, a design concept is proposed and an Al–Mg–Si alloy showing suppression of natural aging for >6 months and significant artificial aging potential is demonstrated. [ABSTRACT FROM AUTHOR]

This study investigates the effect of trace Sn additions on the artificial aging of an Al–Mg–Si alloy at unconventionally high temperatures (> 210 °C), where such additions generate both ultrafast aging kinetics and superior peak hardness. The study shows that the effect of Sn is comparable to the influence of natural pre-aging on high temperature artificial aging, and explains this by solute-vacancy interactions, proposing that during artificial aging Sn-vacancy pairs contribute to diffusion and/or Sn retards the annihilation of quenched-in vacancies. [ABSTRACT FROM AUTHOR]