Design strategy for controlled natural aging in Al-Mg-Si alloys

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. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Diffusion on Demand to Control Precipitation Aging: Application to Al-Mg-Si Alloys

We demonstrate experimentally that a part-per-million addition of Sn solutes in Al-Mg-Si alloys can inhibit natural aging and enhance artificial aging. The mechanism controlling the aging is argued to be vacancy diffusion, with solutes trapping vacancies at low temperature and releasing them at elevated temperature, which is supported by a thermodynamic model and first-principles computations of Sn-vacancy binding. This "diffusion on demand" solves the long-standing problem of detrimental natural aging in Al-Mg-Si alloys, which is of great scientific and industrial importance. Moreover, the mechanism of controlled buffering and release of excess vacancies is generally applicable to modulate diffusion in other metallic systems

Effect of copper addition on the cluster formation behavior of Al-Mg-Si, Al-Zn-Mg, and Al-Mg-Ge in the natural aging

The time-dependent resistivity of Al-Mg-Si(-Cu), Al-Zn-Mg(-Cu), and Al-Mg-Ge(-Cu) alloys are studied over a range of constant temperatures between 255 K and 320 K. The resistivity vs time curves for the samples show three temperature stages associated with solute element–vacancy clustering. Cu addition was found to make the stage transition time longer for the studied samples. Arrhenius plots of the transition time vs temperature provide the activation energy (Q) of clustering from stage I to II and II to III. While the Cu addition increased the Q(I to II) values of Al-1.0 pct Mg2Si-0.20 pct Cu and Al-2.68 pct Zn-3.20 pct Mg-0.20 pct Cu, it was found that the added Cu decreased the Q(I to II) value of Al-0.44 pct Mg-0.19Ge-0.18 pct Cu. The Q(II to III) values of Al-1.0 pct Mg2Si and Al-2.68 pct Zn-3.20 pct Mg were slightly decreased by the Cu addition. The different effect of added Cu on the Q values is discussed in terms of diffusivity and binding energy between vacancies and solute elements