Natural Aging in Al‐Mg‐Si Alloys – A Process of Unexpected Complexity

Abstract

The natural aging behavior of pure ternary Al‐Mg‐Si alloys is investigated by measuring hardness, electrical resistivity and positron lifetime, as well as carrying out thermal analysis and atom probe microscopy. It is found that several distinct temporal stages of natural aging can be distinguished in which one of these quantities shows a characteristic behavior and that these times coincide for many of these measurements. The rate of change in the measured data is correlated with proposed solute dynamics during natural aging for both aging that takes place prior to artificial aging (natural pre‐aging) and after artificial underaging (natural secondary aging) heat treatments. Controlling factors for solute dynamics are discussed.

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

“…In the following clustering stage II, the fraction of vacancy-free clusters continuously increases until these dominate [15]. Both types of clusters are interpreted as predominantly involving Si and result in an increase of electrical resistivity or hardening at an initial rate in stage I and a more rapid increase in stage II [13,14,29,30]. Both linear logarithmic stages are present in the hardening curves in Fig.…”

“…), which starts directly after quenching from solution heat treatment, the material hardness increases due to solute clustering of Mg-and Si-atoms [11][12][13][14][15]. This generates two problems for the transportation industry: first, dynamic hardening during n.a.…”

“…reduces formability [4]; second, clustering results in a negative effect of n.a. on subsequent artificial aging (a.a.) [12,13,[16][17][18]. The transportation industry, however, requires several months of stable formability combined with good a.a. performance so as to obtain reproducibility of designs that have increasing complexity [4,10].…”

Kinetics of natural aging in Al-Mg-Si alloys studied by positron annihilation lifetime spectroscopy

“…In alloy F, the full negative effect is established after about 18 min of NA, 7 suggesting that the processes observed in stage I are a prerequisite for the negative effect.…”

“…One important result is that NA takes place in distinct stages, see for example Ref. 7 and references therein. Direct observations of the structures formed during NA in the transmission electron microscope (TEM) claimed in the literature 8 could not be reproduced later, 9 and just atom probe (AP) analysis seems to have the potential to reveal the clusters formed.…”