Structural and compositional study of precipitates in under-aged Cu-added Al-Mg-Si alloy

Abstract

Atomic scale characterization of fine precipitates in an under-aged Cu added Al-Mg-Si alloy was carried out by combination of atomically-resolved annular dark-field scanning transmission electron microscopy and energy dispersive X-ray spectroscopy. Two types of precipitates were observed in the alloy. In the case of ordered β” precipitates, β” was proposed as Mg5-xAl2+xSi4 (x ≈ 1) with solute Cu atoms replacing Al site of β” precipitate. In the case of disordered precipitates, the precipitates were found to consist of β” sub-unit cells, three-fold symmetric structure without Cu atoms, Cu containing structures termed as “Cu sub-unit cluster”, and Q’ sub-unit cells. Among these structures, the morphologies of three-fold symmetric structure without Cu atoms, Cu sub-unit cluster, and Q’ sub-unit cell were almost the same, so that these structures should be the clusters of Q’ phase. Since the areal density, length and diameter of precipitates were almost equal between Cu free Al-Mg-Si alloy and Cu added Al-Mg-Si alloy, the increase of hardness by Cu addition should be due to the precipitation of Cu related precipitates, such as Cu sub-unit clusters and Q’ sub-unit cells.

Review of the Quench Sensitivity of Aluminium Alloys: Analysis of the Kinetics and Nature of Quench-Induced Precipitation

“…However, during cooling at 0.1 K/s in the ternary laboratory alloys, two further types of MgSi precursor phases were identified, namely hexagonal β'-Mg9Si5 (Figure 45) and U1-MgAl2Si2. From Figure 45, it is particularly interesting to note that these fine particles are polycrystalline, and are likely to contain disordered areas such as those previously reported for fine precipitates formed during ageing in AlMgSi alloys [184][185][186][187]. ageing for pure Al0.6Mg0.8Si shows a maximum at the UCCR determined by DSC, and unlike all other alloys studied in this work, the hardness drops with faster cooling rates.…”

On the effect of interrupted ageing (T6I4) on the mechanical properties of AA6351 and AA7050 alloys

“…The sequence of generic precipitation for the 6xxx series alloys is defined as: SSS (supersaturated solid solution) → AC → GP zones → β'' → β' + B' + U1+ U2 → β (Mg 2 Si) phases (Murayama and Hono, 1999;Van Huis et al, 2006;Vissers et al, 2007;Son et al, 2011;Xiao-song et al, 2011;Simar et al, 2012;Maeda et al, 2018). In this sequence, initially, Mg and Si form atomic clusters (AC) and GP (Guinier-Preston) zones are originated (Vissers et al, 2007), with Si to Mg ratio close to 1 (Murayama and Hono, 1999).…”

“…The 6xxx (Al-Mg-Si) and 7xxx (Al-Zn-Mg-Cu) series aluminum alloys are widely applied in the automotive and aircraft manufacturing due to their high strength to density ratio that allows the weight reduction of structures (Buha et al, 2008;Chen et al, 2013;Maeda et al, 2018). The changes in the aluminum alloys microstructure cause remarkable modifications in their properties (Souza et al, 2012).…”

Silicon Nitride Whisker-Reinforced Aluminum Matrix Composites: Twinning and Precipitation Behavior

“…where SSS stands for the supersaturated solid solution, Al 2 Cu and Mg 2 Si are two main strengtheners in the peak-aged condition [48]. Some other phases (e.g., S (Al 2 CuMg) [50], and Q-Al 5 Cu 2 Mg 8 Si 6 [40] or Q-Al 3 Cu 2 Mg 9 Si 7 [51]) also existed in the aged Al-Si-Cu-Mg alloys. Double aging peaks in the Al-Si-Cu-Mg alloy were also reported by Li et al [50].…”