Effect of Ag addition on the precipitation evolution and interfacial segregation for Al–Mg–Si alloy

Abstract

The effect of Ag addition on the precipitation evolution and interfacial segregation for Al-Mg-Si alloys was systematically investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), atom probe tomography (APT) and density functional theory (DFT) calculation. At the early aging stage, Ag atoms could enter clusters and refine the distribution of these clusters. Then, Ag atoms preferentially segregate at the GP zone/α-Al and β"/α-Al interfaces at the peak aging stage by the replacement of Al atoms in FCC matrix. With prolonging aging time, Ag atoms generally incorporate into the interior of β" precipitate, facilitating the formation of QP lattice (a hexagonal network of Si atomic columns) and the local symmetry substructures, Ag sub-unit (1) and Ag sub-unit (2). At the over-aged stage, the Ag sub-unit (1) and Ag sub-unit (2) could transform to the β′Ag (i.e. β′Ag1 and β′Ag2.) and Q′Ag unit cells, respectively. All the precipitates at the over-aging stage have a composite and disordered structure due to the coexistence of different unit cells (β′Ag1, β′Ag2, Q′Ag and β′) and the non-periodic arrangement of Ag atoms within the precipitate. In the equilibrium stage, the incorporated Ag atoms in the precipitates release into the α-Al matrix as solute atoms or form Ag particles. In general, Ag atoms undergo a process of "segregate at the precipitate/matrix interface → incorporate into the interior of precipitate → release into the α-Al matrix" during the precipitation for Al-Mg-Si-Ag alloys. Besides, Ag segregation is found at the interfaces of almost all metastable phases (including GP zone, β″, β′/ β′Ag phase) in Al-Mg-Si-Ag alloys. The Ag segregation at the β′/α-Al interface could increase the length/diameter ratio of β′ phase and thus promote the additional strengthening potential of these alloys. These findings provide a new route for precipitation hardening by promoting the nucleation and morphology evolution of precipitates.

Newer Developments in Aluminium Alloys Through Ordered Precipitates and Segregation of Transition Elements

“…The alloys were found to have better thermal stability than Al-Mg-Si alloys at 250 °C [110]. A similar effect was reported with Ag addition [62,111]. At initial stages of ageing to Al-Mg-Si-Ag alloys, Ag was reported to segregate at GPZ/a-Al matrix and b''/a-Al matrix interface, which on later stages of ageing were incorporated into the precipitates resulting in QP lattice with disordered structures.…”

“…The addition of Ag to high Al-Cu-Mg alloys with a high Cu/Mg ratio results in the formation of a thermally stable coherent X phase along {111} Al habit plane [61]. Ag has also been found to segregate at precipitate/matrix interface [58,59,62]. However, the addition of Cu results in the suppression of the b'' phase and promotes several new metastable phases.…”

Influence of Aging on Corrosion Behaviour of the 6061 Cast Aluminium Alloy

“…Compared with the samples after solid solution treatment, the 180 °C aging treatment significantly enhanced hardness, and it increased gradually with aged time with a peak value of 116.4 ± 1.5 HV at 5 h. The hardness then decreased slowly with increasing aged time, while the main strengthening stage in the peak aged stage is β”. According to the previous literature [ 25 , 26 , 27 ], the rapid hardness increase was mainly due to clusters and GP zones formation at the under-aged stage, while the main strengthening phase at peak aged stage was β″. Thus, under aged (1 h), peak aged (5 h), and overaged (14 h) conditions were selected to study the precipitate evolution.…”

Advanced imaging and simulations of precipitate interfaces in aluminium alloys and their role in phase transformations

“…the Ω phase in the Al-Cu-Mg-Ag [32], the T 1 phase in Al-Cu-Li-Mg-Ag [36] and the β /β phases in Al-Mg-Si-Ag [37]. The micro-alloying effect of Ag is expected to be deeply related to the clustering behaviour of Ag in the aluminium matrix and its interaction with defects and other solute elements.…”