Nucleation and growth of the γ′(AlAg2) precipitate in Al–Ag(–Cu) alloys

Rosalie, Julian M.; Bourgeois, Laure; Muddle, Barrington Charles

doi:10.1016/j.actamat.2011.08.001

Cited by 12 publications

(7 citation statements)

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“…An Al-1.68%Ag alloy aged at 200 • C for times varying from as-quenched to 7 days exhibits the microstructure expected from previous studies [21,60]: finely distributed Ag-enriched coherent precipitates known as GP zones and sparsely distributed γ precipitates. Fig.…”

Section: Atomic Structure: Haadf-stem Imaging Simulation and Analysissupporting

confidence: 63%

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The bi-layered precipitate phase ζ in the Al-Ag alloy system

et al. 2017

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The Al-Ag system is thought to be a well-understood model system used to study diffusional phase transformations in alloys. Here we report the existence of a new precipitate phase, ζ, in this classical system using scanning transmission electron microscopy (STEM). The ζ phase has a modulated structure composed of alternative bilayers enriched in Al or Ag.Our in situ annealing experiments reveal that the ζ phase is an intermediate precipitate phase between GP zones and γ . First-principles calculations show that ζ is a local energy minimum state formed during Ag clustering in Al. The layered structure of ζ is analogous to the well-known Ag segregation at the precipitate-matrix interfaces when Ag is microalloyed in various aluminium alloys.

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Section: Atomic Structure: Haadf-stem Imaging Simulation and Analysissupporting

confidence: 63%

“…GP zone and γ . The absence of shear in ζ minimises the energy barrier for its formation, which is considered to restrict γ nucleation [60]. Previous calculations also have shown that pure Ag layers lower the stacking fault energy in Al [70], which offers a pathway for a ζ to γ transformation.…”

Section: Discussionmentioning

confidence: 98%

The bi-layered precipitate phase ζ in the Al-Ag alloy system

et al. 2017

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“…3(a), 4(c) and 7(a)) did not show the alternating strong and weak contrast that was expected for the Ag-rich and Al-rich layers in this structure [27,28]. In an earlier report on thickening of γ ′ precipitates, we noted that Ag and Al did not appear to be ordered in single unit cell thickness γ ′ precipitates [29] and an investigation into ordering of silver in γ ′ precipitates in Al-Cu-Ag alloys is underway.…”

Section: Comparison With Other Al Alloy Systemsmentioning

confidence: 47%

Silver segregation to θ′ (Al2Cu)–Al interfaces in Al–Cu–Ag alloys

2012

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θ ′ (Al 2 Cu) precipitates in Al-Cu-Ag alloys were examined using high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The precipitates nucleated on dislocation loops on which assemblies of γ ′ (AlAg 2 ) precipitates were present. These dislocation loops were enriched in silver prior to θ ′ precipitation. Coherent, planar interfaces between the aluminium matrix and θ ′ precipitates were decorated by a layer of silver of two atomic layers in thickness. It is proposed that this layer lowers the chemical component of the Al-θ ′ interfacial energy. The lateral growth of the θ ′ precipitates was accompanied by the extension of this silver bi-layer, resulting in the loss of silver from neighbouring γ ′ precipitates and contributing to the deterioration of the γ ′ precipitate assemblies. Pre-printSilver segregation to θ ′ -Al interfaces in Al-Cu-Ag alloys J.M.Rosalie & L. Bourgeois forms with {111} habit instead of {100} and displays remarkable coarsening resistance. Drift corrected energy dispersive x-ray analysis [7] and 3D atom probe studies [8] have shown that Ag and Mg both segregate to the Ω-Al matrix interface. The Ω phase has a distinctive interface structure, with a bilayer of Ag atoms at the coherent interface with the matrix [9]. Density functional theory calculations indicated while substitution of either Mg or Ag alone was energetically unfavourable, a combination of both elements provided a lower-energy interface [10].Recent work has shown that compositional changes at the θ ′ -matrix interface occur in other alloys. Silicon was found to segregate to the coherent {100} interfaces of the θ ′ phase in Al-Cu-Si alloys, substituting for Cu sites in the precipitate [11]. It has also been demonstrated that the interface composition and structure of θ ′ in binary Al-Cu differs substantially from the bulk structure, with additional Cu atoms occupying octahedral interstices in the precipitate [12]. Despite these studies a detailed understanding of precipitate-matrix interfaces has yet to be developed in the majority of Al alloys.Aluminium-copper-silver (Al-Cu-Ag) alloys are ideal for studying the effect of a third element on the precipitation of the θ ′ phase. Unlike many ternary systems (e.g. Al-Cu-Mg) the Al-rich region of the phase field contains only binary phases [13], eliminating the complexities associated with competing ternary phase precipitation. The interatomic spacing in pure Ag also corresponds very closely to that of pure Al, and substantial additions of silver have little effect on the lattice parameter of Al [14,15]. Solute misfit can therefore be regarded as minimal, and solute-induced strain can be largely neglected. In addition, the chemical affinity between Ag and Cu atoms is weak and the strong co-clustering behaviour exhibited by systems such as Al-Si-Mg is not observed.Early studies on Al-Cu-Ag alloys established that for compositions of ∼1at.%(Ag,Cu) both γ ′ (AlAg 2 ) and θ ′ precipitates were formed [16] and it been shown that both phases can form on disloc...

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“…High angle annular dark field (HAADF) / annular dark field (ADF) scanning transmission electron microscopy (STEM): (262,269,292,301,307,326,333,334,344,348,400,408).…”

Section: Energy-filtered Transmission Electron Microscopy (Eftem) / Ementioning

confidence: 99%

The Crystallography of Aluminum and Its Alloys

Nakashima

2019

Encyclopedia of Aluminum and Its Alloys

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This article begins with pure aluminum and a discussion of the form of the crystal structure and different unit cells that can be used to describe it. Measurements of the face-centered cubic lattice parameter and thermal expansion coefficient in pure aluminum are reviewed and parametrizations are given which allow the reader to evaluate them across the full range of temperatures where aluminum is a solid. A new concept called the “vacancy triangle” is introduced and demonstrated as an effective means for determining vacancy concentrations near the melting point of aluminum. The Debye–Waller factor, quantifying the thermal vibration of aluminum atoms in pure aluminum, is reviewed and parametrized over the full range of temperatures where aluminum is a solid. The nature of interatomic bonding and the history of its characterization in pure aluminum are reviewed with the unequivocal conclusion that it is purely tetrahedral in nature. The crystallography of aluminum alloys is then discussed in terms of all of the concepts covered for pure aluminum, using prominent alloy examples. The electron density domain theory of solid-state nucleation and precipitate growth is introduced and discussed as a new means of rationalizing phase transformations in alloys from a crystallographic point of view.

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Nucleation and growth of the γ′(AlAg2) precipitate in Al–Ag(–Cu) alloys

Cited by 12 publications

References 37 publications

The bi-layered precipitate phase ζ in the Al-Ag alloy system

The bi-layered precipitate phase ζ in the Al-Ag alloy system

Silver segregation to θ′ (Al2Cu)–Al interfaces in Al–Cu–Ag alloys

The Crystallography of Aluminum and Its Alloys

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