The enhanced theta-prime (θ′) precipitation in an Al-Cu alloy with trace Au additions

Chen, Yiqiang; Zhang, Zezhong; Жен, Чен; Tsalanidis, Amalia; Weyland, Matthew; Findlay, Scott; Allen, L. J.; Li, Jiehua; Medhekar, Nikhil V.; Bourgeois, Laure

doi:10.1016/j.actamat.2016.12.012

Cited by 79 publications

(14 citation statements)

References 40 publications

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“…Although the crystal structure of the individual precipitate phases in this alloy system has been well established, to authors' knowledge, such GP zone/Al/θ' sandwich structure has never been reported. While trace alloying additions are effective in enhancing the age hardening response of Al-Cu alloys [3], we show that, with the example of Al-4Cu-0.1Au (wt%) alloy, such sandwich structure can also form in the presence of alloying additions. In the ternary alloy, it is hard to distinguish Au from Cu as they both have higher atomic number than Al.…”

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confidence: 74%

Sandwich Structure in Al-Cu(-Au) Alloys—Characterization by Atomic-Resolution HAADF-STEM and EDXS-STEM

et al. 2019

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The Al-Cu system has been a classical example for studying phase transformations in alloys, and it is also the base for developing Al alloys with high-strength [1]. In this system, the precipitation process usually involves the formation of GP zones, θ'', θ', and θ, and the microstructure is thought to be wellunderstood [1,2]. In our investigation of the distribution of precipitates in an Al-4Cu (wt%) alloy using high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM), we found, surprisingly and interestingly, a GP zone/Al/θ' sandwich structure in which one or multiple GP zone(s) form adjacent to the broad surface of θ' (Figure 1). In such structure, the number of sandwiched {002}α planes between the broad surface of θ' and the GP zone adjacent to it, and that between every two neighboring GP zones, is always three. Density functional theory calculations indicate that this number of sandwiched {002}α planes corresponds to an energy minimum. Although the crystal structure of the individual precipitate phases in this alloy system has been well established, to authors' knowledge, such GP zone/Al/θ' sandwich structure has never been reported. While trace alloying additions are effective in enhancing the age hardening response of Al-Cu alloys [3], we show that, with the example of Al-4Cu-0.1Au (wt%) alloy, such sandwich structure can also form in the presence of alloying additions. In the ternary alloy, it is hard to distinguish Au from Cu as they both have higher atomic number than Al. Although energy-dispersive X-ray spectroscopy (EDXS) is suitable for identifying different elements, for Al alloys, mapping at atomic-scale remains a challenging problem due to electron beam damage. In this work, from the sandwich structure, we have successfully acquired atomic-resolution EDXS maps on an FEI Titan G 2 60-300 ChemiSTEM, and the results show that Au atoms distribute mainly in the central part of θ' but not in GP zones (Figure 2). Density functional theory calculations confirm our experimental observations, and further clarify that it is energetically favorable for Au to substitute for the Cu sites, instead of Al sites, in the central part of θ'. Our findings are unique as they suggest a new way of alloy design-strengthening by tailoring the shape of precipitates [4].

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confidence: 74%

Sandwich Structure in Al-Cu(-Au) Alloys—Characterization by Atomic-Resolution HAADF-STEM and EDXS-STEM

et al. 2019

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“…Under continued ageing, these colonies slowly grow, progressively consuming the θ″-rich regions. In this situation θ″ precipitates appear to dissolve and be replaced by the tetragonal phase θ′, which has a lower free energy 22 . We have found no evidence of a direct structural transformation from θ″ to θ′.…”

Section: Resultsmentioning

confidence: 99%

“…In an attempt to quantify why TDN is promoted in a nanoscale specimen but not in bulk samples subjected to conventional heat treatments, we calculated the energy barrier of nucleation for different situations using classical nucleation theory (CNT) 12 -see Supplementary Note 3. Only nucleation of the θ′ phase is considered, as all its energy parameters required are well known 22,31 , in contrast to the unexpected η′ phase. According to CNT, nucleation of a phase will only take place for precipitates of that phase large enough to overcome the nucleation energy barrier.…”

Section: And Supplementarymentioning

confidence: 99%

Transforming solid-state precipitates via excess vacancies

Bourgeois

Zhang

et al. 2020

Nat Commun

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Many phase transformations associated with solid-state precipitation look structurally simple, yet, inexplicably, take place with great difficulty. A classic case of difficult phase transformations is the nucleation of strengthening precipitates in high-strength lightweight aluminium alloys. Here, using a combination of atomic-scale imaging, simulations and classical nucleation theory calculations, we investigate the nucleation of the strengthening phase θ′ onto a template structure in the aluminium-copper alloy system. We show that this transformation can be promoted in samples exhibiting at least one nanoscale dimension, with extremely high nucleation rates for the strengthening phase as well as for an unexpected phase. This template-directed solid-state nucleation pathway is enabled by the large influx of surface vacancies that results from heating a nanoscale solid. Template-directed nucleation is replicated in a bulk alloy as well as under electron irradiation, implying that this difficult transformation can be facilitated under the general condition of sustained excess vacancy concentrations.

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“…For comparison, Au has almost an identical size to Ag, yet Au displays a very negative defect energy in Al [65]. It is the electronic difference between Ag and Au in aluminium that leads to completely different clustering and precipitation behaviours, either in the binary alloys [65][this work] or when they are added to Al-Cu alloys [38,66]. Our DFT calculations also illustrate the preference of Ag aggregation on {111} Al planes in Al-Ag binary alloys, which also occurs at the early stage of ageing in Al-Cu-Mg-Ag alloys [31,33] and Al-Cu-Li-Mg-Ag alloys [67].…”

Section: Discussionmentioning

confidence: 99%

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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The enhanced theta-prime (θ′) precipitation in an Al-Cu alloy with trace Au additions

Cited by 79 publications

References 40 publications

Sandwich Structure in Al-Cu(-Au) Alloys—Characterization by Atomic-Resolution HAADF-STEM and EDXS-STEM

Sandwich Structure in Al-Cu(-Au) Alloys—Characterization by Atomic-Resolution HAADF-STEM and EDXS-STEM

Transforming solid-state precipitates via excess vacancies

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

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