Aluminium light alloys are employed in commercial automotive and aerospace applications due to their high specific strength and corrosion resistance [1]. Precipitation hardening is one of the most important ways to improve the alloy performance. By tailoring precipitate size, aspect ratio, and distribution, the precipitation hardening can be significantly enhanced. The interfacial structure between precipitates and matrix is the critical factor being thought to manipulate the precipitate growth, but the fundamental understanding of these interfaces remains poor due to both limitations in atomic-resolution compositional characterisation techniques and computational capacity of first principle calculations. 

Al-Cu is a textbook binary alloy having precipitate strengthener θ′ (Al₂Cu) phase, but recent work showed its semi-coherent interfaces is not as simple as previously thought [2]. In fact, a complex metastable θ′_t phase is sandwiched in-between θ′′ and θ′ precipitates, indicating a non-intuitive energetically favourable phenomenon. Gold (Au) has strong negative solute formation enthalpy with aluminium and thereby its precipitation is directly linked to η′ and η phases without precursor Guinier–Preston (GP) zone [3]. How such element affects the interfacial structure is still unclear. In this work, we have used atomic-resolution high angle annular dark-field (HAADF) via aberration-corrected scanning transmission electron microscopy (AC-STEM) for detailed investigations of the influence of Au on the heterophase interfacial structure in an Al-Cu alloy. 

We have experimentally determined the effect of Au and ageing temperature on the complex interfacial structure between solid solution (α) and θ′ (Al₂Cu). We have observed the sandwiched interfacial structure in Al-Cu-Au alloys aged at 200°C (See Fig. 1a) as discovered in Ref. [2], while in comparison some partially direct θ′-α interface was found (see Fig. 1b) in rare probability. However, Au addition was observed to clearly destabilise the complex interfaces (see fig. 1c) at higher temperature ageing (350°C), whereas the corresponding binary Al-Cu alloy still somehow displays complex interfacial structures (Fig. 1d). The complex interface was also proved to provide the first solution to the four-decades-old mystery where experimental precipitate coarsening rate was found to be hundreds of times that of theoretical predictions based on the direct θ′-α interface in the Ref. [4].

References:

[1] Williams JC et al. Acta Materialia 51(2003):5775-5799.

[2] Bourgeois L, et al. Physical Review Letters 111 (2013): 046102.

[3] Bourgeois, L et al. Acta Materialia 105 (2016): 284-293.

[4] Boyd JD et al. Acta Metallurgica 19.12 (1971): 1379-1391.

Figures:

Fig. 1 HAADF-STEM images of θ′ (Al2Cu) phase in an Al-1.7Cu-0.2Au (at.%) alloy along a <100> orientation, a) interfacial structure of the semi-coherent interface showing the sandwiched structure, b) a rarely found interfacial structure showing the partially direct θ′-α structure, for a specimen aged at 200°C for 24 hrs. c) Interfacial structure showing the direct θ′-α interfacial structures for a specimen aged at 350°C for 1min. d) Evenly distributed direct θ′-α and two θ′t-α interfacial structures along with the interface in a specimen of the Al-1.7Cu aged at 350°C for 30min. The inset showing the projected atomic arrangements for different phases along a <100> orientation, where yellow represents Cu or Au and blue represents Al.

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