Transforming solid-state precipitates via excess vacancies
We report how vacancies can enable the direct transformation of solid-state precipitates in aluminium. The product of the transformation is a strengthening precipitate phase commonly found in high-strength lightweight aerospace alloys.
Aluminium alloys are used in a wide range of industrial sectors and are particularly important to the aerospace industry, where a combination of light weight and high strength is essential. This combination of properties is obtained through the development of special alloy microstructures dominated by solid-state precipitate phases, which act a bit like reinforcing bars in the soft aluminium matrix. To date, how to obtain the right precipitate phases and their arrangements in the microstructure remains more akin to black magic than science, thus hindering the development of materials with optimum properties.
To better understand solid-state precipitation, we have focused on the strengthening phase θ′ in an Al-Cu alloy forming the basis of many aerospace alloys. When imaging this alloy in a transmission electron microscope (TEM), we discovered that the electron beam transformed another, unwanted, precipitate phase called θ″, into something that resembled the strengthening θ′ phase. Whereas precipitates with little strengthening ability, like θ″, form with ease, strengthening precipitates usually do not. The lack of such transformation in Al-Cu has been particularly mysterious because both phases are structurally and chemically similar. So directly observing this seemingly difficult transformation was exciting.
However, the transformed product was not exactly the desired strengthening phase: structural disorder was generated by the electron beam of the microscope. To order this phase, we decided to heat it inside the electron microscope. This idea was based on the recollection of a comment made more than twenty years ago by former colleague Prof. D. Golberg: heating to a low temperature could anneal out electron beam induced defects.
The in-situ heating worked: the transformed product ordered into the strengthening θ′ phase. But to our surprise, we observed another phenomenon: nearly all θ″ precipitates in the sample transformed into the desired phase, and, even more surprisingly, into a phase never observed before! – see Figure 1.
Figure 1. The θ′ phase (right) is an important strengthening precipitate phase in many light aluminium alloys. Unfortunately, its homogeneous nucleation is difficult, in contrast to the θ″ phase (left), which has poor strengthening ability. In our recent Nature Communications paper , we show that the θ″ phase can transform into the θ′ phase thanks to vacancies (a type of fundamental crystalline defect). This can take place via a previously unknown intermediate phase η′ (middle). The transformation was captured by atomic-resolution high-angle annular dark field scanning transmission electron microscopy.
Many more experiments later followed by simulations, we concluded that this transformation was enabled by vacancies generated at the surface of the nanoscale thin TEM sample. Vacancies also explained why the electron beam could induce the structural transformation. This provided direct evidence that these defects can alter phase transformation pathways, as hinted in earlier work by former student Dr Yong Zhang .
The above knowledge enabled us to design a process replicating the transformation in a bulk alloy. Our findings  therefore promise improved materials design for many technologically significant situations, from materials in service, nanostructured materials and additive manufacturing, to the processing of numerous engineering alloys.
Our initial observations were obtained through serendipity: the phase transformation was triggered by an electron beam. Even more serendipitous was observing the phase transformation during in-situ heating in an electron microscope. It turns out that, as I recently discovered, my former colleague never used a heating holder to anneal out electron beam induced defects! Heating was caused directly by the electron beam through charge accumulation on the sample .
In our case serendipity was very generous: applying the same methods as for Al-Cu , former student Dr Zezhong Zhang observed a similar phenomenon in the textbook alloy system Al-Ag that also included the discovery of a new phase . This illustrates the universal role played by vacancies in facilitating phase transformations.
For more details, please see our article “Transforming solid-state precipitates via excess vacancies“ published in Nature Communications.
L. Bourgeois, Y. Zhang, Z. Zhang, Y. Chen, N.V. Medhekar
- L. Bourgeois, Y. Zhang, Z. Zhang, Y. Chen, N.V. Medhekar, Nature Communications 11 (2020) 1248.
- Y. Zhang, Z. Zhang, N.V.Medhekar, L. Bourgeois, Acta Materialia 141 (2017) 341.
- D. Golberg, Y. Bando, M. Eremets, K. Takemura, K. Kurashima, K. Tamiya, H. Yusa, Chem. Phys. Lett. 279 (1997) 191.
- Z. Zhang, L. Bourgeois, J.M. Rosalie, N.V. Medhekar, Acta Materialia 132 (2017) 525.