Age-hardening in Al alloys has been used for over a century to improve its mechanical properties. However, the lack of direct observation limits our understanding of the dynamic nature of the evolution of nanoprecipitates during age-hardening. Using in-situ (scanning) transmission electron microscopy (S/TEM) while heating an Al-Cu alloy, we were able to follow the growth of individual nanoprecipitates at atomic scale. The heat treatments carried out at 140, 160, 180 and 200 °C reveal a temperature dependence on the kinetics of precipitation and three kinds of interactions of nano-precipitates. These are precipitate-matrix, precipitate-dislocation, and precipitate-precipitate interactions. The diffusion of Cu and Al during these interactions, results in diffusion-controlled individual precipitate growth, an accelerated growth when interactions with dislocations occur and a size dependent precipitate-precipitate interaction: growth and shrinkage. Precipitates can grow and shrink at opposite ends at the same time resulting in an effective displacement. Furthermore, the evolution of the crystal structure within an individual nanoprecipiate, specifically the mechanism of formation of the strengthening phase, θ′, during heat-treatment is elucidated by following the same precipitate through its intermediate stages for the first time using in-situ S/TEM studies.

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In-situ TEM studies using an environmental cell (nanoreactor) play an important role in not just giving an understanding the corrosion mechanisms at a sub-micron scale, but also on the influence of heat-treatment on the microstructural change and corrosion behaviour of these alloys. One of the main requirements of for these in-situ TEM studies is the leak tightness of the nanoreactor. This is achieved by gluing the top and the bottom chips together with water glass or commercially available cyanoacrylate compounds. The drawback of this method is the chips are inseparable after the in-situ TEM study, making it impossible to carry out any further investigations on the same specimen. To overcome this drawback, we worked on upgrading the nanoreactor by redesigning the TEM holder to avoid gluing. This made it possible not only to assemble the nanoreactors in a more reliable way but also separate the two halves after the in-situ TEM study. This has opened up opportunities to carry out investigations like tomography, AFM measurements and other surface characterization studies on the same specimen, adding more to the mechanisms observed from the in-situ TEM studies.@en