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.

Using this holder, we studied the corrosive attack of an Al alloy that has been heat-treated at 250 °C in an environment of oxygen bubbled through aqueous HCl of pH 3. After corrosion, STEM-tomography has been carried out to understand the propagation of the corrosive attack. Most of the Al alloys have the unique property of improving their strength by the mechanism known as precipitation hardening. This involves a special heat-treatment given to the alloys where alloying elements are added to aluminium, heated to an elevated temperature (usually above 500 °C) to form a single-phase solid solution, and then quenched rapidly to room temperature. On quenching, a super-saturated solid solution is obtained, from which a distribution of numerous fine nano-sized precipitates in the matrix can be obtained by heating at slightly elevated temperatures (typically ranging from 100 to 250 °C). Figure 1 shows the precipitation of numerous S-type nanoprecipitates in the matrix of a FIB specimen of AA2024, heat-treated at 250 °C for 3 minutes. While the formation of the S-type precipitates contributes to a significant increase in the strength of the alloy, the Mg-rich S-type precipitates and the formation of precipitate-free-zones next to the grain boundary have a severely detrimental effect on the corrosion behaviour of this alloy.

The corrosion behaviour of the heat-treated specimen can be investigated by assembling a nanoreactor. The heat-treated specimen was also exposed to a gas mixture of oxygen bubbled through aqueous HCl at room temperature, at a pressure of 1 bar.  The snapshots from a movie recorded during the in-situ corrosion experiment are shown in Figure 2.  In contrast to the specimen prior to heat-treatment, Figure 1, exposure to the reactive gas mixture of oxygen bubbled through aqueous HCl causes an immediate attack as shown in Figure 1(a). The bubble-like features observed all over the specimen indicate this, more prominently next to the grain boundary precipitates. As the exposure time increases, the corrosion attack progresses as observed by the growth of circular features all over the matrix after exposure of 15 min (Figure 2b) and approximately 30 min (Figure 2c). STEM-ADF tilt-series confirms that the circular feature observed all over the specimen are pits initiating from the surface of the sample. The high-density of the S-type precipitates, enriched in Mg and Cu act as numerous galvanic couples leading to such an attack.

Acknowledgement:

The authors gratefully acknowledge the ERC project 267922 for the financial support.

Figures:

Figure 1: STEM-annular dark field images obtained from a FIB specimen of AA2024 (a) before and (b) after heat-treatment at 250 °C for 3 minutes. Prior to heat-treatment, there exist numerous matrix precipitates enriched in Mn, Cu, Fe, Si, whereas heat-treatment results in precipitation of fine nanometre-sized S-type precipitates enriched in Cu, Mg. Also notice the precipitate free zone indicated by the arrows next to the grain boundary.

Figure 2: (a) Surface attack and selective attack next to grain boundary precipitates in a heat-treated FIB specimen of AA2024. The surface attack progresses rapidly after (b) 15 min, as indicated by the increase in the size of the circular features. The attack progresses more and the whole specimen surface is attacked after (c) approximately 30 min. Projections at (c) -30°, (d) 0° and (c) +30° from STEM-ADF tilt series obtained from the region shows that the circular features are in fact pits formed due to the corrosive attack.

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EMC Abstracts - https://emc-proceedings.com/abstract/improved-gas-holder-for-in-situ-tem-studies/