Abstract text (incl. figure legends and references)

Aluminum alloys with predefined alloying elements are chosen regarding their performance in certain areas of industrial applications. Unfortunately, the natural forming protective alumina layer is suppressed in most alloys. Thus, the anodization of aluminum alloys is mandatory to achieve chemical resistance. The existence of segregations and stress fields inside the Al matrix hinders the formation of highly ordered anodic aluminum oxides (AAOs) as achievable using high-purity aluminum samples.

Depending on the thickness of the sample and anodization process, the electron beam damage of AAOs during transmission electron microscopy (TEM) investigations varies at a high level. This leads to severe artefacts and finally to the evaporation of the sample when performing experiments at room temperature. Previous scanning transmission electron microscopy (STEM) studies have shown that cryogenic temperatures can partially reduce the beam damage in electron beam sensitive materials as organics or AAOs [1,2]. In this study, the sample is characterized by nanobeam diffraction pattern (NBDP) analysis and electron dispersive x-ray spectroscopy (EDS) at cryogenic temperatures. The focus is set on the correlation of structural and chemical characterization of the participating segregations inside the AAOs and at the interface. For comparison the same measurements are done inside the Al 6xxx alloy close to the interface to the AAO.

Polished Al 6xxx samples are anodized with various anodization processes. An enhancement of the imaging quality of the high angle annular dark field (HAADF) detector is achieved by coating the pores with ZnO using atomic layer deposition (ALD). The sample preparation and milling are conducted with a focused ion beam using either Ga-ions or a mixture of Si/Au-ions. The samples were analyzed with a Gatan Double Tilt Liquid Nitrogen Cooling Holder 636 using a Titan Themis 60 – 300 at a temperature of ~96K. EDX spectra and energy-filtered NBDP are recorded with a quadrupole SuperXG2 detector and a CCD-GIF camera, respectively.

An exemplified measurement of the correlation between chemical and structural information is given in figure 1. In total, a dataset of 10.000 NBDP is acquired on a part of the subsequently conducted EDX map. The beam damage during acquisition was reduced in comparison to measurements at room temperature. The combination of both methods shows the amorphous AAO tubes which are coated with polycrystalline ZnO. However, the segregations inside the AAOs are remaining crystalline and are only partially anodized.

Based on the results, the segregations remain crystalline inside the amorphous AAOs. The decrease of chemical concentration assumes that one has to distinguish between segregation consisting of valve metals (e.g. Si, Al, Mg, etc.) and other elements (e.g. Mn).

[1] H. Ayoola, C.-H. Li, S. D. House, C. S. Bonifacio, K. Kisslinger, J. Jinschek, W. A. Saidi, and J. C. Yang.Ultramicroscopy 219 (2020), 113127

[2] A. M. Jasim, X. He, Y. Xing, T. A. White, and M. J. Young. ACS Omega 6.13 (2021), 8986–9000.

Fig. 1: STEM – EDX maps, HAADF and NBDP to analyze segregations inside the formed AAOs. As indicated in the HAADF signal, a smaller map for NBDP is recorded along the segregation. Areas which are enriched with Al are amorphous, whereas Zn enriched regions show polycrystalline NBDP. The diffraction pattern of the segregations indicate that the corresponding structure remains crystalline.