Secondary Phase Measurements on a Large Scale in Aluminum Alloys

mechanical properties of wrought aluminum alloys are significantly determined by sec-ondary phase particles. Some of these particles can facilitate material hardening or function as nuclei for recrystallization. Therefore, it is crucial to get detailed information about the type, size and distribution of the precipitates within the aluminum sheet. These sheets can be sev-eral millimeters thick, wherefore an area of several square millimeters has to be observed to get statistically relevant information. This work presents a method for analyzing these large areas and extracting properties of the secondary phase particles. It is carried out using an aluminum sheet alloy AA2024 which primarily contains copper, magnesium and manganese as alloying elements. AA2024 was selected because it is a typical aerospace material where mechanical properties are especially important.
Energy dispersive X-ray spectroscopy (EDX) in combination with the feature mode in the ‘Ox-ford Instruments Aztec’ software is used to automatically measure the chemical composition of the secondary phase particles. First a backscattered electron (BSE) image is recorded, which is then automatically segmented by the Aztec software based on grey color values. The particles of interest are selected by setting a size limit, for which a EDX spectrum is recorded. To obtain results for a very large area, it is subdivided into several smaller areas which are measured separately and stitched together afterwards.
This approach leads to very good chemical information results, whereas the obtained size and shape of the particles is not entirely precise. This is due to the limitation of the grey color value segmentation used by the Aztec software. To overcome this issue, after the finished measure-ment the BSE image is segmented again using the software ilastik, which bases on AI assisted classifiers [1]. This new segmentation, which is much more precise, and the measured chem-ical information are then combined in a custom MATLAB script. As an example, it was found that the S-phase (Al2MgCu) in AA2024 seems to have a considerable size distribution and has no uniform distribution across the thickness of the sheet. This gathered information then helps to further optimize the properties of the material.