Aluminium Microstructure Analysis Gainhub

These analysis techniques and, above all, the enduring expertise of the AMAG Center’s researchers, make it possible to characterize features of the microstructure of aluminium from several millimeters in scale right down to atomic resolution. A scanning electron microscope (SEM) can examine the microstructure over “large” areas of several mm² and also produce high-resolution images of fine secondary phases and defects, such as following mechanical testing. Secondary electron (SE) and backscattered electron (BSE) detectors allow researchers to achieve optimal contrasts in topography, chemical composition and orientation. Energy-dispersive X-ray spectroscopy (EDX) facilitates chemical analysis while electron backscatter diffraction (EBSD) makes it possible to determine crystal orientation, both methods ensuring quantitative analysis of microstructure components beyond area fraction or grain size.The texture (totality of crystallographic orientations) can be used, for example, to indicate deformation and recrystallization processes during production, or in the estimation of certain mechanical properties [3,4,6,7]. The scanning electron microscope with a field emission gun and connected EBSD and EDX detectors are at the cutting edge of technology and facilitate high measurement speeds and signal yield. This means efficient, large-scale and statistically valid analyses as well as detailed, high-resolution examinations of fine structures.

If the features under examination are smaller than ~20-50 nm (hardening phases, dispersoids, nanocrystalline materials, individual dislocations), the scanning electron microscope no longer provides sufficient resolution. In such cases, the transmission electron microscope (TEM) is the instrument of choice. It can achieve atomic resolution. The combination of high-resolution imaging and extremely rapid chemical analysis facilitates efficient, detailed 2D analysis (Figure 3: TEM image and chemical analysis of ultra-fine, core-shell structured  particles in a ternary Al-Zr-Sc alloy [8]). Tilting the sample holder also makes it possible to conduct automated 3D tomography and use scanning TEM and EDX. This makes it possible, for example, to determine the three-dimensional morphology of precipitates and establish a link between heterogeneous nucleation sites (e.g. grain boundaries, dispersoids, etc.) and precipitate distribution.The three-dimensional structure of very fine microstructure features, such as hardening phases precipitates and individual grain boundaries in a material, can be characterized at the nanometer scale with atom probe tomography (APT). This involves applying a large electric field to a needle-shaped sample (diameter < 50 nm), which releases a succession of individual atoms through field evaporation. The local origins of the ions are measured using a two-dimensional, position-sensitive detector, while chemical identification relies on time-of-flight mass spectrometry. This data makes it possible construct a 3D model of the sample. The benefits of this method compared to other analysis techniques include the fact it produces three-dimensional data, its high sensitivity for light elements, and the small sample size required. This has made it possible to demonstrate cluster formation induced by plastic deformation (see Figure 4). [9]

The aluminium research activities successfully established in Leoben are therefore built on several pillars:

• (i) State-of-the-art analysis equipment. This is crucial for efficient, precise, world-leading analyses: the AMAG Center makes it possible for outstanding postdoc researchers to use these pieces of equipment for aluminium-specific research.
• (ii) High-resolution imaging and 3D analyses require efficient, material-specific and, above all, artifact-free sample preparation. In recent years, efficient preparation routines have been established for standard metallographic procedures, with special methods developed for particularly challenging samples.
• (iii) Operators with an extensive understanding of the physical fundamentals of each measurement technique, detailed knowledge of measurement equipment, and experience of working with this equipment and associated data analysis all combine to facilitate high-quality imaging and quantitative analyses. However, it is ultimately the combination of aluminium-specific metallurgical and process engineering expertise that provides a solid foundation for internationally leading research. This focus requires a long-term approach to researchers’ personal and professional development, which the AMAG Center strongly supports. Dr. Weissensteiner, for example, plans to submit the thesis for her habilitation (the highest university degree in German-speaking countries) on the topic of the AMAG Center as the next step in her scientific career.

In an effort to guarantee that the close cooperation between AMAG and MUL endures in the future, Dr. Weissensteiner and AMAG are preparing to submit a proposal for a major research project. The plan includes exploring fundamental issues of deformation in sustainable aluminium alloys and methods to improve microstructure analysis. Given the gap in measurement technology between SEM and TEM, the project’s aims also include implementing scanning precession electron diffraction (SPED) at the TEM. This method would make it possible to capture an almost complete diffraction pattern for every data point on an image grid. Based on this crystallographic information, the phase and orientation could be determined for every data point, with a resolution in the region of a few nanometers. Everyone involved in this research will be able to apply and refine the methods described here, such as by implementing AI-based data analyses.