In situ TEM investigation of dynamic processes has rapidly grown into a valuable tool for materials development. The microscopic insight provided by modern in situ TEM infrastructure eases the advancement of these material systems, since it allows the correlation of macroscopic properties with the microstructural evolution that is triggered by exactly defined stimuli.
However, a key condition for the use of a sample with dimensions in the micro- and nanometer range to illustrate processes in a bulk system is the exact knowledge of the local composition. Consequently, concomitant analytical analysis that is fast enough to observe changes in the chemical details of the sample during the experiment is extremely valuable for the reliable interpretation of in situ TEM results. To cut down on analysis time while simultaneously providing high resolution, and possibly a large field of view, is therefore a key element for the applicability for the spectroscopic method of choice. The combination of HR STEM with Direct Electron Detection EELS (DED EELS) [1], especially when used with suitable post-processing, provides a perfect toolset for this task.
In this study, we illustrate the benefits of DED EELS to perform 2D chemical analysis during STEM heating experiments. One of the materials that are described is AlCu4, an aluminum alloy with 4% of copper that can be strengthened by precipitation hardening [2,3]. We capture and describe the changes of the system during different stages of precipitate formation, using this fast EELS technique to track the chemical composition of the alloy system, and compare it to EDXS and EELS using a common CCD.
This results in both a detailed analysis of precipitate evolution at critical temperatures and an evaluation of the pros and cons of individual spectroscopic techniques for their application in in situ STEM.

Using analytical signals during in situ STEM experiments is an essential factor in understanding the dynamic processes in a changing sample. We will show that DED EELS is the most powerful choice in terms of speed, providing the capability to perform detailed chemical analysis of a complex light element system at various stages of transformation.
The authors acknowledge financial support by Austrian Cooperative Research (ACR) project “INSIGHT” (SP-2021-04).