The power of speed: DED EELS for analytical in situ TEM

With the advent of advanced in situ TEM, speed and precision gained considerably in importancefor dynamic TEM experiments. Collecting information at just the right time became significantlymore crucial. Powerful techniques and hardware that allows capturing images and analytical datawith high speed got into focus to be able to monitor fast dynamic processes with both the lateraland the temporal resolution that is needed for adequate description.Due to various challenges of geometrical and physical nature that might arise in EDXS analysiswith MEMS (microelectromechanical systems) heaters [1,2], using EELS is often beneficial foranalytical analysis during in situ TEM experiments. This is especially true for the investigation ofprecipitation in light alloys. They requires systems capable of tilting along two axes, and detectorsthat provide ample signal not only for the main metallic components, but also for light elements likeLi or B (used as additives for grain refinement or strengthening). The properties of these materialscan be modified by subjecting them to accurately defined temperature treatments that trigger anartificial ageing process [3]. Using certain threshold temperatures, some systems can even be“cycled” – i.e. restored to their original supersaturated state and “aged” again [4]. In situ (S)TEMcan provide detailed insight into various stages of precipitation. However, a key condition for theuse of a structure with dimensions in the micro- and nanometer range to illustrate processes in abulk system is the exact knowledge of the local stoichiometry. Consequently, concomitantanalytical analysis that is fast enough to observe changes in the chemical composition of thesample during the experiment is extremely valuable for the reliable interpretation of in situ TEMresults.In this study, we use direct electron detection EELS (DED EELS) from Gatan (Quantum ERS GIFwith K2 camera) to perform 2D chemical analysis during a STEM heating experiment. The materialthat is described is AlCu4, an aluminum alloy with 4% of copper that can be strengthened byprecipitation hardening. We capture and describe the changes of the system during differentstages of precipitate formation (see Figure 1), using fast EELS techniques to track the chemicalcomposition of the whole system.Especially when compared to EDXS analysis, this approach is more powerful in terms of speed,providing the capability to perform detailed chemical analysis of a complex light element system atvarious stages of transformation