ACOM‐TEM and its application for the investigation of deformation pathways in nanocrystalline Pd and AuPd

Abstract

Most of our current understanding of the deformation mechanisms active in nanocrystalline (nc) metals stems from in‐situ deformation experiments on bulk materials using x‐ray diffraction (XRD). However, XRD cannot directly resolve the local deformation processes, e.g. grain growth or twinning. For a local analysis, these processes are traditionally investigated using BF/DF‐TEM. However, varying contrast due to local orientation changes, bending and defects during in‐situ BF‐TEM straining experiments make an accurate interpretation for nanometer sized grains difficult. On the other hand, Automated Crystal Orientation Mapping (ACOM‐TEM) allows for the identification of the crystallographic orientation of all crystallites with sizes down to around 10 nm, well below the limit of electron back scatter diffraction (EBSD) 1 . Using template matching to reveal the crystal orientation, the ASTAR (Nanomegas) ACOM‐TEM analysis offers an angular resolution that is nearly as good as EBSD 1,2 . Recently, ACOM‐TEM imaging in STEM modus was combined with in‐situ straining inside a TEM 3–5 . This combination was the key to new data evaluation based on orientation maps. By tracking individual crystallites through a straining series the change of their orientation can be evaluated in order to distinguish between an overall crystallite rotation and sample bending. In addition, twinning/detwinning and grain growth can be directly followed and the automatic data evaluation leads to user independent quantitative statistical information such as grain size distribution 3 . Recent investigations revealed some challenges using ACOM‐TEM for in‐situ experiments if there are overlapping crystallites. Overlapping crystallites lead to superimposed diffraction patterns that confuse the matching procedure. Tilting nc Pd in‐situ and tracking the changes using ACOM‐TEM, it became apparent that some orientations are more dominant than others during the matching procedure. Further, we present an ambiguity filter that reduces the number of pixels with a '180° ambiguity problem' (Fig. 1). The challenges discussed here for orientation mapping of nc materials do not only appear with ACOM‐TEM, but are mostly an inherent problem of any TEM projection technique. However, using ACOM‐TEM these limitations become apparent and can be properly analyzed, e.g. by mapping the rotation of many crystallites for a given area of interest. This enables to detect sample bending or tilting in a (in‐situ) series of orientation maps, which cannot be measured by BF/DF‐TEM. The ACOM‐TEM measurement and evaluation routine was applied to magnetron sputtered Pd x Au 1‐x thin film samples of about 50 nm. Grain growth and grain fragmentation as well as twinning and detwinning have been observed to take place simultaneously at different locations. In addition, large angle grain rotations with ~39° and ~60° occur that can be related to twin formation, twin migration and twin‐twin interaction as a result of partial dislocation activity (Fig. 2). Furthermore, plastic deformation in nanocrystalline thin films was found to be partially reversible upon rupture of the film. In conclusion, conventional deformation mechanisms are still active in nanocrystalline metals, however, with different weighting than in conventional materials with coarser grains. We would like to acknowledge Christian Brandl, Edgar Rauch, Florian Bachmann, Ralf Hielscher, Ankush Kashiwar. This work was supported by the DFG under grant FOR714 and Karlsruhe Nano Micro Facility (KNMF).