Imaging the Structural Evolution in Nanocrystalline Metals during Mechanical Deformation

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. 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) [1] allows to identify the crystallographic orientation of all crystallites with sizes down to around 10 nm, well below the limit of electron back scatter diffraction (EBSD). Performing ACOM-TEM in µp-STEM mode allows to image grain orientations in situ during straining inside a TEM [2, 3]. This combination was key to a 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 local crystallite rotation and sample tilting/bending [2, 3]. In addition, twinning/detwinning and grain growth can be directly followed (Fig. 1, 2) and the automatic data evaluation leads to user independent quantitative statistical information such as grain size distribution and grain rotation statistics [3].