PNNL

Abstract

Dislocation imaging using transmission electron microscopy (TEM) has been an invaluable tool for characterizing crystallographic defects in metals. Recent advances in electron microscopy techniques have allowed for new dislocation imaging techniques to be devised. Compared to conventional TEM imaging, diffraction contrast imaging scanning transmission electron microscopy (DCI STEM) can provide better defect contrast with fewer bend contour artifacts, enabling more effective analysis of dislocation structures. Here we outline a simple procedure to help set up DCI STEM experiments, using a body-centered cubic HT-9 ferritic/martensitic alloy as an example. To study the behavior of dislocation imaging in STEM mode, we compared the imaging parameters in DCI STEM mode to corresponding conventional TEM images. We found that using a few milliradians of STEM convergence and collection semi-angles, S and S, alleviates bend contours by averaging out the rocking-curve oscillation in reciprocal space. Practical guidelines regarding STEM parameters and specimen orientation and thickness are provided for DCI STEM dislocation imaging. Lastly, we show that coupling DCI STEM with spectrum images of Electron Energy Loss Spectroscopy and of Energy-Dispersive X-ray Spectroscopy offers a comprehensive characterization of crystallographic defects and chemical information of complex microstructures.