Dislocation Imaging by Precession Electron Diffraction

Abstract

Dislocation is a major plasticity carrier, which also dictates the mechanical properties, in metallic systems[1]. Transmission electron microscopy (TEM) is a powerful technique to allow for the visualization of dislocations. Both two-beam condition and low-index zone axis imaging (multiple-beam condition) have been used to reveal dislocations. However, both techniques have drawbacks. In the two-beam condition, some dislocations are invisible due to the criterion, thus limits the holistic illumination of dislocations. Regarding the low-index zone axis imaging, more dislocations could be illustrated, but the dynamical effect usually overwhelms the contrast from the dislocation lines. In this work, we employed precession electron diffraction (PED) as a tool for dislocation imaging. PED is a diffraction-based technique, where pixel-by-pixel diffraction patterns of the whole scanned area are acquired. The experimentally acquired diffraction patterns are then compared to those in the database to offer crystal orientation and elastic strain information at the nanoscale[2-6]. Here, we realize the beam precession could also be utilized to potentially generate high quality micrographs with enhanced dislocation contrast. One major result of beam precession is to average out the dynamical effect in TEM micrographs and to show high-quality kinematical information[7]. To demonstrate such capability, we used a deformed AZ31 magnesium alloy as the model material. The crystal was tilted to the zone axis and bright-field images were first taken. In the example shown in Fig. 1a, the dislocations are visible, but the contrast is low (appearing to the eyes) due to the strong dynamical effect of the beam-crystal interaction. In the same area, we then acquired the PED map and formed the virtual bright-field (VBF) image using the direct beam.