On the visibility of GP-zones in 6xxx Al alloys in atomic LAADF-STEM

Abstract

In this work, the contrast seen in low-angle annular dark-field (LAADF) scanning transmission electron microscopy (STEM) of Guinier-Preston zones (GP-zones) in Al-Mg-Si alloys (6xxx) is discussed by STEM simulations, conventional STEM and 4D-STEM data. It is found that GP-zones of the 6xxx system can be experimentally imaged even with some elemental contrast when collecting primarily the elastically scattered electrons which fall within the zero-order Laue zone (ZOLZ). It is found that GP-zones give rise to contrast in LAADF-STEM primarily due to static atomic displacement which results in diffraction contrast, i.e. Huang scattering. As a result of lattice distortion from the GP-zones' short-range order, more electrons get elastically scattered within the ZOLZ and can be exploited by LAADF-STEM. Typical challenges which might arise when imaging with a LAADF setup compared to a high-angle annular dark-field setup are discussed, as well as, the limits of interpretability of using coherently scattered electrons are examined. It is found that the effects of contrast reversal and misalignment can be drastically reduced by collecting electrons with a small gap from the bright field disc and using detectors with reasonable collection widths, i.e. larger than 5 mrad. Furthermore, it is found that a reduction of the probe's semi-convergence angle results in a better contrast due to the possibility of collecting more electrons which scatter within the ZOLZ. Probes with semi-convergence angles of around 20 mrad produced the most optimal results.