AUTHOR=Egusa D. , Kawaguchi K. , Abe E. 

TITLE=Direct Observations of Precursor Short-Range Order Clusters of Solute Atoms in a LPSO-Forming Mg-Zn-Gd Ternary Alloy

JOURNAL=Frontiers in Materials

VOLUME=6

YEAR=2019

URL=https://www.frontiersin.org/journals/materials/articles/10.3389/fmats.2019.00266

DOI=10.3389/fmats.2019.00266

ISSN=2296-8016

ABSTRACT=<p>We have investigated short-range order (SRO) clusters of solute atoms in a Mg<sub>97</sub>Zn<sub>1</sub>Gd<sub>2</sub> alloy which forms a long-period stacking/order (LPSO) phase, based on aberration-corrected scanning transmission electron microscopy (STEM) observations and first-principles calculations. High-angle annular dark-field (HAADF) STEM provides directly the individual Gd atom positions through significant atomic-number dependent Z-contrast, and their averaged image-processing have immediately shown up the representative SRO-Gd configurations being in favor of the second-nearest networks in the <italic>hcp</italic>-Mg structure. Interestingly, Zn atoms seem to be essentially placed at the particular sites within the SRO-Gd configurations to form D0<sub>19</sub>-like ZnGd<sub>3</sub> clusters, a validity of which are verified by STEM image simulations and first-principles calculations. These D0<sub>19</sub>-like ZnGd<sub>3</sub> clusters in the <italic>hcp</italic>-Mg matrix are likely to be a precursor sub-structure to the L1<sub>2</sub>-type Zn<sub>6</sub>Gd<sub>8</sub> clusters that are embedded in the local <italic>fcc</italic> layers in the LPSO structure. In fact, we find that these D0<sub>19</sub>-like ZnGd<sub>3</sub> clusters seem to form a layered atmosphere prior to the LPSO formation, providing an important clue on how the LPSO phases nucleate and grow.</p>