Stacking Faults and Growth Twins in Long-Period Stacking-Ordered Structures in Mg-Co-Y Alloys

Herein, by atomic-scale high-angle annular dark field scanning transmission electron microscopy, the structure, symmetry, and distribution of stacking faults (SFs) and growth twins in 12H, 15R, and 18R long-period stacking-ordered (LPSO) structures in Mg88Co5Y7 and Mg92Co2Y6 (at%) as-cast alloys are characterized. The SFs or growth twins in LPSO structures are denoted as the faulted arrangement of n-Mg components, and some of them possess inversion centers or mirror symmetry. The formation of SFs or growth twins in 15R and 18R structures depends on whether the sum of Mg layers in all the n-Mg components of the defect is an even number or an odd number, whereas only SFs form in 12H structures. Various types of SFs or SFs of the same kind can distribute locally in the LPSO structures in Mg-Co-Y alloys, and then some other LPSO structures with a longer period are detected via orderly distribution of SFs.