{101¯1} Twin boundary structures in a Mg-Gd alloy

In this work, we report our direct observations of atomic structures of both end and broad interfaces of{101¯1} twin boundary (TB) in a deformed and annealed Mg-Gd solid solution single phase alloy using atomic-resolution high-angle annular dark-field scanning transmission electron microscopy. The end interface is an asymmetric tilt boundary decorated by a periodic array of clusters comprising ordered Gd-rich columns. The broad interface consists of coherent {101¯1} twin boundaries that are disconnected by various steps and basal/pyramidal (BPy) facets, which all contain characteristic Gd segregation. The twin steps, including S1/1, S2/2, S3/2, S5/4, S7/6 and S7/7, are observed and classified in terms of their heights, orientations, and Burgers vectors. The S1/1 and S2/2 steps are not associated with any misfit dislocations, but S1/1 is always associated with an I1 stacking fault, which is free of Gd segregation, in the twin crystal. The S3/2, S5/4, S7/6 and S7/7 steps contain misfit dislocations but no stacking faults. The BPy facets, observed for the first time in the{101¯1} TB of Mg alloys, exhibit different widths and orientations, and they are associated with elastic strain arising from in-plane mismatch and plane disclination. A double-segregation-layer structure is also observed, with the coherent{101¯1}1} TB lying on one of these two layers. These observations are discussed in terms of existing crystallographic models.

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