Transmission electron microscopy investigation and interpretation of the morphology and interfacial structure of the ′-Mg₅₄Ag₁₇ precipitates in an Mg–Sn–Mn–Ag–Zn alloy

The morphology and interfacial structure of the ′-Mg₅₄Ag₁₇ precipitates in a peak-aged Mg–Sn–Mn–Ag–Zn alloy have been investigated using transmission electron microscopy. A typical ′-Mg₅₄Ag₁₇ precipitate exhibits a (0001)_α habit plane and three pairs of side facets, with two major pairs of facets being in irrational orientations. The orientation of each preferred interface is normal to a vector connecting two adjacent diffraction spots from different phases (Δg). Using the constrained coincidence site lattice (CCSL) model, the interface orientations have been interpreted according to the degree of matching on the interfaces. The detailed stepped structures in the irrational facets and dislocation configurations in the side facets have been predicted using the secondary CCSL (II-CCSL) and the secondary O-lattice model. Both the calculated terrace/ledge and dislocation configurations in side facets are in good agreement with the high-resolution transmission microscopy observations.