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Structural relationships among monoclinic and hexagonal phases and transition structures in Mg-Zn-Y alloys

Isothermal ageing of plastically deformed Mg-Zn-Y alloys resulted in precipitation along {10-12} twin boundaries. The bulky precipitates formed had structures similar to those recently reported for the rod-like β' precipitates, but afforded a more detailed study by high resolution TEM due to their larger size. The core of the precipitates often had the structure of monoclinic Mg4Zn7 phase, and had the orientation [0001]_Mg//[010]; {10-10}_Mg // (201) with either the matrix or the twin. On this Mg4Zn7 phase, hexagonal MgZn2 phase grew in two orientations, both with [010]_Mg4Zn7 // [11-20]_MgZn2. One of these orientations formed a known orientation relationship [0001]_Mg // [11-20]_MgZn2; {11-20}_Mg // (0001)_MgZn2 with the matrix. The part of the precipitate with the MgZn2 structure was usually in direct contact with the twin boundary. Both Mg4Zn7 and MgZn2 phases have layered structures that can be described with similar building blocks of icosahedrally coordinated atoms. The atomic positions of zinc atoms comprise the vertices of these icosahedra and form "thick" rhombic tiles. Orientation of these rhombuses remain unchanged across the interfaces between the two phases. Near the interface with MgZn2, transition structures formed in Mg4Zn7 phase, with Zn:Mg atom ratio between those of Mg4Zn7 and MgZn2 phases. In these transition structures, the unit cell of Mg4Zn7 phase is extended along [100] or [001] by half a unit cell length by continuation of the rhombic tiling. Structures of these extended unit cells are proposed.