  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 \beta'
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.
