Abstract
Isothermal ageing of plastically deformed Mg-Zn-Y alloys resulted in precipitation along twin boundaries. The bulky precipitates so formed had structures similar to those recently reported for the rod-like
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 the monoclinic Mg4Zn7 phase, and had the orientation
;
with either the matrix or the twin. On this Mg4Zn7 phase, the hexagonal MgZn2 phase grew in two orientations, both with
. One of these orientations formed a known orientation relationship
;
with the matrix. The part of the precipitate with the MgZn2 structure was usually in direct contact with the twin boundary. Both the 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. The orientations of these rhombuses remain unchanged across the interfaces between the two phases. Near the interface with MgZn2, transition structures formed in the Mg4Zn7 phase, with the Zn:Mg atom ratio between those of the Mg4Zn7 and MgZn2 phases. In these transition structures, the unit cell of the 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.
Acknowledgements
One of the authors (JMR) gratefully acknowledges the support of the Japan Society for the Promotion of Science (JSPS) through a JSPS fellowship. The authors also thank Reiko Komatsu, Keiko Sugimoto and Toshiyuki Murao for assistance with sample preparation.