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

摘要

Isothermal ageing of plastically deformed Mg-Zn-Y alloys resulted inprecipitation along {10-12} twin boundaries. The bulky precipitates formed hadstructures similar to those recently reported for the rod-like \beta'precipitates, but afforded a more detailed study by high resolution TEM due totheir larger size. The core of the precipitates often had the structure ofmonoclinic Mg4Zn7 phase, and had the orientation [0001]_Mg//[010]; {10-10}_Mg// (201) with either the matrix or the twin. On this Mg4Zn7 phase, hexagonalMgZn2 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 theprecipitate with the MgZn2 structure was usually in direct contact with thetwin boundary. Both Mg4Zn7 and MgZn2 phases have layered structures that can bedescribed with similar building blocks of icosahedrally coordinated atoms. Theatomic positions of zinc atoms comprise the vertices of these icosahedra andform "thick" rhombic tiles. Orientation of these rhombuses remain unchangedacross the interfaces between the two phases. Near the interface with MgZn2,transition structures formed in Mg4Zn7 phase, with Zn:Mg atom ratio betweenthose of Mg4Zn7 and MgZn2 phases. In these transition structures, the unit cellof Mg4Zn7 phase is extended along [100] or [001] by half a unit cell length bycontinuation of the rhombic tiling. Structures of these extended unit cells areproposed.