Orientation relationships between icosahedral clusters in hexagonal MgZn_2 and monoclinic Mg_4Zn_7 phases in Mg-Zn(-Y) alloys

摘要

Intermetallic precipitates formed in heat-treated and aged Mg-Zn and Mg-Zn-Y alloys have been investigated via electron microscopy. Coarse spheroidal precipitates formed on deformation twin boundaries contained domains belonging to either the MgZn_2 hexagonal Laves phase or the monoclinic Mg _4Zn_7 phase. Both phases are structurally related to the quasi-crystalline phase formed in Mg-Zn-Y alloys, containing icosahedrally coordinated zinc atoms arranged as a series of broad rhombohedral units. This rhombohedral arrangement was also visible in intragranular precipitates where local regions with the structures of hexagonal MgZn_2 and Mg _4Zn_7 were found. The orientation adopted by the MgZn _2 and Mg_4Zn_7 phases in twin-boundary and intragranular precipitates was such that the icosahedral clusters were aligned similarly. These results highlight the close structural similarities between the precipitates of the Mg-Zn-Y alloy system.