  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 MgZn2 hexagonal Laves phase or the monoclinic Mg4Zn7 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 MgZn2 and Mg4Zn7 were found. The orientation adopted by the MgZn2
and Mg4Zn7 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.
