Orientation relationships between icosahedral clusters in hexagonal MgZn2 and monoclinic Mg4Zn7 phases in Mg-Zn(-Y) alloys

摘要

Intermetallic precipitates formed in heat-treated and aged Mg-Zn and Mg-Zn-Yalloys have been investigated via electron microscopy. Coarse spheroidalprecipitates formed on deformation twin boundaries contained domains belongingto either the MgZn2 hexagonal Laves phase or the monoclinic Mg4Zn7 phase. Bothphases are structurally related to the quasi-crystalline phase formed inMg-Zn-Y alloys, containing icosahedrally coordinated zinc atoms arranged as aseries of broad rhombohedral units. This rhombohedral arrangement was alsovisible in intragranular precipitates where local regions with the structuresof hexagonal MgZn2 and Mg4Zn7 were found. The orientation adopted by the MgZn2and Mg4Zn7 phases in twin-boundary and intragranular precipitates was such thatthe icosahedral clusters were aligned similarly. These results highlight theclose structural similarities between the precipitates of the Mg-Zn-Y alloysystem.