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

TL;DR

This study investigates the orientation relationships between icosahedral clusters in MgZn2 and Mg4Zn7 phases within Mg-Zn(-Y) alloys, revealing their structural similarities and alignment in precipitates.

Contribution

It provides new insights into the orientation and structural relationships of intermetallic precipitates in Mg-Zn(-Y) alloys, highlighting their close structural similarities.

Findings

Precipitates contain domains of MgZn2 and Mg4Zn7 phases.

Icosahedral clusters are similarly aligned in different precipitate regions.

Structural similarities link precipitates in Mg-Zn-Y alloys.

Abstract

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.