Phase equilibria and transformations in ternary Mg-rich Mg-Y-Zn alloys

The long-period stacking ordered structures 18R and 14H formed in Mg-Y-X (X = Zn, Cu, Ni) systems have received considerable interest over the past decade, but their thermal stability and relationships with other intermetallic phases in the Mg-Y-X systems remain to be unambiguously established. In this study, the occurrence and transformations of long-period stacking ordered structures 18R and 14H are clarified in as-cast and heat-treated Mg-Y-Zn alloys. The 18R structure is a stable equilibrium phase that forms directly from the melt whereas the 14H cannot form directly from the melt but forms in a solid-state transformation. That explains the absence of 14H in the as-cast microstructures of the alloys. These findings are embedded in the complete description of Mg-Y-Zn phase equilibria, generated by Calphad-type thermodynamic calculations and verified for a range of Mg-rich alloys by electron microscopy and thermal analysis. It is found that the 18R is a stable equilibrium phase that exists in the high temperature range from 753 to 483 degrees C, and that the 14H is an equilibrium phase below 537 degrees C. In the small temperature range between 537 and 483 degrees C, the 18R and 14H can co-exist in equilibrium in some special alloy compositions. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.