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

摘要

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 °C, and that the 14H is an equilibrium phase below 537 °C. In the small temperature range between 537 and 483 °C, the 18R and 14H can co-exist in equilibrium in some special alloy compositions.