The thermodynamic tailoring for Mg86Y8Zn6 alloy with sole long-period stacking ordered structures

Abstract The alloy composition and the heat-treatment temperature of a Mg86Y8Zn6 (at.%) alloy with sole long-period stacking ordered (LPSO) structures 18 R and 14H was tailored under the assistance of thermodynamic Mg-Y-Zn phase diagram calculations. Before the tailoring for the Mg86Y8Zn6 alloy, a real alloy Mg86.2Y7.7Zn6.1 was studied for comparison between the thermodynamic calculations and the experimental measurements, indicating the effectiveness of the phase equilibrium design approach. Scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS), transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was used for microstructure and phase analyses. After annealing at 500°C for 240 h, the microstructure of the as-cast Mg86.2Y7.7Zn6.1 alloy composed of α-Mg, W-Mg3Y2Zn3, 18 R and 14H was thoroughly transformed to that solely composed of 18 R and 14H. The validated design approach indicates a narrow composition and temperature window to avoid phases such as α-Mg or W-Mg3Y2Zn3.