Experimental and calculated phases in two as-cast and annealed Mg–Zn–Y alloys

The CALPHAD (Calculation of Phase Diagram) method was used to select ternary alloys from Mg–Zn–Y system, aimed at determining the role of precipitates in the microstructure and texture evolution of Mg during and after deformation. The selected alloys are Mg–6Zn–1.2Y and Mg–5Zn–2Y. The constituent phases in the as-cast Mg–6Zn–1.2Y alloy are α-Mg solid solution phase and I (Mg3YZn6) intermetallic phase. The as-cast Mg–5Zn–2Y alloy is composed of α-Mg, I and W (Mg3Y2Zn3) phases. The intermetallics in the two alloys form by eutectic reaction, which in Mg–5Zn–2Y alloy results in initially W-phase formation and ultimately I-phase formation during solidification. After heat treatment, the Mg–6Zn–1.2Y and Mg–5Zn–2Y alloys contain nearly the same amount of ternary intermetallics (I and W phases, respectively) in equilibrium with α-Mg solid solution phase. The main solute in α-Mg phase is Zn with the same amount in the two alloys. The type and quantity of the phases obtained experimentally disagree with the results obtained from the thermodynamic database. One important discrepancy is that, in Mg–6Zn–1.2Y alloy, the I phase is not stable at the temperature of 430 °C, and that the W phase is the stable phase at this temperature. The differences in the experimental and calculated data indicate that the Mg–Zn–Y system requires to be reassessed with more experimental data.

► Mg–6Zn–1.2Y and Mg–5Zn–2Y alloys were selected by FactSage™ Thermodynamic software.
► The I and W intermetallics in the two alloys form by eutectic reaction.
► The alloys contain similar amounts of different intermetallics in equilibrium with α-Mg.
► In Mg–6Zn–1.2Y, the I phase is not stable at the temperature of 430 °C.
► The hardness of W phase is determined to be lower than that of I phase.