Microstructure transformations in the heat-treated Mg-Zn-Y alloy

Mg-Zn-Y icosahedral quasicrystals have been prepared from the Mg63.5Zn34Y2.5 (at.%) alloy by copper and steel molds casting respectively. The phase transformations under different heat treatments have been investigated successively. Although the cooling rates of the two samples are quite different, the icosahedral quasicrystalline (I-phase) can precipitate from liquid directly almost at the same temperature. The eutectic structure (α-Mg + I-phase) forms at the later stage, while the Mg7Zn3 phase forms under a larger cooling rate (copper mold). Moreover, with a lower cooling speed, micro shrinkage cavities and cracks are easily caused by solute partitioning at the edge of petals-like I-phase. The volume fraction of I-phase is effectively increased after heat treatment at 350 °C for 50 h because I-phase can grow further in a facet manner during the heat treatment process. The I-phase in the Mg63.5Zn34Y2.5 alloy is thermodynamically unstable at 420 °C, and therefore transformations of the I-phase to the W or H-phase occur due to slow transformation kinetics and low thermodynamics driving force. In addition, few decagonal quasicrystalline phases (D-phases) can form on the I-phase/Mg7Zn3 interface. Atomic diffusion and concentration also play a crucial role in these transformations.