The nano-eutectic in the microstructure of semisolid Mg–6Zn–1Y alloy

• An (α-Mg+I-phase) nano-eutectic was discovered in the semisolid Mg-6Zn-1Y alloy under water-cooling conditions.
• The spacing of the nano-eutectic is 0.086 (±0.007±0.007) μm.
• The icosahedral type of I-phase solidified at fast cooling rate is primitive.

The microstructures of Mg–6Zn–1Y (at%) alloys cooling in the 20 °C water and 20 °C air respectively from semisolid state at 540 °C were investigated in this work. An (α-Mg+I-phase) nano-eutectic was discovered in the water-cooled samples. The details of the nano-eutectic were investigated under TEM. Compared with the face-centered icosahedral I-phase solidified at slow cooling rate, the icosahedral type of I-phase solidified at fast cooling rate is primitive. The orientation relationship of the two phase in the nano-eutectic was also studied, namely {011̅1̅}Mg∥{τ10}I−phase and {101̅0}Mg∥2fI−phase. The two phases with different lattice structure matched as semi-coherent, which indicated that the interface was in a low energy state.