• Applied Microscopy
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• v.32 no.1
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• pp.31-37
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• 2002

As-cast microstructure of Mg-rich $Mg_{68}Zn_{28}Y_4$ has been investigated by a detailed transmission electron microscopy. The as-cast $Mg_{68}Zn_{28}Y_4$ alloy consisted of three different types of phases: $10{\sim}20{\mu}m$ size primary solidification phase, dendritic phase grown from the primary phase and a eutectic structure formed at the later stage of solidification. The primary solidification phase has an icosahedral structure with a large degree of phason strain. 1/1 rhombohedral approximant phase with lattice parameters: $a=27.2{\AA}\;and\;{\alpha}=63.43^{\circ}$ is first observed in Mg-Zn-Y system. The rhombohedral structure can be obtained by introducing phason strain in the six dimensional face centered hyper-cubic lattice. The decagonal phase nucleates with orientation relationship with the icosahedral phase, and $Mg_4Zn_7$ nucleates with orientation relationship with the decagonal phase, indicating a close structural similarity between the three phases. Gradual depletion of Y during solidification plays an important role in heterogeneous nucleation of decagonal and $Mg_4Zn_7$ phases from icosahedral and decagonal phases respectively.