Quasicrystals and related approximant phases in Mg-Zn-Y

As-cast microstructure of Mg-rich Mg₆₈Zn₂₈Y₄ has been investigated by a detailed transmission electron microscopy study: The as-cast Mg₆₈Zn₂₈Y₄ alloy consisted of three different types of phases: 10-20 μm 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 Å and α = 63.43° 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 Mg₄Zn₇ 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 Mg₄Zn₇ phases from icosahedral and decagonal phases, respectively.