Quasicrystals and related approximant phases in Mg–Zn–Y

Abstract

As-cast microstructure of Mg-rich Mg 68Zn 28Y 4 has been investigated by a detailed transmission electron microscopy study. The as-cast Mg 68Zn 28Y 4 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 A ̊ 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 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.