Abstract
Precipitation of the ordered icosahedral quasicrystal in Mg-6wt%Zn-1wt%Y alloy has been characterized by transmission electron microscopy observations. The lamellar-type icosahedral qusicrystal phases (I-phase) with the face-centered icosahedral (FCI) structure are observed in alloy after solution treatment at <TEX>$550^{\circ}C$</TEX>. In the alloy annealed at <TEX>$400^{\circ}C$</TEX>, polygon-shaped I-phases are observed in the <TEX>${\alpha}$</TEX>-Mg matrix. The interfaces of the I-phase with the matrix are facetted and the facets are on five-fold and two- fold plane of the I-phase. The orientation relationship of the I-phase with the matrix is determined to be <TEX>$[I5]_I//[001]_{Mg}$</TEX>, <TEX>$(2f)_I//(2\overline{1}0)_{Mg}$</TEX> and <TEX>$[I2]_I//[311]_{Mg}$</TEX>, <TEX>$(5f)_I//(0\overline{1}1)_{Mg}$</TEX>. The icosahedral grains are occasionally found to be twinned with one of the five-fold axis as the twin axis. The twin boundaries appear to be fairly straight and perpendicular to the fivefold twin axis. The icosahedral twin can be expressed as a rotation of <TEX>$63.4^{\circ}$</TEX> or <TEX>$116.62^{\circ}$</TEX> around two fold zone axis.
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