Phase formation involving icosahedral quasicrystals (iQc) in the Mg-Cd-Yb system was investigated. The phase diagrams obtained revealed that the iQc is in equilibrium with either (Mg, Cd)2Yb or an α-Mg phase over a wide composition range at 673 K. A eutectic reaction, where the melt decomposed to a rod-like lamella structure consisting of iQc and α-Mg phases was observed for Mg68Cd24Yb8 at 735 K. High-angle annular dark-field scanning transmission microscopy observation of the iQc in Mg96Cd3Yb1 verified the atomic positions of the Yb icosahedra and confirmed that the i-MgCdYb is isostructural to the i-CdYb. The formation of the eutectic structure is responsible for the high stability of the iQc/α-Mg interfaces because of good lattice matching; which is coincident interplanar spacing over several planes for the two phases. This coincidence in interplanar spacing was further confirmed in the real atomic structure, for which the twofold planes of the iQc, and the [0 0 0 2] and [2 -1 -1 0] planes of α-Mg are dominant factors in determining the stability of the interfaces.
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