Formation of an icosahedral quasicrystal and its approximant in Au-Al-Sc system

Abstract

The formation of the icosahedral (i-) and 1/1-approximant (1/1-) phases in the Au-Al-Sc system has been investigated. A primitive-type i-phase was found to form in an Au 45 Al 40 Sc 15 alloy. This i-phase was shown to be thermodynamically stable at 973 K. A stable 1/1-phase was found to form at 1173K for compositions of Au 44+x Al 41-x Sc 15 (x = 0, 1, 2, 3). High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) revealed that the 1/1-phase consists of Tsai-type icosahedral clusters. Similar local structures were observed in the HAADF-STEM image for the i-phase, indicating that the i-phase consists of Tsai-type clusters identical to the clusters in the 1/1-phase. The composition of the Au-Al-Sc i-phase is consistent with the formation conditions, including those in the valence electrons per atom ratio and in the atomic size factor, which has been shown previously for other Tsai-type i-phases. This fact demonstrates the importance of these conditions for stabilization of Tsai-type i-phases.