Formation of three types of quasicrystal in the Al-Pd-Mg system

Abstract

Abstract In the Al-Pd-Mg system, a Mackay icosahedron (MI) type of icosahedral phase (I-phase) with the F-type superlattice order and a decagonal phase (D-phase) have been found to form in a melt-quenched state, in addition to the Frank-Kasper (FK) type of stable I-phase reported previously. This is the first example in which the three types of quasicrystalline phase are formed in the same alloy system. The formation range of the FK-type I-phase in the melt-quenched state is rather wide; 5–20 at.%Pd and 20–45 at.%Mg. In contrast, the MI-type I-phase and D-phase are formed in small composition ranges around Al52Pd31Mg17 and Al74Pd21Mg5 respectively. Electron diffraction studies have shown that the period along the tenfold axis of the D-phase is about 1.6 nm. The formation and stability of the MI-type and FK-type I-phases have been discussed in terms of a Hume-Rothery rule.