The quasi-crystalline phase in the Mg—Al—Zn system

Abstract

Shechtman et al.¹ have reported a phase in rapidly solidified Al₈₆Mn₁₄ alloy with long-range orientational order, but with icosahedral point-group symmetry, which is inconsistent with lattice translations. However, Pauling² has argued that the apparent icosahedral symmetry in A1₈₆Mn₁₄ alloy is due to directed multiple twinning of cubic crystals with a cube edge of 26.7 Å. We report here the powder X-ray diffraction study of a rapidly solidified Mg₃₂(Al, Zn)₄₉ alloy which shows 5–3–2 symmetry diffraction in transmission electron microscopy (TEM). Also reported is a study of rapidly solidified Mg₂A1₃ alloy. The advantage of these alloys is that β-Mg₂Al₃, which has nearly the same structure as proposed by Pauling for the rapidly quenched Al₈₆Mn₁₄, is an equilibrium phase. This is unlike the Al–Mn system, where, according to Pauling, the cubic phase occurs in a metastable and already twinned form. The present choice of alloy systems makes it possible to compare the X-ray diffraction patterns from the quasi-crystalline phase and a cubic phase with nearly the same structure as proposed by Pauling. We observe that the X-ray diffraction pattern of rapidly solidified Mg₃₂(Al, Zn)₄₉ alloy is distinct from those of the equilibrium phases and that the pattern can be completely indexed to an icosahedral phase. We argue that the above observation is inconsistent with the proposal that the material consists of an aggregate of twinned cubic crystals. Therefore, the alternative is to invoke a quasi-crystalline lattice to explain the observed 5–3–2 symmetry diffraction in TEM.