Non-equilibrium and equilibrium Al-Pd-Mn decagonal quasicrystals and their related crystals: Structural characteristics and relationships revealed by high-resolution electron microscopy

Abstract

A non-equilibrium decagonal quasicrystal and its two related crystal structures, Al₃Mn type and its structural variant called the π-phase, were found to intergrow coherently to form a mixed state in the rapidly solidified Al₇₀Pd₁₀Mn₂₀ alloy. By annealing at 800°C, the non-equilibrium decagonal quasicrystal transforms to an equilibrium quasicrystal. From high-resolution electron microscopy of the as-quenched mixed structure and its annealed state, we directly reveal the successive structural transitions and relationships between the crystalline Al₃Mn and π-phases, and non-equilibrium and equilibrium decagonal quasicrystals. It is shown that the structures of all these phases can be systematically understood in terms of different configurations and tilings of three kinds of structural unit: a squashed hexagon, a star-shaped pentagon and decagon constructed by pentagonal atom columns.