Quasicrystals and quasicrystal-related phases in the AlMn system

The AlMn system is very rich in phases with composition close to Al₄Mn. As solidification conditions change from very slow (casting) to extremely fast (atomized submicron-size droplets) the following phases will form: hexagonal μ phase (a = 1.995nm; c = 2.452nm), hexagonal λ phase (a = 2.841 nm; c = 1.238 nm), decagonal quasicrystal, icosahedral quasicrystal and microquasicrystalline or “amorphous” phase. In the present work, the potential interrelationship between the structures of the crystalline, quasicrystalline and amorphous Al₄Mn phases is investigated by a systematic study of electron diffraction intensities. Analysis of electron diffraction intensity modulations and their spatial relationships suggests that the phases have a structural skeleton of icosahedral units, possibly of the Mackay icosahedron type. Different crystalline and quasi-periodic phases can be formed by different stackings of the same icosahedral clusters and they are not necessarily in a single orientation. The amorphous structure can be described as a network of the randomly oriented clusters.